VSS 4500 doubt

[Fabio Bustamante]

Hi everyone. 

We have the following topology: 

VSS is working fine on both sides. We can ping each 4500-VSS and telnet each other without any problem. Interfaces Te1/1/1 and Te2/1/1 are part of a Portchannel on each VSS. 4500X-A and 4500-D are VSS active switches.

Now, if interface Te2/1/1 of 4500X-D goes down, we cannot ping the VSS (C-D) from VSS (A-B). Why does this happen, if we have a PortChannel and interface Te1/1/1 is still UP, and can even see VSS(A-B) through CDP?  Can't this be seen as if we had two switches connected through a Portchannel, and one interface of the Portchannel goes down? (devices in the middle are provider equipment)

Thanks.

[Reza Sharifi]

Hi,

In order for the fail over to work correctly you need to have a cross connect between 4500-C and the other device above it and 4500-D and the other device above it (fully mesh). The same thing for 4500-A and B.

HTH 

[Fabio Bustamante]

Hi Reza.

You mean something like this right?:

And we should include the four interfaces in one Port-Channel, thus avoiding any STP issue?

Thanks!

[Reza Sharifi]

You mean something like this right?:

That is correct, but instead of using one Portchannel for all 4 links you may have to use two different Portchannels as the switches above and below VSS devices are not stacked and therefore can't share a Portchannel.  So, it would be one Portchannel from one of the switches to both 4500-C and D and the same from the other switch to both 4500-C and D.

HTH

[Fabio Bustamante]

Ok, thanks. But, that would mean that one of the Port-Channels would be in blocking-state due to STP?

[Reza Sharifi]

No, there shouldn't be any loop since the switches above and below VSS are not connected together.  Think of the individual switches as access layer switches that need to connect to distro/core (VSS). So, each access switch has a Porchannel with 2 links in it and each link connect to a different 4500.  Since the links are part of a Portchannel there is no loop.

HTH

[Fabio Bustamante]

But, I guess devices above the VSS are Layer 1 devices; they're not switches, since I can see both VSS (A-B and C-D) through CDP. 

[Reza Sharifi]

Ok, you didn't explain this before and I thought the devices connecting to VSS are switches or at least what the diagram appears to show.

In that case than you are connecting 2 VSS domains together and so all 4 ports can be part of the same Portchannel. And I guess the cloud in the middle is just dark fiber right?

HTH
 

[Fabio Bustamante]

Yes, that's right. I'm sorry if I hadn't been clear before. So, now that it is clear that those devices are not L2 switches, do you still recommend to have a cross-connect from the VSS to these devices (4 interfaces), so the VSS fail over can work properly?

Thanks!

[Reza Sharifi]

Yes, 4 interfaces all in one Portchannel.

Good Luck