Rapid Spanning Tree and Portfast Ion Trunks.

Unanswered Question
May 1st, 2007

All,

As I understand it Portfast should prevent a switch from generating TC BPDUs when the port moves into the forwarding state. There is a feature on the lastest IOS versions that allows you to enable Portfast on Trunk links.

We are having a few issues at the moment with a few cascaded switches that cannot be point to point to the distribution layer due to lack of fibre. When these cascaded switches are powered off, upon rebooting the switch port moves into forwarding state (on both access switches), which generates multiple TCN BPDU's causing mac tables on all switches in the VLAN to flush their MAC tables. This causes our management platforms to temporarily raise alerts for reachability to multiple switches in this VLAN.

I was hoping to configure PORTFAST on these cascaded ports that keep going down / up, however, after testing it, the switches still gnerated TCN's and flood the VLAN.

Is this because the links to the cascaded switches are classed as P2P in Rapid PVST and cannot disable TCN generation ? Or is portfast not working on the trunk link ?

Thanks.

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Francois Tallet Tue, 05/01/2007 - 12:25

Hi Chris,

I guess you are using the portfast trunk command right? Yes, it should prevent the TC from being generated. Be careful that, when you reload the access switch, virtually all the ports in the box can be the cause of a TC indication. So check that you have configured portfast on all the ports that go forwarding on this switch. You can also do a show spanning-tree detail on that switch in order to see the source of the last TC.

Regards,

Francois

cbeswick Sat, 05/05/2007 - 02:39

Francois,

To be honest, I had totally fogotten about making sure the actual UTP ports on the cascaded access switch are configured for portfast. I will investigate and update.

Thanks for reply.

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