BGP Community

jthreadgill · ‎02-26-2002

I have a bgp config that looks like this. (less the IPs).

router bgp xxxxx

bgp log-neighbor-changes

network x.x.x.x mask 255.255.255.0

neighbor x.x.x.x remote-as 1111

neighbor x.x.x.x version 4

neighbor x.x.x.x weight 50

neighbor x.x.x.x distribute-list 2100 out

neighbor x.x.x.x route-map setaspath out

neighbor x.x.x.x remote-as 2222

neighbor x.x.x.x ebgp-multihop 255

neighbor x.x.x.x update-source Loopback0

neighbor x.x.x.x version 4

neighbor x.x.x.x weight 50

neighbor x.x.x.x distribute-list 2100 out

There is only 2 neighbors, and the problem is that all of the traffic is coming in through AS 2222. What I want to do is to take a subnet out of one of the networks, and force it to only come in and out AS 1111. I have tried adjusting the weights and padding with the result being all the traffic coming in 1111. Bandwidth is limited on AS 1111.

Thanks for your help.

amdcent · ‎02-26-2002

Outbound traffic:

You can certainly manage outbound traffic in worst case using policy routing

Inbound traffic:

Hard to manage inbound traffic however you can try advertising more specific routes thru AS2222

jthreadgill · ‎02-26-2002

I understand that it is hard to manage inbound traffic. I just need a little advise in how to get the subnet to think the only route is through the other AS. Would the community command work for this? Is there a better way?

Could I use this?

neighbor x.x.x.x send-community

neighbor x.x.x.x route-map setcommunity out

route-map setcommunity

match ip address 8

set weight 2000

access-list 8 permit x.x.x.x 255.255.255.128

access-list 8 permit ip any

Would this match the subnet and then tell it the path through that neighbor has a higher weight? Then use the other route for outgoing and incoming. Am I even on the right track?

Thanks for your help.

amdcent · ‎02-26-2002

Two things.

1. I don't see any community reference here. (Community will not help you in this case)

2. Weight is local to the router. You can't send this to another router.

Here is a url might give you more info. on BGP.

http://www.cisco.com/pcgi-bin/Support/PSP/psp_view.pl?p=Internetworking:BGP

-amdcent

rais · ‎02-26-2002

You should add AS to your Advertised AS-PATH.

mshao · ‎02-26-2002

you can always use command "neighbor filter-list" to block network be advertised to specific neighbor router.

ruwhite · ‎02-27-2002

Take a look at the looking glasses and see what the actual routes look like a couple of hops out--you're probably getting a /24 or something through one provider, and the other provider is most likely summarizing the address into a shorter prefix aggregate. If this is true, then no amount of padding or setting of anything will help--the longer prefix length route will always be preferred by other as' into your as.

So, take a look at this before trying to get the traffic to balance out in any way.

Russ

bryan.king · ‎02-28-2002

If your neighbors use filter lists for what they will accept from you, you can try and ask AS 2222 to ignore more specific and ask AS 1111 to accept more specific on the network you want to force on to AS 1111. I had a similar need and had no problem getting my neighbor AS'es to do this for me. I've been doing this for a specific /25 out of a /19 on our network for about 6 mos. with out any problems.

ashok_boin · ‎03-04-2002

Hi,

Initial step you should take is as by "ruwhite" to confirm with your neighbors that is there any difference in your networks advertisement. If there is no difference, then it's quite easy. Observe your routes advertisement through your providers through "route-servers" and observe how many AS hops the AS 2222 path is preferring one compared with AS 1111. Then create prefix lists for your networks individually and create route-maps for both neighbors like this. According to the bandwidth of your both links, advertise some networks without any AS-path prepend and all other networks with AS-path prepend to lower it's best match. And do the same for second neighbor. This will work even when one of your links down, you will get all traffic with another link completely.

Let me know if you want help during configuration...

Regards...

--Ashok.

With best regards...
Ashok