BGP how to prefer eBGP over iBGP

[neil_titchener]

Hi All,

I have a customer who has two Data Center sites running iBGP between them.  We will be adding a new WAN and we want the subnets local to each site to be advertised across the new WAN which will have an eBGP relationship two each of the Data centers.  The diagram shows an example of the new setup.  I'm unable to get the actual configurations at the moment as the setup has not yet been configured so I'm looking for options.  I've tried applying a route map to each DC to block the advertisement of the local subnets but that doesn't work.  I can see those local subnets advertise by eBGP to all the remote BGP peers but it's not populated in the Data center routing tables, I'm guessing that's because the iBGP peers see their own AS listed in the AS_paths.  I believe I could apply a route-map setting the next hop and pointing that to the eBGP peer but where would I apply a route-map stating the next hop?  Alternatively I could stick with blocking the route advertisement and add static routes pointing to the next hop towards the eBGP neighbours.  I don't know the local architecture at this moment but if that's the only option I can go to the customer and work on that as a solution. 

Any suggestions would be welcome :-)

[Jon Marshall]

I am not following what the problem is especially based on the title. 

Could you give a specific example of what you want to happen ? 

Jon

[neil_titchener]

Hi Jon,

I want traffic to use the EBGP route rather than the iBGP route. The route map option doesn't seem to work when the route is advertised by eBGP the receiving DC sees it's own AS in the path so drops the route. If I use static routing that works for one hop but I don't think it's viable over multiple hops.

Cheers

Cheers

[Jon Marshall · Accepted solution]

You can use the "allowas-in" option to allow the routers to accept route advertisements with their own AS in but you need to be careful you do not create a routing loop. 

I am sorry but I still don't exactly follow the issue. 

You say you want traffic to use the EBGP route and not the IBGP route but traffic from where to where ie. an example of specific subnets advertised from where to where and what the BGP and IP routing tables show would help. 

Jon

[paul driver]

Hello

so if I understand this - presently your dcs are ibgp peered to each other and also each have a ebgp connection for remote sites?

as it stands each DC wan rtr will select it’s connected ebgp peer for remote site connectivity unless that is either one of these rtrs is receiving a better path metric than the other over it’s own ebgp then either will select its ibgp neigbour as it next hop ( providing this is configured) - So is it this part that you wish to manipulate?

[neil_titchener]

The two DCs have both iBGP and eBGP peering, and each site has a number of local subnets advertised via iBGP.  we will eventually migrate away from the iBGP links over to a new WAN which will use an eBGP relationship.  I would like to force the local subnets to use the eBGP routes rather than iBGP.  I can prevent the routes being advertised between the iBGP peers using route-maps but the problem I have is that as the routes are now seen advertised from an eBGP peer they are dropped because the original AS is seen in the route path.  I will try the allowas-in command and see how that works.  I can't do the until monday so I'll provide an update then.

Thanks for all suggestions, they are very much appreciated.

[Larry Sullivan]

Look into the "BGP local-as" option.  As stated, you have to be careful.  Lab it up first.  Why exactly do you want the iBGP traffic to prefer the WAN again?  

[neil_titchener]

Thanks to everyone for their input.  Following your suggestions I've done some research and learned a lot more about BGP including allowas-in, rewrite and local-as :-).  I think in this situation the allowsas-in is the most appropriate solution.

Thanks again