🚀 go-pugleaf

In article <3B39DA75.B5BEAB3B@inrete.it>,
Marco Meinardi  <marcom@inrete.it> wrote:
>A question on BGP announces.
>On a WORLD EBGP4 router, I will see the followning announcements.
>
>CARRIER1 = AS4
>CARRIER1 announce 192.168.0.0/16 path 1 2 3 4
>CARRIER1 announce 192.168.100.0/24 path 1 2 3 4 5
>
>CARRIER2 = AS6
>CARRIER2 announce 192.168.100.0/24 path 11 2 8 6 5
>
>CARRIER3 = AS7
>CARRIER3 announce 192.168.100.0/24 path 12 10 7 5
>
>The first /16 announce, given by CARRIER1, contains the more specific
>one given by CARRIER1,2,3.
>Which announce will other BGP routers consider? I think it's the most
>specific announce, so everyone will get the /24 announce with 3
>different paths, EVEN if the /16 announce contains that network, and its
>path is shorter than other ones.
>If AS5 link to CARRIER1 is down, WORLD will consider announces given by
>CARRIER2,3.
>The first announce (192.168.0.0/16) will be considered ONLY if all
>CARRIERS withdraw /24 announces.
>
>Is it a fair explanation? Or, excluding all those BGP routers which do
>not accept small announcement, there may be situations in which the /16
>announcement is taken as the best path one?

The question you're asking is not about BGP, but about how a router
searches the routing table when it's making forwarding decisions.  It
always uses the most specific route it can find.

BGP only considers attributes like AS path length when choosing among
routes with the *same* prefix and prefix length, to decide which one to
enter into the routing table.

-- 
Barry Margolin, barmar@genuity.net
Genuity, Burlington, MA
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