Transient VLAN

[JavaJunkie]

Hello all!  I have a Cisco 3800 series MLS, on this switch I have a transient VLAN(Vlan 400) that connects to a unrelated customer network.  

Attached to the 3800 on my side of the network I have multiple switches using VLANs 100, 200 and 300.  

The traffic from VLANs 100, 200 and 300 are routed over the transient VLAN(400) where they are processed and returned via the same route.

My question is, do I need to specify VLAN 400, the transient VLAN on all my switches and trunks that are behind the customer network for the traffic to successfully go from VLAN 100, 200 and 300 across the transient VLAN(400) and then return to VLANs 100,200,300?

If so, why?

[Reza Sharifi]

Hi,

No, the transit vlan 400 is just a layer-3 boundary between your switch and the customer (2 devices) and does not need to be define anywhere else.

HTH 

[JavaJunkie]

So, help me understand why.  Is the VLAN 400 traffic not tagged from the customer side?

[Reza Sharifi · Accepted solution]

No, vlan 400 most likely is configured as untagged (access port). Is just like a layer-3 point-to-point link with IP an each side. In this case it is vlan with SVI and IP configured on each side of the connection.

HTH

[JavaJunkie]

So, do I even need the VLAN 400 for the transient ports?  I guess I'm not understanding why these ports would need to be configured this way.  Why not just create layer 3 ports with an IP address?

Thanks for the explanation.

[Joseph W. Doherty]

"So, do I even need the VLAN 400 for the transient ports?"

Possibly not.

One reason it might be done, if using a switch stack or multi-blade chassis switch, you can pre-define a port in the same VLAN and in case of a port failure on the original port/ you can just move the port connection without needing to reconfigure the other port. (Can be a wonderful thing if the switch is at a remote site with only one link and no out-of-band management. You only need to talk someone at the site though moving the physical connection. [At one job I worked, no one saw the benefit until a remote site went off-line for six hours ;until they could get an engineer to the site.])