VSS Spanning tree

[abimadaro4462]

Hello,

I've the following topology

I've noticed that VLANs in the port channel (port 7 on VSS-02 nodes) are in blocking states, I believe this is not normal behavior as all links should be in forward state between both nodes.

spanning tree configuration:

VSS-01

spanning-tree mode rapid-pvst
spanning-tree portfast edge bpduguard default
spanning-tree extend system-id
spanning-tree vlan 1-4094 priority 0

VSS-02

spanning-tree mode rapid-pvst
spanning-tree portfast edge bpduguard default
spanning-tree extend system-id
spanning-tree vlan 1-4094 priority 4096

VLANs that are allowed in the trunk between both VSS nodes are:

VLAN 10, 20, 30

I have OSPF configured between both VSS nodes using VLAN 10.

Advise please.

Best Regards,

[julian.bendix]

Hey!

Could you tell me what switches you exactly have and what version of code is running on them?

Then pls share the following output from both sides:

show spanning-tree blockedports 
show spanning-tree brief
show spanning-tree summary

show etherchannel summary

Thanks and br
Juls

[Joseph W. Doherty]

VSS, logically, behaves as one device. So, take your two VSS pairs, diagram them as one device (for each pair), and then see if you still believe all those links should be unblocked.

[Scott Hodgdon · Accepted solution]

abimadaro4462,

The Best Practice in a setup like this is to have all 4 links in a single Multi-Chassis EtherChannel . That way it is like have two switches with one link between them.

What you have now is two switches with two links between them, passing the same VLANs over both links. I think if you change to a single 4-port MEC between the two, this will go away.

Cheers,

Scott Hodgdon

Senior Technical Marketing Engineer

Enterprise Networking Group

[Joseph W. Doherty]

Scott makes an excellent suggestion.

If you do go with a 4 link MEC, besides splitting the number of links between the VSS pairs, you might want to also avoid having all the links use the same line card on an individual chassis, or the same ASIC on the same line card.

Keep in mind, unlike non-VSS Etherchannel, VSS will not load balance, Etherchannel across a VSS pair. It keeps to the general rule of VSS, to avoid using the cross VSL links, for the VSS virtual device, if possible.

Again, an excellent suggestion by Scott, as one of the advantages of VSS Etherchannel is avoiding the need to use STP for multiple links going from same VSS pair to another device. (Much like you might see for a set of stackable switches.)