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.