Announce less specific prefix in routeserver environment for upstream purpose

[Arnold Nipper]

on 16.12.2011 19:49 Ondrej Zajicek wrote:
> 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)
> 

Well, RS would prepend AS65123 towards Peer1 and Peer2, not GW toward
RS. This is intended iBGP behaviour, isn't it.

> 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.
> 

Well, is that really simpler?

> 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.
> 

That also would need additional config as yyou would have to set
next-hop to GW (actually the whole truth is that there are two GW)

Wouldn't the original proposal (% the update I gave) reflect standard
behaviour?


Arnold
-- 
Arnold Nipper / nIPper consulting, Sandhausen, Germany
email: arnold at nipper.de      phone: +49 6224 5593407 2
mobile: +49 152 53717690     fax:   +49 6224 5593407 9

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