[c-nsp] loop guard still useful?

Michele Bergonzoni bergonz at labs.it
Mon Jan 18 14:50:40 EST 2016

First of all, I have to admit that I confused loop guard with keepalives (the one that errdisables self looped switchport interfaces, and then people do "no keepalives"). Sorry.

> So it seems like loop guard isn't needed if rstp is enabled.

I have no operational experience with loop guard, but from the description it seems to me that in order to trigger it the interface must become unidirectional *after* link up. Thus, if your Joe Average while troubleshooting does a shut/no shut, he actually gets the loop.

So it will protect you on the other unidirectionality side, but not in all possible sequences of events.

If you are operating an all-cisco net you might take a look at bridge assurance. I have no operational experience with it as well (apart from disabling it in the nexus), but looks much more like a bidirectional keepalive at the STP layer. It is proprietary and violates the standard as I understand it.

> No, I don't like UDLD at all - too many bad experiences with it

In fact after what Saku said I would consider trusting the layer 1, but I usually work in a multivendor environment, YMMV.


Ing. Michele Bergonzoni - Laboratori Guglielmo Marconi S.p.a.
Phone:+39-051-6781926 e-mail: bergonz at labs.it

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