BGP Route Selection (local preference is equal)

[Steph1963]

Hi to All,

I am trying to understand the followign BGP route selection rule that says:

Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.

Local paths that are sourced by the network or redistribute commands are preferred over local aggregates that are sourced by the aggregate-address command.

q) I am trying to find a topology that would produce a situation where a router can locally generate a BGP route and also receive the same route from a neighbor. It seems to me that this rule violate the iBGP rules saying that an iBGP router will not propagate routes learaned from one iBGP neighbor to other iBGP neighbor.

The only way that I can imagine this situation is in the case where a locally originated route would come back from another AS via a dual home topology but this route should never be announce since it contains the local AS where this route was originated. Loop detection should prevent from announcing a BGP route that contains it`s own AS.

Thanks for your help

Stephane

[Giuseppe Larosa]

Hello Stephane,

>> q) I am trying to find a topology that would produce a situation where a router can locally generate a BGP route and also receive the same route from a neighbor. It seems to me that this rule violate the iBGP rules saying that an iBGP router will not propagate routes learaned from one iBGP neighbor to other iBGP neighbor.

the IBGP rule says that RB cannot advertise to RC what it learns from RA (given that all these routers are in same AS) and mandates for full mesh of IBGP sessions not that two routers cannot exchange routes over a DIRECT iBGP session

>> Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.

in Cisco BGP implementation locally originated BGP routes have a weigth of 32,768 and next-hop 0.0.0.0 and this provides also usually for picking up local route over the one received by a BGP peer.

It is enough to have two routers both advertising in BGP a  connected Vlan interface where both are connected to see this.

Hope to help

Giuseppe