Spanning tree on a network with catalyst and small business switches

Hi Rene

I have a network which consists mainly of Catalyst switches (a Catalyst 4510 as the root) sprinkled also with a number of small business switches (SG300 switches). I am investigating TCN alerts as it seems when there is a topology change our VMware infrastructure sends out an alert. Our network is running RSTP across all our switches however whilst all the Catalyst switches on the network are in synch with when a topology change took place on the main Vlan. The small business switches are totally out of synch regarding topology changes on the network. I would expect all the switches to be totally in synch with each other. Am I correct ??? what can I do to modify this as it is restricting me locating what ports may cause a topology change. Thanks Simon.

Hi Simon,

Keep in mind that every interface triggers a topology change, including access interfaces unless you enable portfast. Here’s an example:

Did you enable portfast on all your access mode interfaces? The SG300 switches also support this.

When the root bridge receives a TCN, it will set the TC (topology change) bit in its BPDUs so all non-root switches should receive this. On Cisco IOS, you can see the last time when a topology change occurred and also on what interface:

SW1#show spanning-tree vlan 1 detail                 

 VLAN0001 is executing the rstp compatible Spanning Tree protocol
  Bridge Identifier has priority 24576, sysid 1, address fa16.3e52.ca3b
  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6
  We are the root of the spanning tree
  Topology change flag not set, detected flag not set
  Number of topology changes 7 last change occurred 00:07:42 ago
          from GigabitEthernet0/2

I’m not sure if you can also see this on a SG300. You could also capture BPDUs to see which devices are sending TCNs. For example, here is a capture from a non-root switch that sends a TCN:

Hope this helps!