BGP question about learning routes

[Steven Williams]

iBGP uses the the same AS throughout your network from end to end right.

So I have three routers connected to each other all using the same AS number and all peering, how does router 1 know about the networks that router 3 is advertising?                 

[Jon Marshall]

Steven

An IBGP peer cannot advertise a route to another IBGP peer if it learnt the route from IBGP. So if i understand this is the point of your question.

The ways round this -

1) have full connectivity between all IBGP peers ie. each router peers with every other router using IBGP but this is often impractical in a network with a lot of IBGP peers.

2) use route reflectors where the restriction of IBGP route advertisements is relaxed. Basically you pick a router to be a route reflector and you then configure other routers to be route reflector clients. The route reflector can then advertise a route learnt from one of it's clients to the other clients. The clients only need to peer with the route reflector using IBGP.

Jon

[Steven Williams]

Just a quick drawing for ease of understanding.

The router at the top cannot see the networks advertised from the bottom router (10.171.140.0/24)

The Nexus devices that connect to the top router can see these networks so thats all good. The issue is there are other locations on the other side of the MPLS that cannot get to the bottom routers networks because it doesnt know where to go.

Do i need to enter a command in the top router to set another neighbor with 192.168.192.1 on the nexus devices?

[Jon Marshall · Accepted solution]

Steven

Just to clarify. The box at the bottom is a router. The box at the top is router. The 2 boxes connected to the top box are routers.

You are running IBGP on all these routers ?

What are the boxes between the 2 two at the top and the one at the bottom ?

Jon

[Steven Williams]

Cisco 3750 switches operating in layer 2 mode.

[Steven Williams]

and the subnet at the bottom is 10.171.140.0/24 not 10.170...my mistake

[Jon Marshall]

Okay if they are peering using IBGP then yes that is your problem. The 2 boxes cannot advertise IBGP leant routes from the bottom router to the top MPLS router.

You can either create another peering between the bottom and top MPLS router for IBGP or use one or both of the 2 boxes as route reflectors and make the MPLS and bottom boxes clients.

Jon

[Steven Williams]

So from the top router I would enter neighbor 192.168.192.4 remote-as 64707?

I have no clue what router reflectors are or how to do it.

[Jon Marshall]

Steven

With only 4 routers it's probably easier to do it with another peering. You would simply copy the config you have used for the other routers peerings obviously replacing the IP address with the address of the bottom box. And you need to setup the peering on the bottom box as well.

You should be fine to do this but whenever i made changes like this i tended to do them out of key hours just in case.

Jon

[Steven Williams]

so the bottom router would have a neighbor relationship with each interface on the top router? even though they are not connected?

Also if the two 3750's acting in layer 2 mode were layer 3 enabled also running ibgp everything would work as I envision it correct?

[Jon Marshall]

so the bottom router would have a neighbor relationship with each interface on the top router? even though they are not connected?

Yes they don't need to be physcially connected as long as they can route to each other.

If the 3750s were L3 and running IBGP that would just make the situation worse because now you have more IBGP peerings to setup.

Jon

[Steven Williams]

so looking at my diagram each 5k connecting to the top mpls router have two different subnets. So on the bottom router I am going to want to create a peer relationship with both IPs in each subnet, but am I going to want to prefer one over the other so traffic doesnt get confused on which way to go?

[Jon Marshall]

Steven

each 5k connecting to the top mpls router have two different subnets.

That is not what your diagram is showing.  Each 5k has one connection to the MPLS router using a separate subnet.

When you peer with the MPLS router the routes received will point to the MPLS router. So the bottom router needs to know how to route to the MPLS 10.170.199.x interface addresses. Does it know this ?

Traffic won't get confused, it will just pick one of the routes. You should see equal cost paths so the bottom router will see 2 equal cost paths to the remote networks via the MPLS router.

Note, i have never configured 2 IBGP peerings between the same 2 routers and i can't test but i see no reason why it won't work.

Again, i suggest you do this out of hours.

Jon

[Steven Williams]

Ok yes the bottom router can get to 10.170.199. networks, these are /29 networks so on the bottom router i set up a neighbor relationship with 10.170.199.6 and now I see the routes being advertised in the MPLS cloud. But when I create a neighbor relationship with 10.170.199.14 which is the other interface on the MPLS router (top) the bgp status is idle and I never see messages being sent or received. The bottom router can ping this.

[Jon Marshall]

Steven

It may be because the IP address is the same on the bottom router for each peering so it is not allowing a second IBGP peering. Apologies, i was only thinking of the MPLS router and forgot that the bottom router has only one connection. You can run "debug ip bgp neighbor" on the bottom router to see what is happening but i suspect that is the issue.

This may or may not be an issue for you. Does the 5k on the left have a route to that next hop or is it just the 5k on the right ?

Jon