Re: [trill] I-D Action: draft-ietf-trill-clear-correct-05.txt

Thomas Narten <> Wed, 18 July 2012 18:47 UTC

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To: Donald Eastlake <>
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Comments: In-reply-to Donald Eastlake <> message dated "Mon, 16 Jul 2012 21:12:39 -0400."
Date: Wed, 18 Jul 2012 14:47:39 -0400
From: Thomas Narten <>
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Subject: Re: [trill] I-D Action: draft-ietf-trill-clear-correct-05.txt
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Hi Donald.

> This draft has relatively small changes that are responding to a
> Routing Directorate review. The most significant of these is that it
> clarifies that packets are not forwarded through overloaded RBridges.
> If you have a problem with any of these changes, please tell me or
> post to the list.

I took the above as an opportunity to review what the various TRILL
specs say about handling "overload" situations.

I gather that an overloaded RB sets the overload bit in its IS-IS
advertisement to indicate "I have problems and may not work right".

OK. But what exactly are other RBs supposed to do with this info?

I would assume that for correctness/consistency, the safest thing to
do is simply remove the RB from the topology and pretend it doesn't
exist. i.e, stop using it or trying to forward traffic through
it. Yes, this may result in suboptimal routes (or worse), but at least
you'll get consistent behavior. If TRILL continues to forward traffic
through RBs that aren't working right, you'll likely see more
random/intermittent problems that are hard to debug/diagnose.

That said, the clear-correct draft does say if the AF is the only RB
connected to a particular VLAN at an edge, if you remove that RB from
TRILL, the VLAN becomes unreachable. So it says don't do that.

But going through the various spec, the published RFCs don't seem to
say anything about the overload bit. (I didn't do an exhaustive
search, but searching for the string "overload" in the RFCs shows
almost nothing.)

Where is the TRILL behavior wrt to using the overload flag documented?