IPv4 Addressing and Subnetting in cisco.

IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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- IPv4 Addressing and Subnetting -
Hardware Addressing
The hardware address is used by devices to communicate on the local
network. Hardware addressing is a function of the data-link layer of the OSI
model (Layer-2).
The hardware address for Ethernet networks is the MAC address, a 48-bit
hexadecimal address that is usually hard-coded on the network card. In
theory, this means the MAC address cannot be altered; however, the MAC
address is often stored in flash on the NIC, and thus can be changed with
special utilities.
MAC addresses can be represented in two formats (either notation is
acceptable):
00:43:AB:F2:32:13
0043.ABF2.3213
The MAC address has one shortcoming – it contains no hierarchy. There is
no mechanism to create boundaries between networks.
Instead, the first six hexadecimal digits of a MAC identify the manufacturer
of the network card (referred to as the OUI (Organizational Unique
Identifier)), and the last 6 digits identify the host device (referred to as the
host ID). Still, there is no way to distinguish one network from another.
Imagine the difficulties this poses. If only hardware addressing existed, all
devices would technically be on the same network. Modern internetwork
systems like the Internet could not exist, as there would be no way to
separate my network from your network.
Furthermore, imagine if the entire Internet was a purely switched, data-link
layer environment. Switches, as a rule, forward broadcasts out all ports.
Guesstimating that there are billions devices on the Internet, with each
device sending out a broadcast on average every few seconds, the resulting
broadcast storms would be devastating. The Internet would simply collapse.
The need for logical addressing, and routers, became apparent.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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Logical Addressing
Logical addressing is a function of the network layer of the OSI Model
(Layer-3).
Logical addresses, unlike hardware addresses, provide a hierarchical
structure to separate networks. A logical address identifies not only a unique
Host ID, but also the network that host belongs to. Additionally, logical
addresses are rarely hard-coded onto hosts, and can be changed freely.
Two common logical addressing protocols are IPX (Internetwork Packet
Exchange) and IP (Internet Protocol). IPX was predominantly used on
Novell networks, but is mostly deprecated. IP is the most widely-used
logical address today.
Internet Protocol (IP)
IP was developed by the Department of Defense (DoD) during the late
1970’s. It was included in a group of protocols that became known as the
TCP/IP protocol suite.
The DoD developed their own networking model to organize and define the
TCP/IP protocol suite. This became known as the DoD Model, and consists
of four layers:
OSI Model DoD Model Example Protocols
7 Application
6 Presentation
5 Session
4 Application FTP, HTTP, SMTP
4 Transport 3 Host-to-Host TCP, UDP
3 Network 2 Internet IP
2 Data-link
1 Physical
1 Network Access Ethernet
IP provides two core functions:
• Logical addressing of hosts
• Routing of packets between networks.
IP has undergone several revisions. IP Version 4 (IPv4) is currently in
widespread deployment, but will eventually be replaced with IP Version 6
(IPv6). This guide will concentrate on IPv4, and IPv6 will be covered
extensively in a separate guide.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

IPv4 Addressing
One of IP’s core functions is to provide logical addressing for hosts. An IP
address provides a hierarchical structure to separate networks. Consider the
following address as an example:
158.80.164.3
An IP address is separated into four octets:
First Octet
Second Octet Third Octet Fourth Octet
158 .80 .164 .3
Each octet is 8 bits long, resulting in a 32-bit IP address. A computer
understands an IP address in its binary form; the above address in binary
would look as follows:
First Octet
Second Octet Third Octet Fourth Octet
10011110 .01010000 .10100100 .00000011
Part of the above IP address identifies the network. The other part of the
address identifies the host. A subnet mask helps make this distinction.
Consider the following:
158.80.164.3 255.255.0.0
The above IP address has a subnet mask of 255.255.0.0. The subnet mask
follows two rules:
• If a binary bit is set to a 1 (or on) in a subnet mask, the corresponding
bit in the address identifies the network.
• If a binary bit is set to a 0 (or off) in a subnet mask, the corresponding
bit in the address identifies the host.
Looking at the above address and subnet mask in binary:
Address: 10011110.01010000.10100100.00000011
Subnet Mask: 11111111.11111111.00000000.00000000
The first 16 bits of the subnet mask are set to 1. Thus, the first 16 bits of the
address (158.80) identify the network. The last 16 bits of the subnet mask are
set to 0. Thus, the last 16 bits of the address (164.3) identify the unique host
on that network.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

IPv4 Addressing (continued)
Hosts on the same logical network will have identical network addresses,
and can communicate freely. For example, the following two hosts are on
the same network:
Host A: 158.80.164.100 255.255.0.0
Host B: 158.80.164.101 255.255.0.0
Both share the same network address (158.80), which is determined by the
255.255.0.0 subnet mask. Hosts that are on different networks cannot
communicate without an intermediating device. For example:
Host A: 158.80.164.100 255.255.0.0
Host B: 158.85.164.101 255.255.0.0
The subnet mask has remained the same, but the network addresses are now
different (158.80 and 158.85 respectively). Thus, the two hosts are not on
the same network, and cannot communicate without a router between them.
Routing is the process of sending packets from one network to another.
Consider the following, trickier example:
Host A: 158.80.1.1 255.248.0.0
Host B: 158.79.1.1 255.248.0.0
The specified subnet mask is now 255.248.0.0, which doesn’t fall cleanly on
an octet boundary. To determine if these hosts are on separate networks, first
convert everything to binary:
Host A Address: 10011110.01010000.00000001.00000001
Host B Address: 10011110.01001111.00000001.00000001
Subnet Mask: 11111111.11111000.00000000.00000000
Remember, the 1 (or on) bits in the subnet mask identify the network portion
of the address. In this example, the first 13 bits (the 8 bits of the first octet,
and the first 5 bits of the second octet) identify the network. Looking at only
the first 13 bits of each address:
Host A Address: 10011110.01010
Host B Address: 10011110.01001
Clearly, the network addresses are not identical. Thus, these two devices are
on separate networks, and require a router to communicate.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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IP Address Classes
The IPv4 address space has been structured into several classes. The value
of the first octet of an address determines the class of the network:
Class First Octet Range
Default Subnet Mask
Class A 1 - 127 255.0.0.0
Class B 128 - 191 255.255.0.0
Class C 192 - 223 255.255.255.0
Class A networks range from 1 to 127. The default subnet mask is
255.0.0.0; thus, by default, the first octet defines the network, and last three
octets define the host. This results in a maximum of 127 Class A networks,
with 16,777,214 hosts per network!
Example of a Class A address:
Address: 64.32.254.100
Subnet Mask: 255.0.0.0
Class B networks range from 128 to 191. The default subnet mask is
255.255.0.0; thus, by default, the first two octets define the network, and the
last two octets define the host. This results in a maximum of 16,384 Class B
networks, with 65,534 hosts per network.
Example of a Class B address:
Address: 152.4.12.195
Subnet Mask: 255.255.0.0
Class C networks range from 192 to 223. The default subnet mask is
255.255.255.0; thus, by default, the first three octets define the network, and
the last octet defines the host. This results in a maximum of 2,097,152 Class
C networks, with 254 hosts per network.
Example of a Class C address:
Address: 207.79.233.6
Subnet Mask: 255.255.255.0
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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CIDR (Classless Inter-Domain Routing)
Classless Inter-Domain Routing (CIDR) is simplified method of
representing a subnet mask. CIDR identifies the number of binary bits set to
a 1 (or on) in a subnet mask, preceded by a slash.
Consider the following subnet mask: 255.255.255.240
Looking at the above subnet mask in binary:
11111111.11111111.11111111.11110000
The first 28 bits of the above subnet mask are set to 1. To represent this in
CIDR notation: /28
Consider this next example:
192.168.1.1 255.255.255.0
The above address/subnet mask can be represented as follows using CIDR:
192.168.1.1 /24
Address “Classes” vs. Subnet Mask
Remember the following three rules:
• The first octet on an address dictates the class of that address.
• The subnet mask dictates what portion of an address identifies the
network, and what portion identifies the host.
• Each class has a default subnet mask.
Thus, the address 10.1.1.1 is a Class A address, and its default subnet mask
is 255.0.0.0 (or in CIDR, /8). However, it is possible to use subnet masks
other than the default, such as applying a Class B mask to a Class A address:
10.1.1.1 /16
However, this does not change the class of the above address. It remains a
Class A address, which has been subnetted using a Class B mask.
Remember, the only thing that determines the class of an IP address is the
first octet of that address. Likewise, the subnet mask is the only thing that
determines what portion of an address is the network, and which portion is
the host.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

Subnet and Broadcast Addresses
Two addresses have been reserved on each network for special use. Each
network must have a subnet (or network) address, and a broadcast address.
Neither of these addresses can be assigned to a host device.
The subnet address is used to identify the network itself. Routing tables
contain lists of networks, and each network is identified by its subnet
address. Subnet addresses contain all 0 bits in the host portion of the
address.
For example, the following is a subnet address: 192.168.1.0/24
The broadcast address identifies all hosts on a particular network. A packet
sent to the broadcast address will be received and processed by every device
on that network. Broadcast addresses contain all 1 bits in the host portion
of the address.
For example, the following is a broadcast address: 192.168.1.255/24
Broadcasts are one of three types of IP packets:
• Unicasts are packets sent from one host to another host
• Multicasts are packets sent from one host to a group of hosts
• Broadcasts, as stated earlier, are packets sent from one host to all
other hosts on the local network
A router, by default, will never forward a multicast or broadcast packet
from one interface to another.
A switch, be default, will forward a multicast or broadcast out every port,
except for the port that sent the multicast/broadcast.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

Subnetting
Subnetting is the process of creating new networks (or subnets) by stealing
bits from the host portion of a subnet mask. There is one caveat: stealing bits
from hosts creates more networks but fewer hosts per network. Thus, every
time a network is subnetted, addresses are lost.
Consider the following Class C network:
192.168.254.0
The default subnet mask for this network is 255.255.255.0. This single
network can be segmented, or subnetted, into multiple networks. For
example, assume a minimum of 10 new networks are required. Resolving
this is possible using the following magical formula:
2n – 2
The exponent ‘n’ identifies the number of bits to steal from the host portion
of the subnet mask. The default Class C mask (255.255.255.0) looks as
follows in binary:
11111111.1111111.1111111.00000000
There are a total of 24 bits set to 1, which are used to identify the network.
There are a total of 8 bits set to 0, which are used to identify the host, and
these host bits can be ‘stolen.’
Stealing bits essentially involves changing host bits (set to 0 or off) in the
subnet mask to network bits (set to 1 or on). Network bits in a subnet mask
must always be sequential, skipping bits is not allowed.
Consider the result if three bits are stolen. Using the above formula:
2n – 2 = 23 – 2 = 8 – 2 = 6 new networks created
However, a total of six new networks does not meet the original requirement
of at least 10 networks. Consider the result if four bits are stolen:
2n – 2 = 24 – 2 = 16 – 2 = 14 new networks created
A total of fourteen new networks does meet the original requirement.
Stealing four host bits results in the following new subnet mask:
11111111.11111111.11111111.11110000 = 255.255.255.240
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

Subnetting (continued)
In the previous example, a Class C network was subnetted to create 14 new
networks, using a subnet mask of 255.255.255.240 (or /28 in CIDR). Four
bits were stolen in the subnet mask, leaving only four bits for hosts.
To determine the number of hosts this results in, for each of the new 14
networks, the same formula can be used: 2n – 2
Consider the result if four bits are available for hosts:
2n – 2 = 24 – 2 = 16 – 2 = 14 usable hosts per network
Thus, subnetting a Class C network with a /28 mask creates fourteen new
networks, with fourteen usable hosts per network.
The “-2” Rule of Subnetting
There is a specific purpose for the ‘– 2’ portion of the 2n – 2 formula.
Previously, it was unacceptable to use an address that contained all ‘0’ or all
‘1’ bits in the network portion of the address.
However, this is no longer true on modern systems. Specifically, on Cisco
IOS devices, the following command is now enabled by default:
Router(config)# ip subnet-zero
The ip subnet-zero commands allows for the use of networks with all ‘0’ or
all ‘1’ bits in the network portion of the address. Thus, the formula for
calculating the number of new network is slightly altered, to simply 2n.
Consider if four bits are stolen for networks:
2n
= 24
= 16 new networks created
However, it is never possible to assign an address with all ‘0’ or all ‘1’
bits in the host portion of the address. These are reserved for the subnet
and broadcast addresses, respectively. Thus, the formula for calculating
usable hosts is always 2n – 2.
Some have questioned whether CCNA/CCNP simulations and questions
have ip subnet-zero enabled. It is generally accepted that having this enabled
is now default behavior, and test questions should be answered accordingly.
All future examples in this guide will assume the command is enabled.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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Determining the ‘Range’ of Subnetted Networks
Determining the range of the newly created networks can be accomplished
using several methods. The ‘long’ method involves some binary magic.
Still looking at the example 192.168.254.0 network, which was subnetted
using a 255.255.255.240 mask:
192.168.254.0: 11000000.10101000.11111110.00000000
255.255.255.240: 11111111.11111111.11111111.11110000
Subnetting stole four bits in the fourth octet, creating a total of 16 new
networks (assuming ip subnet-zero is enabled). Looking at only the fourth
octet, the first newly created network is 0000. The second new network is
0001. Calculating all possible permutations of the four stolen bits:
Binary Decimal Binary Decimal Binary Decimal
.0000 xxxx .0 .0110 xxxx .96 .1100 xxxx .192
.0001 xxxx .16 .0111 xxxx .112 .1101 xxxx .208
.0010 xxxx .32 .1000 xxxx .128 .1110 xxxx .224
.0011 xxxx .48 .1001 xxxx .144 .1111 xxxx .240
.0100 xxxx .64 .1010 xxxx .160
.0101 xxxx .80 .1011 xxxx .176
Note that this equates to exactly 16 new networks. The decimal value
represents the first (or the subnet) address of each newly created network. To
determine the range for the hosts of the first new network:
Binary Decimal Binary Decimal Binary Decimal
.0000 0000 .0 .0000 0110 .6 .0000 1100 .12
.0000 0001 .1 .0000 0111 .7 .0000 1101 .13
.0000 0010 .2 .0000 1000 .8 .0000 1110 .14
.0000 0011 .3 .0000 1001 .9 .0000 1111 .15
.0000 0100 .4 .0000 1010 .10
.0000 0101 .5 .0000 1011 .11
The binary value has been ‘split’ to emphasize the separation of the network
bits from the host bits. The first address has all 0 bits in the host portion
(0000), and thus is the subnet address for this network. The last address has
all 1 bits in the host portion, and thus is the broadcast address for this
network. Note that there are exactly 14 usable addresses to assign to hosts.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

Determining the ‘Range’ of Subnetted Networks (continued)
Calculating the ranges of subnetted networks can quickly become tedious
when using the ‘long’ binary method. The ‘shortcut’ method involves taking
the subnet mask (255.255.255.240 from the previous example), and
subtracting the subnetted octet (240) from 256.
256 – 240 = 16
Assuming ip subnet-zero is enabled, the first network will begin at 0. Then,
simply continue adding 16 to list the first address of each new network:
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
Knowing the first address of each new network makes it simple to determine
the last address of each network:
First address of network 0 16 32 48 64 80 96 112 128 144
Last address of network 15 31 47 63 79 95 111 127 143 159
Only the first 10 networks were calculated, for brevity. The first address of
each network becomes the subnet address for that network. The last address
of each network becomes the broadcast address for that network.
Once the first and last address of each network is known, determining the
usable range for hosts is straightforward:
Subnet address 0 16 32 48 64 80 96 112 128 144
Usable Range
1
14
17
30
33
46
49
62
65
78
81
94
97
110
113
126
129
142
145
158
Broadcast address 15 31 47 63 79 95 111 127 143 159
Hosts on the same network (such as 192.168.254.2 and 192.168.254.14) can
communicate freely.
Hosts on different networks (such as 192.168.254.61 and 192.168.254.66)
require a router to communicate.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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Class A Subnetting Example
Consider the following subnetted Class A network: 10.0.0.0 255.255.248.0
Now consider the following questions:
• How many new networks were created?
• How many usable hosts are there per network?
• What is the full range of the first three networks?
By default, the 10.0.0.0 network has a subnet mask of 255.0.0.0. To
determine the number of bits stolen:
255.0.0.0: 11111111.00000000.00000000.00000000
255.255.248.0: 11111111.11111111.11111000.00000000
Clearly, 13 bits have been stolen to create the new subnet mask. To calculate
the total number of new networks:
2n
= 213
= 8192 new networks created
There are clearly 11 bits remaining in the host portion of the mask:
2n – 2 = 211 – 2 = 2048 – 2 = 2046 usable hosts per network
Calculating the ranges is a bit tricky. Using the ‘shortcut’ method, subtract
the third octet (248) of the subnet mask (255.255.248.0) from 256.
256 – 248 = 8
The first network will begin at 0, again. However, the ranges are spread
across multiple octets. The ranges of the first three networks look as follows:
Subnet address 10.0.0.0 10.0.8.0 10.0.16.0
Usable Range
10.0.0.1
10.0.7.254
10.0.8.1
10.0.15.254
10.0.16.1
10.0.23.254
Broadcast address 10.0.7.255 10.0.15.255 10.0.23.255
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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Private vs Public Addresses
The rapid growth of the Internet resulted in a shortage of IPv4 addresses. In
response, the powers that be designated a specific subset of the IPv4 address
space to be private, to temporarily alleviate this problem.
A public address can be routed on the Internet. Thus, devices that should be
Internet accessible (such as web or email servers) must be configured with
public addresses.
A private address is only intended for use within an organization, and can
never be routed on the internet. Three private addressing ranges were
allocated, one for each IPv4 class:
• Class A - 10.x.x.x
• Class B - 172.16-31.x.x
• Class C - 192.168.x.x
NAT (Network Address Translation) is used to translate between private
addresses and public addresses. NAT allows devices configured with a
private address to be stamped with a public address, thus allowing those
devices to communicate across the Internet. NAT is covered in-depth in
another guide.
NAT is only a temporarily solution to the address shortage problem.
Eventually, IPv4 will be replaced with IPv6. This also is covered extensively
in another guide.
Two other ranges, while not considered “private,” have been reserved for
specific use:
• 127.x.x.x - reserved for diagnostic purposes. One such address
(127.0.0.1), identifies the local host, and is referred to as the loopback
or localhost address.
• 169.254.x.x - reserved for Automatic Private IP Addressing (APIPA).
A host assigns itself an APIPA address if a DHCP server is
unavailable to dynamically assign an address.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
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The IPv4 Header
The IPv4 header has 12 required fields and 1 optional field¸ and is 160 bits
long.
Field Length Description
Version 4 bits Version of IP (in this case, IPv4)
Header Length 4 bits Specifies the length of the IP header (minimum 160 bits)
Type of Service 8 bits Classifies traffic for QoS
Total Length 16 bits Specifies the length of both the header and data payload
Identification 16 bits Uniquely identifies fragments of a packet
Flags 3 bits Flags for fragmentation
Fragment Offset 13 bits Identifies the location of a fragment in a packet
Time to Live 8 bits Decremented by each router traversed
Protocol 8 bits Specifies the next upper layer protocol
Header Checksum 16 bits Checksum for error checking
Source Address 32 bits Source IPv4 address
Destination Address 32 bits Destination IPv4 address
Options 32 bits Optional field for various parameters
The Identification, Flags, and Fragment Offset fields are used in
conjunction with each other. An IP packet larger than the MTU size of a link
must be fragmented. Each fragment of the packet is marked with the same
Identification number. The Fragment Offset allows the destination device to
reassemble the fragments in the proper order.
The Flags field can dictate two conditions:
• Don’t Fragment (DF) – indicates the packet cannot be fragmented. If
the packet reaches a link with a small MTU, the packet is then
dropped, and an ICMP error message is sent back to the source.
• More Fragments (MF) – all fragments have this bit set to one, except
for the last fragment, where the bit is set to zero. This allows the
destination device to know it has received all fragments.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

IPv4 Protocol Numbers
The Next Header field is of some importance. This field identifies the next
upper-layer header (for example, UDP, TCP or ICMP). These upper layer
protocols are identified using IP Protocol Numbers.
The following is a list of common IP Protocol Numbers:
Protocol
Number
Upper-Layer Protocol
1 ICMP
2 IGMP
6 TCP
9 IGRP
17 UDP
46 RSVP
47 GRE
50 IPSEC ESP
51 IPSEC AH
88 EIGRP
89 OSPF
(Reference: http://www.iana.org/assignments/protocol-numbers)
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

Resolving Logical Addresses to Hardware Addresses
Hosts cannot directly send data to another device’s logical address. Network
communication occurs across the data-link layer, using hardware addresses.
A mechanism is required to map logical addresses to hardware addresses.
When using IP over an Ethernet network, the Address Resolution Protocol
(ARP) provides this function for us. ARP allows a host to determine the
MAC (hardware) address for a particular IP (logical) address.
Observe the above diagram. Following the step-by-step path a packet travels
from HostA to the 10.2.1.5 address (HostB):
• First, HostA determines if the 10.2.1.5 address is itself. If the address
is configured on a local interface, the packet never leaves HostA.
• Second, HostA determines if the 10.2.1.5 address is on the same
network (or subnet). If it is, HostA will broadcast an ARP request, and
wait for the appropriate host to reply with its MAC address.
• HostA determines that the 10.2.1.5 address is indeed on a separate
network. It now parses its local routing table for a route to this remote
network. Usually, hosts will be equipped with a default route (or,
default gateway), to reach all other networks.
• Host A determines that RouterA is its default gateway. The host
broadcasts an ARP request for RouterA’s MAC address, and then
forwards the packet to RouterA’s MAC (4444.5555.6666).
• RouterA receives the packet, and parses at its own routing table. It
determines that the 10.2.x.x network is directly attached off of its fa1
interface. The router then broadcasts an ARP request for the 10.2.1.5
address.
• HostB responds to the router’s ARP request with its MAC address
(AAAA.BBBB.CCCC). RouterA is then able to forward the packet to
HostB.
IPv4 Addressing and Subnetting v1.32 – Aaron Balchunas
* * *

Troubleshooting IP using ICMP
Internet Control Message Protocol (ICMP) is used for a multitude of
informational and error messaging purposes.
The following is a list of common ICMP types and codes:
Type Code Description
0 0 Echo Reply
- Destination Unreachable
0 Network Unreachable
1 Host Unreachable
2 Protocol Unreachable
3 Port Unreachable
4 Fragmentation Needed – Don’t Fragment Flag Set
6 Destination Network Unknown
7 Destination Host Unknown
9 Destination Network Administratively Prohibited
10 Destination Host Administratively Prohibited
3
5 Redirect
8 Echo
11 TTL Exceeded
Several IP troubleshooting tools utilize ICMP, including Packet Internet
Groper (ping) and traceroute.
Ping utilizes the Echo Request and Echo Reply ICMP messages to
determine if a host is responding on a particular address.
Traceroute determines the routing path a packet takes to reach its
destination.

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what is ethernet in networking for basic CCNA configuration

Types of Twisted-Pair Cables
Various types of twisted-pair cables can be used. A straight-through cable
is used in the following circumstances:
• From a host to a hub (or switch)
• From a router to a hub (or switch)
The pins (wires) on each end of a straight-through cable must be identical.
The most common straight-through standard is wired as follows:
1
2
3
4
5
6
7
8
White Orange
Orange
White Green
Blue
White Blue
Green
White Brown
Brown
------------------------
------------------------
------------------------
------------------------
------------------------
------------------------
------------------------
------------------------
White Orange
Orange
White Green
Blue
White Blue
Green
White Brown
Brown
1
2
3
4
5
6
7
8
A cross-over cable is used in the following circumstances:
• From a host to a host
• From a hub to a hub
• From a switch to a switch
• From a hub to a switch
• From a router to a router
To make a crossover cable, we must swap pins 1 and 3, and pins 2 and 6 on
one end of the cable. The most common crossover standard is as follows:
1
2
3
4
5
6
7
8
White Orange
Orange
White Green
Blue
White Blue
Green
White Brown
Brown
------------------------
------------------------
------------------------
------------------------
------------------------
------------------------
------------------------
------------------------
White Green
Green
White Orange
Blue
White Blue
Orange
White Brown
Brown
3
6
1
4
5
2
7
8
Remember, when connecting anything into a hub or switch, except for
another hub or switch, we use a straight-through cable. When connecting
“like” devices, we use a crossover cable.
Ethernet Standards v1.10 – Aaron Balchunas
* * *

Types of Twisted-Pair Cables (continued)
Finally, a roll-over cable is used to connect a PC into a Cisco router’s
console or auxiliary port.
The pins are completely reversed on one end to make a rollover cable:
1
2
3
4
5
6
7
8
White Orange
Orange
White Green
Blue
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basic internetworking models,osi referance model TCP/IP configuration in cisco

Introduction to Networks -
What is a Network?
A network is defined as devices connected together to share information
and services. The types of data/services that can be shared on a network is
endless - documents, music, email, websites, databases, printers, faxes,
telephony, videoconferencing, etc.
Protocols are “rules” that govern the method by which devices share data
and services. Protocols are covered in great detail in subsequent sections.
Basic Network Types
Networks are generally broken down into two types:
LANs (Local Area Networks) - a high-speed network that covers a
relatively small geographic area, usually contained within a single building
or campus. A LAN is usually under the administrative control of a single
entity/organization.
WANs (Wide Area Networks) – The book definition of a WAN is a
network that spans large geographical locations, usually to interconnect
multiple LANs.
A more practical definition describes a WAN as a network that traverses a
public network or commercial carrier, using one of several WAN
technologies. Thus, a WAN can be under the administrative control of
several entities or organizations, and does not need to “span large
geographical distances.”
Note: Occasionally, books will define a third type of network known as a
MAN (Metropolitan Area Network). A MAN is defined as a network that
spans several LAN’s across a city-wide geographic area. The term “MAN”
is less prevalent than either LAN or WAN.
Introduction to Networks v1.01 – Aaron Balchunas
* * *

Network “Architectures”
A host refers to any device that is connected to your network. Some define a
host as any device that has been assigned a network address.
A host can serve one or more functions:
• A host can request data (often referred to as a client)
• A host can provide data (often referred to as a server)
• A host can both request and provide data (often referred to as a peer)
Because of these varying functions, multiple network “architectures” have
been developed, including:
• Peer-to-Peer networks
• Client/Server networks
• Mainframe/Terminal networks
When using a peer-to-peer architecture, all hosts on the network can both
request and provide data and services. For example, configuring two
Windows XP workstations to share files would be considered a peer-to-peer
network.
Though peer-to-peer networks are simple to configure, there are several key
disadvantages to this type of architecture. First, data is spread across
multiple devices, making it difficult to manage and back-up that data.
Second, security becomes problematic, as you must configure individual
permissions and user accounts on each host.
When using a client/server architecture, hosts are assigned specific roles.
Clients request data and services stored on Servers. Connecting Windows
XP workstations to a Windows 2003 domain controller would be considered
a client/server network.
While client/server environments tend to be more complex than peer-to-peer
networks, there are several advantages. With data now centrally located on
a server or servers, there is only one place to manage, back-up, and secure
that data. This simplified management allows client/server networks to scale
much larger than peer-to-peer. The key disadvantage of client/server
architecture is that it introduces a single point of failure.
Introduction to Networks v1.01 – Aaron Balchunas
* *


OSI Reference Model v1.12 – Aaron Balchunas
* * *

- OSI Reference Model -
Network Reference Models
As computer network communication grew more prevalent, the need for a
consistent standard for vendor hardware and software became apparent.
Thus, the first development of a network reference model began in the
1970’s, spearheaded by an international standards organization.
A network reference model serves as a blueprint, dictating how network
communication should occur. Programmers and engineers design products
that adhere to these models, allowing products from multiple manufacturers
to interoperate.
Network models are organized into several layers, with each layer assigned
a specific networking function. These functions are controlled by protocols,
which govern end-to-end communication between devices.
Without the framework that network models provide, all network hardware
and software would have been proprietary. Organizations would have been
locked into a single vendor’s equipment, and global networks like the
Internet would have been impractical or even impossible.
The two most widely recognized network reference models are:
• The Open Systems Interconnection (OSI) model
• The Department of Defense (DoD) model
The OSI model was the first true network model, and consisted of seven
layers. However, the OSI model has become deprecated over time, replaced
with more practical models like the TCP/IP (or DoD) reference model.
Network models are not physical entities. For example, there is no OSI
device. Devices and protocols operate at a specific layer of a model,
depending on the function. Not every protocol fits perfectly within a specific
layer, and some protocols spread across several layers.
OSI Reference Model v1.12 – Aaron Balchunas
* * *

OSI Reference Model
The Open Systems Interconnection (OSI) model was developed in the
1970’s and formalized in 1983 by the International Organization for
Standardization (ISO). It was the first networking model, and provided the
framework governing how information is sent across a network.
The OSI Model (ISO standard 7498) consists of seven layers, each
corresponding to a particular network function:
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data-link
1 Physical
Various mnemonics have been devised to help people remember the order of
the OSI model’s layers:
7 Application All Away
6 Presentation People Pizza
5 Session Seem Sausage
4 Transport To Throw
3 Network Need Not
2 Data-link Data Do
1 Physical Processing Please
The ISO further developed an entire protocol suite based on the OSI model;
however, this OSI protocol suite was never widely implemented. More
common protocol suites can be difficult to fit within the OSI model’s layers,
and thus the model has been mostly deprecated.
A more practical model was developed by the Department of Defense
(DoD), and became the basis for the TCP/IP protocol suite (and
subsequently, the Internet). The DoD model is explained in detail later in
this guide.
The OSI model is still used predominantly for educational purposes, as
many protocols and devices are described by what layer they operate at.
OSI Reference Model v1.12 – Aaron Balchunas
* * *

The Upper Layers
The top three layers of the OSI model are often referred to as the upper
layers. Thus, protocols that operate at these layers are usually called upperlayer
protocols, and are generally implemented in software.
The function of the upper layers of the OSI model can be difficult to
visualize. The upper layer protocols do not fit perfectly within each layer;
and several protocols function at multiple layers.
The Application layer (Layer 7) provides the actual interface between the
user application and the network. The user directly interacts with this layer.
Examples of application layer protocols include:
• FTP (via an FTP client)
• HTTP (via a web-browser)
• SMTP (via an email client)
• Telnet
The Presentation layer (Layer 6) controls the formatting of user data,
whether it is text, video, sound, or an image. The presentation layer ensures
that data from the sending device can be understood by the receiving device.
Additionally, the presentation layer is concerned with the encryption and
compression of data.
Examples of presentation layer formats include:
• Text (RTF, ASCII, EBCDIC)
• Music (MIDI, MP3, WAV)
• Images (GIF, JPG, TIF, PICT)
• Movies (MPEG, AVI, MOV)
The Session layer (Layer 5) establishes, maintains, and ultimately
terminates connections between devices. Sessions can be full-duplex (send
and receive simultaneously), or half-duplex (send or receive, but not
simultaneously).
The four layers below the upper layers are often referred to as the lower
layers, and demonstrate the true benefit of learning the OSI model.
OSI Reference Model v1.12 – Aaron Balchunas
* * *

The Transport Layer
The Transport layer (Layer 4) is concerned with the reliable transfer of
data, end-to-end. This layer ensures (or in some cases, does not ensure) that
data arrives at its destination without corruption or data loss.
There are two types of transport layer communication:
• Connection-oriented - parameters must be agreed upon by both
parties before a connection is established.
• Connectionless – no parameters are established before data is sent.
Parameters that are negotiated by connection-oriented protocols include:
• Flow Control (Windowing) – dictating how much data can be sent
between acknowledgements
• Congestion Control
• Error-Checking
The transport layer does not actually send data. Instead, it segments data
into smaller pieces for transport. Each segment is assigned a sequence
number, so that the receiving device can reassemble the data on arrival.
Examples of transport layer protocols include Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP). Both protocols are
covered extensively in another guide.
Sequenced Packet Exchange (SPX) is the transport layer protocol in the
IPX protocol suite.
The Network Layer
The Network layer (Layer 3) has two key responsibilities. First, this layer
controls the logical addressing of devices. Logical addresses are organized
as a hierarchy, and are not hard-coded on devices. Second, the network layer
determines the best path to a particular destination network, and routes the
data appropriately.
Examples of network layer protocols include Internet Protocol (IP) and
Internetwork Packet Exchange (IPX). IP version 4 (IPv4) and IP version 6
(IPv6) are covered in nauseating detail in separate guides.
OSI Reference Model v1.12 – Aaron Balchunas
* * *

The Data-Link Layer
The Data-Link layer (Layer 2) actually consists of two sub-layers:
• Logical Link Control (LLC) sub-layer
• Media Access Control (MAC) sub-layer
The LLC sub-layer serves as the intermediary between the physical link and
all higher layer protocols. It ensures that protocols like IP can function
regardless of what type of physical link is being used.
Additionally, the LLC sub-layer can use flow-control and error-checking,
either in conjunction with a transport layer protocol (such as TCP), or
instead of a transport layer protocol (such as UDP).
The MAC sub-layer controls access to the physical medium, serving as
mediator if multiple devices are competing for the same physical link.
Specific technologies have various methods of accomplishing this (for
example: Ethernet uses CSMA/CD, Token Ring utilizes a token).
The data-link layer packages the higher-layer data into frames, so that the
data can be put onto the physical wire. This packaging process is referred to
as framing or encapsulation. The encapsulation type used is dependent on
the underlying data-link/physical technology (such as Ethernet, Token Ring,
FDDI, Frame-Relay, etc.)
Included in this frame is a source and destination hardware (or physical)
address. Hardware addresses usually contain no hierarchy, and are often
hard-coded on a device. Each device must have a unique hardware address
on the network.
The Physical Layer
The Physical layer (Layer 1) controls the transferring of bits onto the
physical wire. Devices such as network cards, hubs, and cabling are all
considered physical layer equipment.
Physical-layer devices are covered extensively in other guides.
OSI Reference Model v1.12 – Aaron Balchunas
* * *

Explanation of Encapsulation
As data is passed from the user application down the virtual layers of the
OSI model, each of the lower layers adds a header (and sometimes a
trailer) containing protocol information specific to that layer. These headers
are called Protocol Data Units (PDUs), and the process of adding these
headers is called encapsulation.
For example, the Transport layer adds a header containing flow control and
sequencing information (when using TCP). The Network layer header adds
logical addressing information, and the Data-Link header contains physical
addressing and other hardware specific information.
The PDU of each layer is identified with a different term:
Layer PDU Name
Application -
Presentation -
Session -
Transport Segments
Network Packets
Data-Link Frames
Physical Bits
Each layer communicates with the corresponding layer on the receiving
device. For example, on the sending device, hardware addressing is placed
in a Data-Link layer header. On the receiving device, that Data-Link layer
header is processed and stripped away before it is sent up to the Network
and other higher layers.
Specific devices are often identified by the OSI layer the device operates at;
or, more specifically, what header or PDU the device processes. For
example, switches are usually identified as Layer-2 devices, as switches
process hardware (usually MAC) address information stored in the Data-
Link header of a frame.
Similarly, routers are identified as Layer-3 devices, as routers look for
logical (usually IP) addressing information in the Network header of a
packet.
OSI Reference Model v1.12 – Aaron Balchunas
* * *

OSI Reference Model Example
The following illustrates the OSI model in more practical terms, using a web
browser as an example:
• At the Application layer, a web browser serves as the user interface for
accessing websites. Specifically, HTTP interfaces between the web
browser and the web server.
• The format of the data being accessed is a Presentation layer function.
Common data formats on the Internet include HTML, XML, PHP, GIF,
JPG, etc. Additionally, any encryption or compression mechanisms used
on a webpage are a function of this layer.
• The Session layer establishes the connection between the requesting
computer and the web server. It determines whether the communication
is half-duplex or full-duplex.
• The TCP protocol ensures the reliable delivery of data from the web
server to the client. These are functions of the Transport layer.
• The logical (in this case, IP) addresses configured on the client and web
server are a Network Layer function. Additionally, the routers that
determine the best path from the client to the web server operate at this
layer.
• IP addresses are translated to hardware addresses at the Data-Link
layer.
• The actual cabling, network cards, hubs, and other devices that provide
the physical connection between the client and the web server operate at
the Physical layer.
OSI Reference Model v1.12 – Aaron Balchunas
* * *

IP and the DoD Model
The Internet Protocol (IP) was developed by the Department of Defense
(DoD) during the late 1970’s. It was included in a group of protocols that
became known as the TCP/IP protocol suite.
The DoD developed their own networking model to organize and define the
TCP/IP protocol suite. This became known as the DoD Model, and consists
of four layers:
OSI Model DoD Model
7 Application
6 Presentation
5 Session
4 Application
4 Transport 3 Host-to-Host
3 Network 2 Internet
2 Data-link
1 Physical
1 Network Access
The DoD model’s streamlined approach proved more practical, as several
protocols spread across multiple layers of the OSI Model.
The following chart diagrams where protocols fit in the DoD model:
Layer Example Protocols
Application FTP, HTTP, SMTP
Host-to-Host TCP, UDP
Internet IP
Network Access Ethernet

Read more »

how to configure VLAN in cisco switch basic vlan configuration pdf

VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
1
- VLANs and VTP -
Review of Collision vs. Broadcast Domains
In a previous guide, it was explained that a “collision domain” is a segment
where a collision can occur, and that a Layer-2 switch running in Full
Duplex breaks up collision domains. Thus, Layer-2 switches create more
collision domains, which results in fewer collisions.
However, Layer-2 switches do not break up broadcast domains, and thus
belong to only one broadcast domain. Layer-2 switches will forward a
broadcast or multicast out every port, excluding the port the broadcast or
multicast originated from.
Only Layer-3 devices can break apart broadcast domains. Because of this,
Layer-2 switches are not well suited for large, scalable networks. Layer-2
switches make forwarding decisions solely based on Data-Link layer MAC
addresses, and thus have no way of differentiating between one network and
another.
Virtual LANs (VLANs)
Virtual LANs (or VLANs) separate a Layer-2 switch into multiple
broadcast domains. Each VLAN is its own individual broadcast domain
(i.e. IP subnet).
Individual ports or groups of ports can be assigned to a specific VLAN.
Only ports belonging to the same VLAN can freely communicate; ports
assigned to separate VLANs require a router to communicate. Broadcasts
from one VLAN will never be sent out ports belonging to another VLAN.
Please note: a Layer-2 switch that supports VLANs is not necessarily a
Layer-3 switch. A Layer-3 switch, in addition to supporting VLANs, must
also be capable of routing, and caching IP traffic flows. Layer-3 switches
allow IP packets to be switched as opposed to routed, which reduces
latency.
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
2
VLAN Example
Consider the following example:
Four computers are connected to a Layer-2 switch that supports VLANs.
Computers A and B belong to VLAN 1, and Computers C and D belong to
VLAN 2.
Because Computers A and B belong to the same VLAN, they belong to the
same IP subnet and broadcast domain. They will be able to communicate
without the need of a router.
Computers C and D likewise belong to the same VLAN and IP subnet. They
also can communicate without a router.
However, Computers A and B will not be able to communicate with
Computers C and D, as they belong to separate VLANs, and thus separate IP
subnets. Broadcasts from VLAN 1 will never go out ports configured for
VLAN 2. A router will be necessary for both VLANs to communicate.
Most Catalyst multi-layer switches have integrated or modular routing
processors. Otherwise, an external router is required for inter-VLAN
communication.
By default on Cisco Catalyst switches, all interfaces belong to VLAN 1.
VLAN 1 is considered the Management VLAN (by default).
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
3
Advantages of VLANs
VLANs provide the following advantages:
Broadcast Control – In a pure Layer-2 environment, broadcasts are
received by every host on the switched network. In contrast, each VLAN
belongs to its own broadcast domain (or IP subnet); thus broadcast traffic
from one VLAN will never reach another VLAN.
Security – VLANs allow administrators to “logically” separate users and
departments.
Flexibility and Scalability – VLANs remove the physical boundaries of a
network. Users and devices can be added or moved anywhere on the
physical network, and yet remain assigned to the same VLAN. Thus, access
to resources will never be interrupted.
VLAN Membership
VLAN membership can be configured one of two ways:
• Statically – Individual (or groups of) switch-ports must be manually
assigned to a VLAN. Any device connecting to that switch-port(s)
becomes a member of that VLAN. This is a transparent process – the
client device is unaware that it belongs to a specific VLAN.
• Dynamically – Devices are automatically assigned into a VLAN
based on its MAC address. This allows a client device to remain in the
same VLAN, regardless of which switch port the device is attached to.
Cisco developed a dynamic VLAN product called the VLAN Membership
Policy Server (VMPS). In more sophisticated systems, a user’s network
account can be used to determine VLAN membership, instead of a device’s
MAC address.
Catalyst switches that participate in a VTP domain (explained shortly)
support up to 1005 VLANs. Catalyst switches configured in VTP
transparent mode support up to 4094 VLANs.
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
4
Static VLAN Configuration
The first step in configuring VLANs is to create the VLAN:
Switch(config)# vlan 100
Switch(config-vlan)# name MY_VLAN
The first command creates VLAN 100, and enters VLAN configuration
mode. The second command assigns the name MY_VLAN to this VLAN.
Naming a VLAN is not required.
The list of VLANs is stored in Flash in a database file named vlan.dat.
However, information concerning which local interfaces are assigned to a
specific VLAN is not stored in this file; this information is instead stored in
the startup-config file of each switch.
Next, an interface (or range of interfaces) must be assigned to this VLAN.
The following commands will assign interface fa0/10 into the newly created
MY_VLAN.
Switch(config)# interface fa0/10
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 100
The first command enters interface configuration mode. The second
command indicates that this is an access port, as opposed to a trunk port
(explained in detail shortly). The third command assigns this access port to
VLAN 100. Note that the VLAN number is specified, and not the VLAN
name.
To view the list of VLANs, including which ports are assigned to each
VLAN:
Switch# show vlan
VLAN Name Status Ports
---- -------------------------- --------- -----------
1 default active fa0/1-9,11-24
100 MY_VLAN active fa0/10
1002 fddi-default suspended
1003 token-ring-default suspended
1004 fddinet-default suspended
1005 trnet-default suspended
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
5
VLAN Port “Types”
There are two types of ports supported on a VLAN-enabled switch, access
ports and trunk ports.
An access port belongs to only one VLAN. Host devices, such as computers
and printers, plug into access ports. A host automatically becomes a member
of its access port’s VLAN. This is done transparently, and the host is usually
unaware of the VLAN infrastructure. By default, all switch ports are access
ports.
VLANs can span multiple switches. There are two methods of connecting
these VLANs together. The first requires creating “uplink” access ports
between all switches, for each VLAN. Obviously, in large switching and
VLAN environments, this quickly becomes unfeasible.
A better alternative is to use trunk ports. Trunk ports do not belong to a
single VLAN. Any or all VLANs can traverse trunk links to reach other
switches. Only Fast or Gigabit Ethernet ports can be used as trunk links.
The following diagram illustrates the advantage of using trunk ports, as
opposed to uplinking access ports:
VLAN A
VLAN B
VLAN C
VLAN A
VLAN B
VLAN C
VLAN A, B, C VLAN A, B, C
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
6
VLAN Frame-Tagging
When utilizing trunk links, switches need a mechanism to identify which
VLAN a particular frame belongs to. Frame tagging places a VLAN ID in
each frame, identifying which VLAN the frame belongs to.
Tagging occurs only when a frame is sent out a trunk port. Consider the
following example:
If Computer 1 sends a frame to Computer 2, no frame tagging will occur.
The frame never leaves the Switch 1, stays within its own VLAN, and will
simply be switched to Computer 2.
If Computer 1 sends a frame to Computer 3, which is in a separate VLAN,
frame tagging will still not occur. Again, the frame never leaves the switch,
but because Computer 3 is in a different VLAN, the frame must be routed.
If Computer 1 sends a frame to Computer 5, the frame must be tagged
before it is sent out the trunk port. It is stamped with its VLAN ID (in this
case, VLAN A), and when Switch 2 receives the frame, it will only forward
it out ports belonging to VLAN A (fa0/0, and fa0/1). If Switch 2 has
Computer 5’s MAC address in its CAM table, it will only send it out the
appropriate port (fa0/0).
Cisco switches support two frame-tagging protocols, Inter-Switch Link
(ISL) and IEEE 802.1Q.
VLANs and VTP v1.21 – Aaron Balchunas
* * *

Inter-Switch Link (ISL)
ISL is Cisco’s proprietary frame-tagging protocol, and supports Ethernet,
Token Ring, FDDI, and ATM frames.
ISL encapsulates a frame with an additional header (26 bytes) and trailer (4
bytes), increasing the size of an Ethernet frame up to 30 bytes. The header
contains the 10 byte VLAN ID. The trailer contains an additional 4-byte
CRC for data-integrity purposes.
Because ISL increases the size of a frame, non-ISL devices (i.e. non-Cisco
devices) will actually drop ISL-tagged frames. Many devices are configured
with a maximum acceptable size for Ethernet frames (usually 1514 or 1518
bytes). ISL frames can be as large as 1544 bytes; thus, non-ISL devices will
see these packets as giants (or corrupted packets).
ISL has deprecated in use over time. Newer Catalyst models may not
support ISL tagging.
IEEE 802.1Q
IEEE 802.1Q, otherwise known as DOT1Q, is the standardized frametagging
protocol supported by most switch manufacturers, including Cisco.
Thus, switches from multiple vendors can be trunked together.
Instead of adding an additional header and trailer, 802.1Q actually embeds a
4-byte VLAN ID into the Layer-2 frame header. This still increases the
size of a frame from its usual 1514 bytes to 1518 bytes (or from 1518 bytes
to 1522 bytes). However, most modern switches support 802.1Q tagging and
the slight increase in frame size.
Neither ISL nor 802.1Q tagging alter the source or destination address in the
Layer-2 header.
Manual vs. Dynamic Trunking
ISL or 802.1Q tagging can be manually configured on Catalyst trunk ports.
Catalyst switches can also dynamically negotiate this using Cisco’s
proprietary Dynamic Trunking Protocol (DTP).
VLANs and VTP v1.21 – Aaron Balchunas
* * *

Configuring Trunk Links
To manually configure a trunk port, for either ISL or 802.1Q tagging:
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk encapsulation isl
Switch(config-if)# switchport mode trunk
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk
The first line in each set of commands enters interface configuration mode.
The second line manually sets the tagging (or encapsulation) protocol the
trunk link will use. Always remember, both sides of the trunk line must be
configured with the same tagging protocol. The third line manually sets the
switchport mode to a trunk port.
The Catalyst switch can negotiate the tagging protocol:
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk encapsulation negotiate
Whichever tagging protocol is supported on both switches will be used. If
the switches support both ISL and 802.1Q, ISL will be selected.
By default, trunk ports allow all VLANs to traverse the trunk link. However,
a list of allowed VLANs can be configured on each trunk port:
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk allowed vlan remove 50-100
Switch(config-if)# switchport trunk allowed vlan add 60-65
The first switchport command will prevent the trunk port from passing
traffic from VLANs 50-100. The second switchport command will re-allow
the trunk port to pass traffic from VLANs 60-65. In both cases, the
switchport trunk allowed commands are adding/subtracting from the current
list of allowed VLANs, and not replacing that list.
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk allowed vlan all
Switch(config-if)# switchport trunk allowed vlan except 2-99
Certain VLANs are reserved and cannot be removed from a trunk link,
including VLAN 1 and system VLANs 1002-1005.
VLANs and VTP v1.21 – Aaron Balchunas
* * *
Native VLANs
A native VLAN can also be configured on trunk ports:
Switch(config)# interface fa0/24
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk native vlan 42
Frames from the native VLAN are not tagged when sent out trunk ports. A
trunking interface can only be assigned one native VLAN. Only 802.1Q
supports native VLANs, whereas ISL does not. (More accurately, ISL will
tag frames from all VLANs, even if a VLAN is configured as native). The
native VLAN should be configured identically on both sides of the 802.1Q
trunk).
Native VLANs are often configured when plugging Cisco VoIP phones into
a Catalyst Switch (beyond the scope of this section). Native VLANs are also
useful if a trunk port fails. For example, if an end user connects a computer
into a trunk port, the trunking status will fail and the interface will
essentially become an access port. The user’s computer will then be
transparently joined to the Native VLAN.
Native VLANs provide another benefit. A trunk port will accept untagged
frames and place them in the Native VLAN. Consider the following
example:
Assume that both 802.1Q switches have trunk links configured to the non-
802.1Q switch, and that the trunk ports are configured in Native VLAN 42.
Not only will the 802.1Q switches be able to communicate with each other,
the non-802.1Q switch will be placed in Native VLAN 42, and be able to
communicate with any device in VLAN 42 on any switch.
(Please note, that the author of this study guide finds the “benefit” of the
above example of Native VLANs to be……dubious at best, and confusing
as hell at worst).
By default on all trunking interfaces, the Native VLAN is VLAN 1.
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
10
Dynamic Trunking Protocol (DTP) Configuration
Not only can the frame tagging protocol of a trunk port be auto-negotiated,
but whether a port actually becomes a trunk can be negotiated dynamically
as well using the Dynamic Trunking Protocol (DTP).
To manually set a port to be a trunk:
Switch(config)# interface fa0/24
Switch(config-if)# switchport mode trunk
To allow a port to dynamically decide whether to become a trunk, there are
two options:
Switch(config)# interface fa0/24
Switch(config-if)# switchport mode dynamic desirable
Switch(config)# interface fa0/24
Switch(config-if)# switchport mode dynamic auto
If a switchport is set to dynamic desirable (the default dynamic setting), the
interface will actively attempt to form a trunk with the remote switch. If a
switchport is set to dynamic auto, the interface will passively wait for the
remote switch to initiate the trunk.
This results in the following:
• If both ports are manually set to trunk - a trunk will form.
• If one port is set to dynamic desirable, and the other is set to manual
trunk, dynamic desirable, or dynamic auto - a trunk will form.
• If one port is set to dynamic auto, and the other port is set to manual
trunk or dynamic desirable - a trunk will form.
• If both ports are set to dynamic auto, the link will never become a
trunk, as both ports are waiting for the other to initialize the trunk.
Trunk ports send out DTP frames every 30 seconds to indicate their
configured mode.
In general, it is best to manually specific the trunk link, and disable DTP
using the switchport nonegotiate command:
Switch(config)# interface fa0/24
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport nonegotiate
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
11
Troubleshooting Trunks
When troubleshooting a misbehaving trunk link, ensure that the following is
configured identically on both sides of the trunk:
• Mode - both sides must be set to trunk or dynamically negotiated
• Frame-tagging protocol - ISL, 802.1Q, or dynamically negotiated
• Native VLAN
• VTP Domain
• Allowed VLANs
If the above parameters are not set identically on both sides, the trunk link
will never become active.
To view whether a port is an access or trunk port (such as fa0/5):
Switch# show interface fa0/24 switchport
Name: Fa0/24
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 42
<snip>
To view the status of all trunk links:
Switch# show interface trunk
Port Mode Encapsulation Status Native VLAN
Fa0/24 on 802.1q trunking 42
Port Vlans allowed on trunk
Fa0/24 1,100-4094
Port Vlans allowed and active in management domain
Fa0/24 1,100
Port Vlans in spanning tree forwarding state and not pruned
Fa0/24 1,100
If no interfaces are in a trunking state, the show interface trunk command
will return no output.
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
12
VLAN Trunking Protocol (VTP)
In large switching environments, it can become difficult to maintain a
consistent VLAN database across all switches on the network. The Ciscoproprietary
VLAN Trunking Protocol (VTP) allows the VLAN database to
be easily managed throughout the network.
Switches configured with VTP are joined to a VTP domain. Only switches
belonging to the same domain will share VLAN information, and a switch
can only belong to a single domain. When an update is made to the VLAN
database, this information is propagated to all switches via VTP
advertisements.
By default, VTP updates are sent out every 300 seconds, or anytime a
change to the database occurs. VTP updates are sent across VLAN 1, and
are only sent out trunk ports.
There are three versions of VTP. The key additions provided by VTP
Version 2 are support for Token Ring and Consistency Checks.
VTP Version 1 is default on Catalyst switches, and is not compatible with
VTP Version 2.
Cisco describes VTP Version 3 as such: “VTP version 3 differs from earlier
VTP versions in that it does not directly handle VLANs. VTP version 3 is a
protocol that is only responsible for distributing a list of opaque databases
over an administrative domain.”
(If you are confused, don’t be alarmed. The author of this guide is not
certain what that means either).
Cisco further defines the enhancements that VTP version 3 provides:
• Support for extended VLANs
• Support for the creation and advertising of private VLANs
• Support for VLAN instances and MST mapping propagation instances
• Improved server authentication
• Protection from the “wrong” database accidently being inserted into a
VTP domain.
• Interaction with VTP version 1 and VTP version 2
• Ability to be configured on a per-port basis.
(Reference: http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml,
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/8.x/configuration/guide/vtp.html#wp1017196)
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
13
VTP Modes
VTP-enabled switches can operate in one of three modes:
• Server
• Client
• Transparent
Only VTP Servers can create, modify or delete entries in the shared VLAN
database. Servers advertise their VLAN database to all other switches on the
network, including other VTP servers. This is the default mode for Cisco
Catalyst switches. VTP servers can only advertise VLANs 1 - 1005.
VTP Clients cannot make modifications to the VLAN database, and will
receive all of their VLAN information from VTP servers. A client will also
forward an update from a server to other clients out its trunk port(s).
Remember, VTP switches must be in the same VTP Domain to
share/accept updates to the VLAN database.
A VTP Transparent switch maintains its own separate VLAN database,
and will neither advertise nor accept any VLAN database information from
other switches (even a server). However, transparent switches will forward
VTP updates from servers to clients, thus acting as a pass-through.
Transparent switches handle this pass-through differently depending on the
VTP version:
• VTP Version 1 – the transparent switch will only pass updates from
the same VTP domain.
• VTP Version 2 – the transparent switch will pass updates from any
VTP domain.
As a best practice, a new switch should be configured as a VTP client in the
VTP domain, before being installed into a production network. Recall that
the default VTP mode on a Cisco Catalyst switch is server. If by some
circumstance the configuration revision number (explained in the next
section) is higher than that of the existing production switches, a new VTP
server could conceivably advertise a blank VLAN database to all other
switches.
Configuring the new switch as a VTP client will allow it to learn the current
VLAN database, and poses no risk to your existing infrastructure.
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
14
VTP Updates
VTP updates contain a 32-bit configuration revision number, to ensure
that all devices have the most current VLAN database. Every change to the
VLAN database increments the configuration revision number by 1.
A VTP switch will only accept or synchronize an update if the revision
number is higher (and thus more recent) than that of the currently installed
VLAN database. Updates with a lower revision number are ignored.
The simplest way to reset the configuration revision on a VTP server is to
change the VTP domain name, and then change it back to the original name.
VTP utilizes three message types:
• Summary Advertisement – sent out every 300 seconds, informing all
VTP switches of the current configuration revision number.
• Subset Advertisement – sent out when there is a change to the
VLAN database. The subset advertisement actually contains the
updated VLAN database.
• Advertisement Request – sent out when a switch requires the most
current copy of the VLAN database. A switch that is newly joined to
the VTP domain will send out an Advertisement Request.
A switch will also send out an Advertisement Request if it receives a
Summary Advertisement with a configuration revision number higher than
its current VLAN database. A Subset Advertisement will then be sent to that
switch, so that it can synchronize the latest VLAN database.
A Subset Advertisement will contain the following fields:
• VTP Version
• VTP Domain
• VTP Configuration Revision
• VLAN IDs for each VLAN in the database
• VLAN-specific information, such as the VLAN name and MTU
(Reference: http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml)
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
15
Configuring VTP
To configure the VTP domain (the domain name is case sensitive):
Switch(config)# vtp domain MYDOMAIN
To configure the VTP mode:
Switch(config)# vtp mode server
Switch(config)# vtp mode client
Switch(config)# vtp mode transparent
The VTP domain can be further secured using a password:
Switch(config)# vtp password PASSWORD
All switches participating in the VTP domain must be configured with the
same password. The password will be hashed into a 16-byte MD5 value.
By default, a Catalyst switch uses VTP version 1. VTP Version 1 and 2 are
not compatible. If applied on a VTP server, the following command will
enable VTP version 2 globally on all switches:
Switch(config)# vtp version 2
To view status information about VTP:
Switch# show vtp status
VTP Version : 2
Configuration Revision : 42
Maximum VLANs supported locally : 1005
Number of existing VLANs : 7
VTP Operating Mode : Server
VTP Domain Name : MYDOMAIN
VTP Pruning Mode : Disabled
VTP V2 Mode : Enabled
VTP Traps Generation : Disabled
MD5 digest : 0x42 0x51 0x69 0xBA 0xBE 0xFA 0xCE 0x34
Configuration last modified by 0.0.0.0 at 3-12-09 4:07:52
To view VTP statistical information and error counters:
Switch# show vtp counters
VLANs and VTP v1.21 – Aaron Balchunas
* * *
All original material copyright © 2009 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
16
VTP Pruning
VTP pruning is a process of preventing unnecessary VLAN broadcast or
multicast traffic throughout the switching infrastructure.
In the following example, VTP pruning would prevent VLAN C broadcasts
from being sent to Switch 2. Pruning would further prevent VLAN A and B
broadcast traffic from being sent to Switch 3.
With VTP pruning, traffic is only sent out the necessary VLAN trunk ports
where those VLANs exist.
VTP pruning is disabled by default on Catalyst IOS switches. If applied on
a VTP server, the following command will enable VTP pruning globally on
all switches:
Switch(config)# vtp pruning
On trunk ports, it is possible to specify which VLANs are pruning eligible:
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk pruning vlan add 2-50
Switch(config-if)# switchport trunk pruning vlan remove 50-100
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk pruning vlan all
Switch(config-if)# switchport trunk pruning vlan except 2-100
VLAN 1 is never eligible for pruning. The system VLANs 1002-1005 are
also pruning-ineligible.

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