Solved: BGP multihop

Antonio_1_2 · ‎04-11-2008

Hello,

I use BGP mutihop for BGP peering to my customers. In path between my BGP router and customer router I have usually 2 more routers with static routes for customers prefixes(these two routers are not included in BGP session). Problem is when customer put BGP session to shut my static routes still exists in my network and I can't reach customer prefixes through internet via his second provider. Does anyone have idea how to solve this problem?

Thanks in advance

marikakis · ‎04-11-2008

Hello,

The solution depends on what you are willing to run on those 2 intermediate routers (depending on their CPU, memory and your general setup).

For example, you could run iBGP between those 2 routers and the BGP router where your client BGP sessions terminate. You could filter the routes from BGP router towards those 2 and only allow customer prefixes (partial BGP routes instead of whole Internet routing table means a lot less memory requirements). This way the 2 routers will learn the customer prefixes from the main BGP router (which knows those only if BGP client session is up). You can then remove the static routes for client subnets and put a default route (suppose you already have such a route) in those 2 towards the main BGP router. When the client shuts the BGP session, routes will be removed from the 2 routers, default route will be used towards the main BGP router and from there on (hopefully) you have a proper BGP core that will forward packets towards the Internet and reach client via its secondary connection.

Kind Regards,

M.

View solution in original post

marikakis · ‎04-11-2008

Hello,

The solution depends on what you are willing to run on those 2 intermediate routers (depending on their CPU, memory and your general setup).

For example, you could run iBGP between those 2 routers and the BGP router where your client BGP sessions terminate. You could filter the routes from BGP router towards those 2 and only allow customer prefixes (partial BGP routes instead of whole Internet routing table means a lot less memory requirements). This way the 2 routers will learn the customer prefixes from the main BGP router (which knows those only if BGP client session is up). You can then remove the static routes for client subnets and put a default route (suppose you already have such a route) in those 2 towards the main BGP router. When the client shuts the BGP session, routes will be removed from the 2 routers, default route will be used towards the main BGP router and from there on (hopefully) you have a proper BGP core that will forward packets towards the Internet and reach client via its secondary connection.

Kind Regards,

M.

Antonio_1_2 · ‎04-11-2008

Thanks a lot.

if those two intermediate routers run iBGP with main router then next-hop will remain as it is and route for prefixes will point to the client router.

I was considering simmilar approach with redistribute BGP to eigrp or ospf on main BGP router and then sending prefixes to intermediate routers via EIGRP but then next-hop for client prefixes would be main BGP router (I would have loop)

marikakis · ‎04-11-2008

Hello,

In most cases it is better to keep your IGP "clean". Ideally your IGP carries only your internal prefixes and not customer prefixes. BGP is preferred for carrying the customer prefixes. More clean and scalable solution. Problem is that some routers might not have the necessary resources for BGP. Still with some partial BGP route feed tricks one can make things work with minimal or no routing loops at all. Only make sure that you put the filters in place and always remember that there exists a possibility of "remote killing" a low resource router by entering the wrong configuration on the main BGP router (i.e. send full routes accidentaly).

Kind Regards,

M.

Richard Burts · ‎04-11-2008

Antonio

I wonder if the feature of Reliable Static Routing Backup Using Object Tracking would be a solution for your issue. If you have static routes in your intermediate routers for customer prefixes it seems that this feature would allow you to remove these statics if the preferred path to the customer stopped forwarding traffic.

For information on this feature you can start with this link:

http://www.cisco.com/en/US/docs/ios/12_4/dial/configuration/guide/hdbackup.html

HTH

Rick

HTH

Rick

Antonio_1_2 · ‎04-14-2008

Hi,

yes that would be also a solution. Thank you. The only drawback is the case when customer put peering in shut (i.e. neighbour x.x.x.x shutdown). Then all IP addresses would be up and static routes would still remain in routing table.

marikakis · ‎04-12-2008

Hello,

Just to clarify: The solution with customer routes only for some BGP routers is not bad in itself. You only need to be careful that you have the filters in place and everytime you are in the process of changing the filters you should check again before clearing softly the session (this is not a difficult thing to do, you only need to NOT trust yourself very much, mistakes happen when people, usually experienced, are too confident in themselves). It is a solution that provider networks use to get along with low resource routers that cannot run full BGP.

Kind Regards,

M.

Antonio_1_2 · ‎04-14-2008

Hi,

I agree with you.Thanks