vlan 1 on dot1q trunks, cross-vtp-domain

[jvreemann]

Hello all,

trying to transport vlan 1 via .1Q between a c6500 (IOS) and a c35xx. The idea is to access the c35xx for management purposes.

Vlan 1 does not seem to work; neither the c35xx nor any devices attached to it can be seen in the vlan 1 cloud.

The 2 catalysts are members of different vtp domains. Both domains do include their vlan 1 definitions.

Any ideas?

[Georg Pauwen]

Hello,

switches in different VTP domains do not share VLAN information, hence VLAN 1 in both domains are practically two different networks. Configure both switches with the same VTP domain name (and make sure that the 6500 is the vtp server switch)...

Regards,

GP

[jvreemann]

Hi GP,

well, any vlan will work across a vtp domain boundary. Provided they have been defined within both vtp domains.

Maybe vlan1 is the one and only exception to this rule?

JV

[jselman]

When using dot1q trunking you must specify what the native vlan is on BOTH sides of the trunk if it is different from 1. If you want the Interface VLAN1 on your 3750 be accessible then it must be in the same network as VLAN1 on your 6509. If you have a different vlan on your 3750 for users but want to keep VLAN1 for management this is how you need to set it up: Using VLAN 25 for users and VLAN 1 for management Example below using GiX/Y for uplink interface to 3750:

interface Vlan25

ip address 172.30.25.2 255.255.255.0

ip helper-address 172.30.X.Y

no ip redirects

standby 1 priority 110 preempt

standby 1 ip 172.30.25.1

no shutdown

!

Interface GiX/Y

switchport

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 1,25

switchport mode trunk

no shutdown

On 3750:

interface VLAN1

decription "Management Interface"

ip address 172.30.25.5 255.255.255.0

interface GigabitEthernet1/0/1

description "Trunk to 6509"

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 1,25

switchport mode dynamic desirable

By NOT including: "switchport trunk native vlan 25"

on both sides of the trunk you are making VLAN 1 the native VLAN but still allowing VLAN 25 traffic

[ybajpai]

it should work.

Like the previous post suggested, you need to make sure that you have a matching "native vlan" on the trunk. Do a "show interface trunk" (IOS) and a "show trunk"(catos) to figure out two important things:

1) what is the native vlan on both ends? Do they match?

2) is vlan 1 forwarding and not pruned on that trunk on both switches.

You may have to set your VTP as transparent and disable CDP on the port to get rid of the VTP mismatch CDP message.