Subject: IPFRR: Link protection for Bcast/NBMA links

From: Alex Zinin <[email-masked]>
Date: Wed, 16 Feb 2005 23:43:43 -0800

In section 2.3, the spec currently says that "For an alternate to be considered
link-protecting, it must be loop-free with regard to the pseudo-node." I'd like
to discuss this a bit.

LAN segments today are implemented using some sort of active equipment, most
often L2 switches. Hence, we have two types of failure cases, associates with
LANs: a) failure of a link between a router and the switch, and b) failure of
the switch. The current text essentially suggests to always be pessimistic and
assume that a link failure disables the whole segment.

Alia, could you comment on why you think we should be more pessimistic here
and always shoot for PNode-protecting in this case, rather than distinguish
between link protection and, say, segment protection?

Thanks.

-- 
Alex
http://www.psg.com/~zinin

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From: Alia Atlas <[email-masked]>
Date: Thu, 17 Feb 2005 03:17:47 -0500

>between link protection and, say, segment protection?

The worst-case failure associated with the LAN is that the whole link 
fails.  So, on one hand, an alternate that avoids the pseudo-node is the 
most protective that one can get in ref to that link.

It would be possible to have an alternate that just avoids that particular 
pseudo-node to primary neighbor link (as seen in the translated topology); 
I assume that this could only be triggered by BFD reporting the inability 
to reach the primary neighbor via that link.

In the absence of BFD, one can detect an ethernet failure if it is local or 
via the RFI notification, if there are no L2 switches in the way.

I don't think that it's straightforward to provide an alternate that avoids 
a particular L2 switch - simply because the topology information isn't 
known.  One could use SRLGs to provide this.  I'm not certain how much 
benefit having the segment protection would provide without this.

In general, we want the most protective alternate that can be found.  In 
the event of a worst failure than was protected for, we have the concerns 
about forwarding loops via the alternates being possible.

If you think it is useful, we could discuss segment protection as well in 
the draft.  I didn't because we want the best protection possible, because 
we don't know the L2 topology, and because supporting segment protection 
requires BFD or a mechanism for detecting which segment has failed.

On another note, the only type of alternate that could go through the 
pseudo-node would be an alternate next-hop explicitly through the 
pseudo-node to a different neighbor or one that doesn't provide node 
protection.  In other words, the alternate next-hop itself would have to be 
across that pseudo-node; the path from the alternate neighbor can't go 
through the pseudo-node, because the alternate neighbor is using the SPT 
and the SPT from the pseudo-node is to that primary neighbor.

Alia

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From: Curtis Villamizar <[email-masked]>
Date: Thu, 17 Feb 2005 11:36:49 -0500

> between link protection and, say, segment protection?

The forwarding card needs an alternate route that covers three cases
(you missed one).  One case is where the near end link fails, the
second is where the switch fails, the third is where the far end link
fails (the link from the switch to the IP next hop).  Even if you
decided the L2 switch was infalible, you'd need to protect for the
links on both sides of the pseudonode.  Since the cost across an
ethernet switch if used as a POP interconnect (common) is very low,
this would have the same effect as protecting against a failure of the
pseudonode even though theoretically it is a distinct case.

Curtis

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From: Alex Zinin <[email-masked]>
Date: Thu, 17 Feb 2005 15:42:04 -0800

Alia, Curtis,

 Makes sense to me. It was more of a clarification question, so let's leave it
 as it is.