Dot1q tag & vlan

[krishnadig]

Hello,

Below is the scenario; can you please help me.

Rtr A gig0/0 --> gig0/0 Rtr B gig0/1  --> Rtr C gig0/1

( --> denotes physically connected)

Rtr B has a trunk interface to Rtr A & C and allowing all vlans (including 70)

Rtr C has gi0/1.70 with encap dot1q 70

I want to extend dot1q 70 on Rtr C to Rtr A gig0/0.70 (encap dot1q 70) and make it a Layer3 sub interface (the main interface is a routed port)

However, Rtr A already has vlan 70 (Layer 2 & 3) created for some other purpose.

Query is:

1. On Rtr A if I create gig0/0.70 with dot1q 70, will it ignore the vlan 70 and work as extension of Rtr C gi0/1.70? 

If no, then

2. As there is overlap of same vlan id, how can I extended dot1q 70 from Rtr C to A?

[John Blakley]

I'm not entirely sure I understand your requirement, but to extend a vlan like that you'd need to bridge the connection on Rtr B for vlan 70...the problem that I see is that you've already stated you have vlan 70 on Rtr A configured for another reason, so you're looking at two different subnets that you'd be contending with (one subnet on Rtr A and one subnet on Rtr C). 

What are you trying to accomplish?

[krishnadig]

Hi John,

inteface vlan 70 already exists on Rtr A; for some different purpose. Without disturbing it, I want to check what can be done to extend the dot1q tunnel 70 from Rtr C to Rtr A. 

The bridging is already in place on Rtr B.

[Richard Burts]

You have not told us what kind of router this is and not how vlan 70 is configured on RtrA. It might make some difference if we knew that. But in general if vlan 70 already exists on RtrA then it is not possible to make a second vlan 70 exist on RtrA.

HTH

Rick

[krishnadig]

Hi Rick, John,

Its a Cisco 6500. Thanks for your assistance.

I used a different vlan, to overcome this. 

Ofcourse, I would still interested to know if there could be a way out in this scenario.

[Richard Burts · Accepted solution]

It is nice to know that it is 6500. Is RtrB also a 6500? If RtrC has Gig0/1.70 with encap dot1q then am I correct in assuming that it is not a 6500?

We do not know how the VLANs and the connections between them are configured. But I will stick with my original answer that in general if vlan 70 already exists on RtrA then it is not possible to extend a second vlan 70 to it.

HTH

Rick

[Richard Burts]

I am glad that our responses were helpful. Thank you for using the rating system to mark this question as answered. This will help other readers of the forum to identify discussions with helpful information.

HTH

Rick

[Jason M.]

Reading your description leaves me with one question. Do you want router A to interact with VLAN 70 on layer 2 or layer 3 only?

If you just want it to interact on layer 3 like a regular routed port then you can have dot1q 70 on router A interface gi0/0.70 without a problem as long as dot1q 70 is not already configured under that particular interface (gi0/0). Because it's a layer 3 interface the tag is removed at that point anyway and treated like regular layer 3 traffic so it will not interact with the VLAN 70 on router A.