[c-nsp] loop guard still useful?

Michele Bergonzoni bergonz at labs.it
Mon Jan 18 03:57:58 EST 2016

>  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.

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. 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.

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. My personal best practice when using this errdisable features is to always enable auto recovery after a suitable time.


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

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