Have a network that has been experiencing a L2 issues and got an example of a mis-configuration below that I'm not sure how the network would react. There are a lot of other design issues around this scenario that’s not correct, but just want to understand the outcome of this example. The network fails once in a while with the core switchs going into 99.9% CPU utilisation. This seems to be around when there is a topology change in the network, yet some parts of the network a topology change will have no effect, which I think maybe done to some specific vlans. Reboot of the cores rectifies the problem.
There are 2 x 6509 cores that are connected together but the vlan in question hasn't been shared between the two. The core switchs are running rapid-pvst and the edge switchs are running a mixture of rapid-pvst and ieee pvst.
For the particular vlan in question, say 195, has been configured from each of the cores to each of the edge switchs. Each edge switch as 2 port, port-channel with vlan 195 added to each port-channel / trunk from both sides of the switch, each one going to a separate core.
When looking into spanning tree, on the root bridge for vlan 195 of one of the cores you get for each port-channel / trunk to each edge switch the following:
Swicth 1) Alt Blk, 2) Alt Blk, 3) Alt Blk, 4)Root Fwd, 5) Desg FWD
Now switchs 1), 2), 3), each show Desg Fwd to the backup bridge and Root FWD the root bridge.
Swicth 4) shows Root Fwd and Desg Fwd
Switch 5) Does not have vlan 195 created, therefore no spanning tree instance for it.
This is the strange part and what I need clarification on. If this switch has no spanning tree instance for Vlan 195 running on it and Vlan 195 is being piped down to either side of the switch and both the core switches show Desg FWD for the spanning tree instance of Vlan 195 to this switch – would the switch pass through BPDU’s for the spanning tree instance of Vlan 195 and therefore potentially create a loop? Or does it simply not pass any BPDU’s for that Vlan?
XMER1#sh spanning-tree vlan 195
VLAN0195 Spanning tree enabled protocol rstp Root ID Priority 32768 Address 0015.2cb3.ccc3 Cost 6 Port 1677 (Port-channel33) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 Address 0015.c760.20c3 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300
I don't have a lab setup to confirm on at the moment but my understanding is the switch 5 would not introduce a loop for vlan 195 as it would simply discard the BPDUs tagged for that VLAN when they come in on the two trunk ports.
Since it has no spanning-tree instance for that VLAN, it would never encapsulate or forward BPDUs with the vlan id 195 in any of its outgoing trunk ports.
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