Spanning-Tree Bridge Assurance

[lagapidis (Lazarus Agapidis)]

Hello Ignacio

@ignacio.rodriguez.or

2.loopguard is ineffective at detecting a port that has been unidirectional since link-up.”) situation. If you can give an example.

Such a situation would occur if a fibre optic port is initially connected with only one of the two fibres resulting in unidirectional link from the moment the link goes up. Loopguard will not “kick in” unless both fibres were initially connected and then one of them subsequently failed.

@ignacio.rodriguez.or

From my understanding only root or alternate ports are the ones who can change the state, for the same reason they are the ones that not send but receive bdpu, and its where you should have loop guard enable.

Let’s think about ports that rely on timely arrival of BPDUs to maintain their current role and state. These are root ports, alternate ports and backup ports. Loopguard is especially useful for the alternate and backup ports because these ports are in a discarding state. If they cease to receive BPDUs, they will move into the forwarding state possibly creating a loop.

So, Loopguard should be applied at least to alternate ports. However, because in a per-VLAN environment each trunk port can have various diverse roles or states for individual VLANs, it is good practice to simply configure loopguard on all ports using the global configuration level command spanning-tree loopguard default.

I hope this has been helpful!

Laz