[c-nsp] VRF randomly, stably install only one from multiple route sets

[Oliver Boehmer (oboehmer)]

> Desired scenario:
> 
>   Remote VRF-1 exports routes R1.
>   Remote VRF-2 exports routes R2.
>   Remote VRF-3 exports routes R3.
> 
>   Central VRF-C imports only one from (R1, R2, R3),
>   without any preference. Once C selects Ri, it
>   stably sticks with Ri as long as Ri remains available.
>   Only if Ri becomes unavaiable, C would switch
>   to another Rj available and distinct from Ri;
>   afterwards, if Ri availability is ever restored,
>   C still stably keeps Rj as long as Rj remains available.

R1, R2 and R3 represent the same IPv4 prefix(es), but from different
VRFs? 

> Questions:
> 
>   1) Is that possible to achieve by properly
>   configuring VRF's statements? If so, how?

Possibly not. VRF-C PE will compare all paths using the standard BGP
decision algorithm (http://www.cisco.com/warp/public/459/25.shtml) and
will import the best path into the VRF.

>   2) Would disabling "multipath ibgp" on the
>   VRF-C's PE, so that only the oldest active Ri
>   get installed on VRF-C's local forwarding table,
>   provide the behavior described above?

I don't think so. For eBGP paths, we don't compare the router-id and
stick with the "oldest" path. But for iBGP we can't do this and perform
a deterministic decision which will, given all other attributes like
localpref, as-path, igp-metric/etc. being equal, compare the router-id. 

>   3) Can anyone point another way to achieve
>   the same behavior?

Why do you need this? Maybe there are alternatives?

	oli