Best external route on iBGP

[Apollon Oikonomopoulos]

Hi all,

I've noticed that BIRD doesn't advertise the best external route to iBGP 
peers (as it "should" according to RFC 1771) if there is already an iBGP 
route for the same destination. I realize this is a point where most BGP 
implementations deviate from the standard, although many of them offer 
knobs to enable best external route propagation[1][2]. Still, this is a 
useful behaviour, especially in active-backup setups where backup paths 
are completely hidden from the rest of the AS.

In essence, it looks as if BIRD can do this with sorted tables and the 
secondary BGP option, but unfortunately sorted tables are incompatible 
with iBGP's multihop operation due to the recursive gateway calculation 
strategy. Another option is of course add-paths, but then BIRD 
advertises all received paths, leading to a larger memory consumption 
than necessary.

So, in case I'm missing something, is there another way to achieve best 
external route propagation over iBGP?

Regards,
Apollon

[1] http://www.juniper.net/documentation/en_US/junos13.2/topics/example/policy-advertise-external.html
[2] http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/irg-best-external.html