Spanning-Tree LoopGuard and UDLD

Lessons Discussion
15

Hi Thomas,

  1. Nope, once something happens on either side…the link will go down. There’s no separate transmit or receive connect with copper links. One topic that is related that you might like is BFD:

https://networklessons.com/ip-routing/bidirectional-forwarding-detection-bfd/

  1. You should use it on the blocking (alternate) ports but also on root ports, basically any port that could be in blocking mode if the topology changes. You can enable it globally like I did.

  2. Loopguard works based on BPDUs (L2 information). If a loopguard enabled non-designated interface doesn’t receive BPDUs anymore then the interface will be in a blocking state. Loopguard doesn’t know why or how BPDUs went missing, it just acts upon missing BPDUs. Protocols like UDLD are used as a “keepalive” to ensure that L1 is working (a working physical interface that can transmit and receive).loop-inconsistent blocking state instead of moving through the listening, learning, and forwarding states.

  3. UDLD only helps to have a working link between two switches, that’s it. When something is wrong with the link, UDLD will bring it down. When it does, STP can do its job to create a loop-free topology. It’s also possible that there is no loop in the topology anymore one one of the physical links fails.

Rene