BGP Announcements

[StarTAC]

hi guys..

is it possible to advertise networks to your peers, for which you have no "network" statement in your BGP configs..?.. even when you aren't doing any redistribution..?..

for instance, if u have such a config..

router bgp xxxx
network 1.1.1.0
network 2.2.2.0
neighbor x.x.x.x remote-as yyyy

say you learn x.x.x.x's networks from his BGP session to you, and you learn some more networks from other peers, is it possible that you may announce other networks that u have no network statement for..

theoretically, u shouldn't, as BGP will only announce what it already knows, and if it doesn't know of any other networks, it won't announce them.. but i just wanna be sure that there is no possible way a network can be announced if it doesn't exist in the BGP table..

all help appreciated..

 

[Bobg1]

Basicly its saying I can handle stuff going to here and I know you can handle stuff going to there. If a network is not in a table somewhere and it is not using bgp routing, it should never be seen.

 

[Frenchie2069]

With this configuration you will be advertising to x.x.x.x anything that you learn from other BGP peers. If you want to stop this then you can set up an as-path filter to prevent advertising extra networks. This would be done like this:

router bgp xxxx
neighbor x.x.x.x filter-list 10 out

ip as-path access-list 10 permit ^$
ip as-path access-list 10 deny .$

this allows networks with a zero length AS path- ^$ (ie, originated within your AS) and denies anything else .$

If x.x.x.x is an ISP then they should be filtering networks that you are advertising to them anyway - otherwise you could find that you are becoming a transit network for 2 ISPs!!

HTH,
Michael.