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Re: IPFRR: Link protection for Bcast/NBMA links

Subject: Re: IPFRR: Link protection for Bcast/NBMA links
From: Alex Zinin
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.

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

Thursday, February 17, 2005, 8:36:49 AM, Curtis Villamizar wrote:

> In message <[email protected]>
> Alex Zinin writes:
>>  
>> 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



> 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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