trunk with vlan mismatch

Unanswered Question
Apr 3rd, 2008

A flat network client is moving to new a Data center(DC). currently a non-cisco shop. Old and new DC will be connected via ISP layer 2 gig link. Client wants to move servers to the new DC on the legacy /16 ip space and wants the flexibility to move to a new segmented IP scheme on their own leisure.

i want to configure the ISP gig link as trunk carrying legacy (untaggged)vlan and a new vlan which would be used to route b/w new ip space and legacy ip space.

Provider has assigned two vlans to client vlan A and Vlan B.

I donot want to use vlan 1 at the new DC as a native vlan for the Trunk as it is bad practice to use VLAN 1 and i also do not want to associate in the new enviornment all flat-n/w-/16 traffic to vlan 1.

so i would like to create a trunk as follows:

New-DC[vlan-10 with Vlan 18 as native]------old-DC[vlan 10 with vlan 1 as native]

new DC

int vlan 10

192.168.1.1/30 advertised in ospf

old DC

int vlan 10

192.168.1.2/30 advertised in ospf

my first question is that would the trunk work with different native vlans. cdp carries the native vlan information however i am not sure if cdp would work as the switches in the old DC are non-cisco and i would also disable it on the cisco switch in the new DC.

My reasoning here is that clients flat n/w traffic is untagged today and would be carried in the native vlan ( which by default should be vlan 1) and should be able to communicate with other side native vlan which is non-routable and not vlan1.

My second question is that the ISP has assigned us two vlans and one of them is called primary which i think means native vlan. i would have to put a switch in between to change the native vlan from vlan 1 to Vlan A or is there another way with out putting a switch in between.

i know that that each trunk could have a different native vlan as it is local to the link and so for each trunk link a vlan which is used as native is sent across as untagged.

I have this problem too.
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hadbou Wed, 04/09/2008 - 09:29

1)Trunk will work only with same native vlan on both the ends of the trunk.If different native vlans are present it results in spanning tree loops.

2)To change the native vlan from vlan 1 to Vlan A just use the following command

switchport trunk native vlan vlan-id(vlan A)

For details about vlan trunk configuration use (based on the your device type):

http://www.cisco.com/en/US/tech/tk389/tk689/tech_configuration_guides_list.html

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