Jaycee's Networking

May 7, 2009

VLAN Trunking Protocol

Filed under: IOS — Tags: , , , — Jaycee @ 2:48 am

VTP allows VLAN configurations to be managed on a single switch. The changes propagated to every switch in the VTP domain.

1. A switch can only be in one VTP domain.

2. VTP domain is null by default.

3. Switches with mismatched VTP domains will not negotiate trunk protocols.

4. Changes are made on VTP servers, propagated to VTP clients and any other VTP servers in the domain. VTP transparent switch receives and forwards VTP updates, but does not update its configuration. VLANs cannot be locally configured on a switch in client mode.

5. Some switches default to VTP server, while others default to VTP transparent.

6. VTP updates called summary advertisements which contains the revision numbers. It’s followed by subset advertisements which contains specific VLAN information.

7. When a switch receives a VTP update:

a. compare the VTP domain name

b. compare revision number

(1) If the revision number is lower than or equal to it’s own revision number, the update is ignored.

(2) If the update has a higher revision number, the switch sends an advertisement request.
=> The response: another summary advertisement, followed by subset advertisement.

8. When a switch’s VTP domain is null, if it receives a VTP advertisement over a trunk link, it’ll inherit the VTP domain and VLAN configuration from the switch on the other end of the trunk.  This will happen only over manully configured trunks, as DTP negotiations cannot take place unless a VTP domain is configured.
*On some swithces, such as Cisco 6500, the null domain will be overwirtten if a VTP advertisement is received over a trunk link, and the switch will inherit the VTP domain from the advertisement.

VTP Pruning prevents traffic originating from a particular VLAN from being sent to switches on which that VLAN is not active.

1. VTP can create a problem when excess traffic is sent across trunks needlessly.

On a busy VLAN, broadcast can amount to a significant percentge of traffic which is being needlessly sent over the entire network, and is taking up valuable bandwidth on the inter-switch trunks.

2. Pruning is not designed to work with switches in VTP transparent mode.

3. VTP pruning must be enabled or disabled throughout the entire VTP domain. Failure to configure VTP pruning properly can result in instability in the network.

Dangers of VTP:

1. In smaller networks that are very stable, VTP should NOT be used.

2. E-commerce website should not use VTP.

3. In larger, more dynamic environments where VTP is of use, proper procedures must be followed to ensure that unintended problems do not occur.

4. Connecting rogue switches to a production network should not be allowed without change-control procedure being followed.

Configuring VTP:

1. VTP domain:

IOS(config)#vtp domain LAB

2. VTP mode:

IOS(config)#vtp mode transparent

3. VTP Password:

IOS(config)#vtp password cisco

4. VTP Pruning:

a. Enable VTP pruning:

IOS(config)#vtp pruning
Pruning switched on

IOS#sh int f0/15 switchport

Name: Fa0/15
Switchport: Enabled
Administrative Mode: trunk
Pruning VLANs Enabled: 2-1001

b. Only VLAN 100 should be eligible for pruning:

IOS(config-if)#switchport trunk pruning vlan 100

IOS#sh int f0/15 swi

Name: Fa0/15
Switchport: Enabled
Administrative Mode: trunk
Pruning VLANs Enabled: 100


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