Solved: BGP :- Why are recvd routes being re-advertised back ?

shahedvoicerite · ‎11-05-2009

Sorry I am totally new to BGP.

I have a VPN connection, that uses BGP as a routing protocol (Amazon VPC).

When I look at my bgp neighbour, and issue the following commands :-

sh ip bgp all neighbors xx received-routes

followed by

sh ip bgp all neighbors xx advertised-routes

I see the same route that I recieved, begin advertised back to the neighbour.

Is there no split-horizon type rule in BGP ?

I thought that if the same AS path exists in a route then BGP will not advertise it back to the neighbour with that AS.

c.captari · ‎11-05-2009

Indeed you shouldn't if you really receive that route from the same neighbor you are trying to advertise it to.

But as i mentioned above, if you receive the route from 2 routers, both in the same AS, you will advertise that upstream to the other one!

The router that receives that will drop it on it's side as it sees it's own AS path in the list.

If you want to control that use as path access-lists

View solution in original post

c.captari · ‎11-05-2009

Hi. My guess is that you are receiving that route from 2 EBGP sources in the same AS probably.

Run:

show ip bgp and see if it's coming from 2 sources.

If it does and you have no as path filtering or any other type of filtering you are going to advertise that to the neighbor that you did not receive the route from, even if it's in the same AS as the one you received it from.

That neighbor will receive the route, look at the AS PATH and will not install it in the bgp/routing table if it has the same AS as it's own AS in the path.

Remember when "YOU" send the route back upstream there is no loop yet. The AS path looks normal until it hits the upstream router.

shahedvoicerite · ‎11-05-2009

Hi,

Thanks.. I do get the route from 2 neighbours.

BGP routing table entry for 172.16.99.0/24, version 4

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Advertised to update-groups:

1

7224, (received & used)

169.254.255.1 from 169.254.255.1 (169.254.255.1)

Origin IGP, metric 100, localpref 100, valid, external

Extended Community: RT:7224:2762

7224, (received & used)

169.254.255.5 from 169.254.255.5 (169.254.255.5)

Origin IGP, metric 100, localpref 100, valid, external, best

Extended Community: RT:7224:2762

I shut down one of the interfaces, and did a clear ip bgp *

However, it still shows the route being recieved, and re-advertised back.

I did a bit of more searching, and it seems eBGP (as opposed to iBGP) does not have a split horizon type behaviour

Thanks

Reza Sharifi · ‎11-05-2009

Shahed,

Can you post your BGP config?

You should not be advertising the same routes back to your neighbor.

Reza

c.captari · ‎11-05-2009

Indeed you shouldn't if you really receive that route from the same neighbor you are trying to advertise it to.

But as i mentioned above, if you receive the route from 2 routers, both in the same AS, you will advertise that upstream to the other one!

The router that receives that will drop it on it's side as it sees it's own AS path in the list.

If you want to control that use as path access-lists

shahedvoicerite · ‎11-05-2009

I removed one of the neighbours, and now it works as expected.

I see only the remote route as received, and am advertising only the default route

Thanks !!