In message <[email protected]>
Alex Zinin writes:
> In section 2.3, the spec currently says that "For an alternate to be
> link-protecting, it must be loop-free with regard to the pseudo-node." I'd
> 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?
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.
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