Backup L2 link to legacy network

[brlehigh]

Greetings,

Let's say I have the following network topology.  There is currently L2 connectivity to the legacy VSS switch pair via a VPC.  On the VSS side; the port-channel is configured as "spanning-tree link-type shared":

I would like to add a "backup" layer 2 VPC between the VSS and ACI so that L2 connectivity will continue to function in the event that Leaf1/2 are offline.  Something like the following:

If the new port channel is also configured as "link-type shared" I would expect that STP would simply block the second port-channel and treat it as a backup, without disrupting the primary link.  Is this true, or, would it trigger the forwarding delay on both port-channels?

Is there a better way of accomplishing something like this (flex link, etc)?

Any other caveats that I should be aware of?

[Sergiu.Daniluk · Accepted solution]

Hi @brlehigh 

First, for the initial diagram, if the VSS is the only switch connected to your ACI, then you do not need the "spanning-tree link-type shared" command, simply because the two interfaces are part of the same port-channel. So logically speaking, the VSS and ACI are interconnected via one interface (Po X).

After you connect the second link, and assuming the new interfaces are having same priority/costs, but higher if number, then yes, the ports will go into blocking state.

I think leaving RSTP as a backup mechanism is not that bad.  

Stay safe,

Sergiu

[brlehigh]

Thank you, @Sergiu.Daniluk 

You are correct.  The VSS is the only switch directly connected to the fabric.  The VSS is the legacy core so all downstream switches connect to it.

In your opinion, would it best to stick with "link-type shared" for these ACI-to-VSS port-channels or change them to the default of "point-to-point"?

The major drawback with "shared" is the long forwarding delay, which I'd love to avoid if possible.

Thanks,

-Brian

[Sergiu.Daniluk]

Hello,

As long as the VSS is the only switch connected to the fabric then yes, you are safe with p2p.

Cheers,

Sergiu