[c-nsp] BGP routes co-existing with different local-preference

[Bruce Pinsky]

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David J. Hughes wrote:
> 
>> An alternative to session timer tweaking is running a version that 
>> supports
>> BGP Next Hop Tracking and having your next hops propogated in your IGP.
>> Then a box failure or intermediate path failure that results in your 
>> next
>> hop becoming unreachable will cause the removal of routes that use that
>> next hop in just over the time it takes your IGP to converge and 
>> result in
>> the triggering of best path selection for those routes.
>>
>> http://www.cisco.com/en/US/products/ps6350/
>> products_configuration_guide_chapter09186a008045561e.html
> 
> 
> 
> Yup, nice step forward for some situations.  Correct me if I'm wrong, 
> but it doesn't help your session to a router if that router is reached 
> via ethernet and there's a switch between them.  You will always think 
> you are directly connected so there's a valid next hop even when the 
> box has fallen over.  Other than using BGP's timers or BFD if/when it 
> becomes commonly deployed, I can't see a way to detect a silent peer 
> failure.  I doubt BFD is ever going to be an option between AS'es so 
> timers look like the only option to an eBGP neighbor.
> 

Yep, but IMHO any interface/link that stays up when it can't pass traffic
is broken, even if that is the result of an intermediate path failure at
layer 1 or 2.  If you need to detect path failures, choose the appropriate
media type that can do it.

BFD between AS'es is a policy choice and not an implementation choice.
There should be no reason that a provider shouldn't be able to offer BFD
capability if they are offering ethernet access (once the feature is
available in the software/platform they use).  Lowering your timers for
eBGP sessions with your providers may not be an option either if 1) they
notice and kill your sessions (again a policy thing); or 2) they run a
version that enforces a minimum holdtime.

- --
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bep

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