[c-nsp] loop guard still useful?

Adam Vitkovsky Adam.Vitkovsky at gamma.co.uk
Tue Jan 19 03:21:53 EST 2016

> Lukas Tribus
> Sent: Monday, January 18, 2016 11:16 PM
> This assumption breaks when you have some kind of RX or TX stall, which I
> saw first-hand on a 7600 linecard that suddenly became faulty (and caused a
> major layer 2 loop because the links had neither UDLD nor loop guard, just
> plain old rapid-pvst and autonegotiation).
> Also, all of this is not true if you have EoMPLS, EoSDH, radio-links and other
> nasty stuff that can suddenly break a single direction and/or both directions
> without RFI'ing the failure end-to-end.
> Unless you operate a network exclusively on dark-fibers or passive WDM,
> you can't assume that everything will go physically down nice and easy.
> You carriers layer 2 circuit signaled RFI once? That doesn't mean it will
> continue to do so after (or within) the next maintenance window or if the
> carrier has a different kind of failure within his network.
> Would you deploy a routed or MPLS switched network without BFD and
> without IGP hello timeout, just relying on the interfaces operational state?
> I can't accept a layer 2 loop because of a single hardware defect. I need the
> control plane to protect me from that and this is what UDLD and loop guard
> can do.
Yeah for a long time I thought the BDF sessions I have enabled on dark-fibre links are unnecessary because during testing the L1 took the circuit down way under 45ms which was the BFD timeout.
Until once NP got frozen keeping the L1 up but no TX -and that's where the BFD saved the day.

So I see now that the UDLD is sort of a BFD for L2 switches.

And yes I'll always rely on BFD across carriers L2 circuit.
However the correct approach is that one should participate in CFM/LFM/E-LMI with L2 carrier.
And as a carrier you should be using CFM, LFM and E-LMI to have end-to-end visibility over the L2 service you provide.


        Adam Vitkovsky
        IP Engineer

T:      0333 006 5936
E:      Adam.Vitkovsky at gamma.co.uk
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