RSTP MAC Address Flushing in TC Instances

[Craddockc]

Community,

Im currently studying for the CCNP SWITCH exam and had a question regarding the RSTP Topology Change mechanism. I understand that the only instance where a TCN is sent from a switch running RSTP is when a non edge port goes forwarding. The switch experiencing the change sets the TC Bit in its BPDU and propagates it on its Root and Designated ports. This in turn causes the upstream or downstream switches to flush the mac table for all non edge ports except the one it received the TC BPDU on. I read this is because in this situation that it cant do anything but learn new MAC Addresses from the switch it got the TC BPDU so no need to flush it if its not going to lose any MAC Addresses. What I dont understand is why flush the CAM entries for all ports it forwards the TC BPDU on? What is the logic behind that? Why get rid of MAC entries for all other upstream and downstream switches? I cant see any scenarios where a port going forwarding on a downstream switch would cause an upstream switch to lose mac addresses.

Can anyone help me?

Thanks.

[acampbell]

Hi,

Here are acouple of links which will interest you:-

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html#newtop

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html

From the 1st link you can see that the flushing is to remove stale MAC table information.

In just a few seconds, or a small multiple of hello-times, most of the entries in the CAM tables of the entire network (VLAN) flush. This approach results in potentially more temporary flooding, but on the other hand it clears potential stale information that prevents rapid connectivity restitution.

All the best with your study.
Regards
Alex

[paul driver]

Hello
My understanding of the topology change in RSTP. is the new bpdu tc flooding concept relates to only rstp non-edged ports transitioning from a non-forwarding state to a forwarding state, Which then initiates a topology change. - This can be a port on the switch itself or due to the switch receiving bdpu  with the tc flag

Now the reason why the cam tables are flushed is the belief by rstp that due to this port now forwarding ,this port now has superior paths to the mac- addresses the old forwarding ports had so the cam tables need to be flushed on those old ports

res
Paul