Thanks for the response. On 1/18/16, Michele Bergonzoni <berg...@labs.it> wrote: >> Using the dispute mechanism included in the IEEE 802.1D-2004 RSTP >> standard... I'm wondering if there's any reason to keep loop guard >> configured > > I think the dispute mechanism can detect unidirectionality where data out of > the designated bridge is lost (which is enough to prevent loops), not the > unidirectionality in the other direction.
Which is my point .. or question - enable RSTP on all the switches in the network and you don't need loop guard. Correct? > So the dispute does half of what UDLD does, if I got it right. > > Loop guard is different, it protects only from self-looped ports. My understanding is that it keeps stp blocked ports blocking if the other side stops sending BPDUs: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10596-84.html The loop guard feature makes additional checks. If BPDUs are not received on a non-designated port, and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state, instead of the listening / learning / forwarding state. Without the loop guard feature, the port assumes the designated port role. The port moves to the STP forwarding state and creates a loop. and a lot further down loop guard does not work on shared links or in situations where the link has been unidirectional since the link-up. So it seems like loop guard isn't needed if rstp is enabled. > I don't > know if the wording of RSTP are written in a way to protect you from that, > but I'm sure that the original STP standard was written in such a way that > any compliant implementation was unable to block the loop caused by a > self-looped port. If self-looped means the port sends a frame and then receives the same frame, you're right, stp doesn't protect you from that. > Most vendors quietly worked around this, and I don't know if 802.1d > corrected this error in the previous standard. I know that it is very > unlikely to find a switch whose STP can't protect you from such a > situation. > > So I bet that if you use RSTP you can disable loopguard, and if you like > UDLD there is still a reason to use it. No, I don't like UDLD at all - too many bad experiences with it. It was a necessary evil with cat5500s and 100Mb fiber connections, but you don't need UDLD on 1Gb fiber links. Thanks, Lee _______________________________________________ cisco-nsp mailing list cisco-nsp@puck.nether.net https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/