[j-nsp] Juniper best path selection process

[Stacy W. Smith]

Joe,

This is actually documented at the link you quoted below:
 
"For EBGP only, prefer the current active route when a route is received from different neighboring ASs, by default. To disable this default behavior, specify the external-router-id option at the [edit protocols bgp path-selection] hierarchy level."

--Stacy

On Apr 1, 2010, at 4:53 PM, Joe Metzger wrote:

> Does anybody have a reference to the real best path
> selection process actually used by an M7i running 9.3
> that documents all of the tie-breaking criteria, and
> when it might decide to just blow off recomputing a new
> best route if the important attributes didn't change?
> 
> I was poking around today on a router that is excepting
> 2 identical iBGP feeds of routes to black hole. Approximately 70%
> of the routes from 1 server were marked as best and 30%
> from the other.
> 
> The published selection algorithm docs only include deterministic
> criteria, and so I would have expected deterministic results and
> that the routes from the black-hole server with the lowest router
> ID would be preferred 100% of the time.
> 
> http://www.juniper.net/techpubs/en_US/junos/information-products/topic-collections/nog-baseline/topic-20357.html
> 
> The underlying system feeding routes to the 2 black hole
> servers guarantees that both servers are sending identical
> lists of prefixes, with identical local-pref, as-path,
> origin code, med value, source protocol, etc. The import
> policy explicitly sets the next-hop to discard for all
> routes from both servers so IGP distance shouldn't be a factor.
> 
> The only differences between the routes announced from each
> black hole server visible in show route extensive are router
> ID, Task, and the time that the route was announced. It appears
> that it is showing the oldest route as the 'best' one.
> 
> This isn't causing me any trouble, but it looks wrong, and making
> routing decisions based on age has bit me in the past. :-(
> 
> 
> 
> Joe Metzger
> metzger at es.net
> 
> 
> 
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