[j-nsp] help with routing bypassing bgp path selection

[James Bensley]

On Mon, 1 Oct 2018 at 06:49, tim tiriche <tim.tiriche at gmail.com> wrote:
>
> hello,
>
> i have 5 PE routers running with full iBGP/RSVP-TE MPLS Mesh.
>
> There is a CE connected to PE5 and PE4.
>
> Based on BGP Path selection all of the PE {1,2,3,4} are preferring route to
> PE5 due to BGP Path selection based on AS PATH tiebreaker.
>
> However, i would like PE1 to prefer PE4 and the rest to PE5.  What is the
> best way to go about doing this?

Hi Tim,

I'd say that the preferred way to control routing protocol behavior
dynamically (e.g. per site or CPE) is to use BGP communities. So in
this case you could set up some communities and have the CE advertise
it's routes with a specific community attached.

E.g. To have a per-PE preference policy you can have the CPE advertise
it's prefixes with a specific community "X". Either PE4 could match
that community and increase it's local-pref for the route before
sending it on to it's iBGP neighbors (this would affect all iBGP
neighbours making them all prefer the prefix via PE4) or have PE1
match the community on ingress (this would only affect PE1). The
problem with the later is that if PE4 doesn't have the best path he
won't advertise it without some BGP add-path / best-external knobs
being twiddled.

You can use an iBGP outbound policy on PE4 e.g. PE4 matches community
X which sets a higher local pref only on advertisements to PE1 but
per-PE outbound policies don't scale well.

Communities work great for manipulating eBGP policy but with iBGP
policy manipulation it quickly becomes messy so you need to plan with
care.

Cheers,
James.