Announce less specific prefix in routeserver environment for upstream purpose

[Ondrej Zajicek]

On Fri, Dec 16, 2011 at 02:14:48PM +0100, Stefan Jakob wrote:
> Dear Bird Users,
> 
> I have the following setup:
> 
> Peer1 - AS65001
> Peer2 - AS65002
> 
> RS - AS65123
> GW - AS65123
> 
> 
> All are connected via a layer 2 infrastructure and share next hop
> addresses out of a /22. This /22 is part of a /20.
> 
> RS works as a routeserver with "rs client" in the protocol definition of
> Peer1, Peer2.
> 
> My goal is, to announce the /20 over  GW to the routeservers GW hasn't
> the "rs client" option set but "gateway direct".
> 
> To solve the iBGP and empty AS path of GW, I want to prepend AS65123 for
> all GW incoming prefixes, so that the as path isn't empty.
> 
> Do you expect this as a working design to announce the /20 from GW via
> RS to the peers?

This is a bit tricky. I see a possible problem - received route on RS
would be probably rejected as loopy (received AS PATH contains local ASN)

I see a simpler idea - just use a different 'local as' (perhaps some
private ASN) on the BGP config on RS directed to GW (i.e. just on that one
connection). In that case the session would be handled as eBGP, like all
other sessions. Because the session would be also configured with 'rs
client' on RS, the configured (private) ASN would not appear anywhere,
it does not matter.

BTW, why not just announce that /20 directly on RS? I think that should
work too (this is probably not documented, but locally originated routes
receives ASN to their path even when propagated through the session with
'rs client' enabled.

-- 
Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."
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