anycast load balancing issue

Johannes Resch jr at xor.at
Thu Jan 5 09:12:33 CST 2012


Hi,

On 04.01.2012 13:02, Måns Nilsson wrote:
>
 > [..snipped..]
 >
> Trouble is, we find that (untweaked) cost and metric are such that all
> nodes are equal. The last resort (peer router ID) gets invoked and all
> traffic goes to one single instance. Of course, when that instance falls
> off the net recalculation takes place and another node steps in, but
> I'd like true path lengths (IGP hop count) to influence more than iBGP
> (route-reflector-style) selection.
>
> Any clues?
>
> Oh, all-cisco, all ASR1000 series. All links GE. ~90 routers in IGP.
>

Since you mention route-reflector route selection - are you already 
using per-VRF, per-PE route distinguishers for that L3VPN instance?

If not, I'd recommend doing so - this will cause your RR to see all 
paths as unique routes, distributing all of them (instead just the best 
one from the RR perspective) to RR clients. As result all PEs will 
always have all paths for this particular prefix (and can then take the 
best path decision based on local IGP metric to the respective BGP next 
hops).

Doing that can also significantly improve reconvergence times for 
certain failure scenarios (e.g. ingress PE failure), as PEs can start 
using alternative paths (already available in local BGP RIB) as soon as 
the IGP nexthop for the failed PE is invalidated and do not need to wait 
for BGP RR reconvergence.

cheers,
-jr



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