BGP not updating with loopback address

[acameron12]

I am staging two 2800's with dual T1 access. The routers exchange updates using the serial interfaces as neighbor addresses.

When I added a second T1 and referenced the loopback address as the neighbor, updates stopped.

What have I missed ?

[dwyerr · Accepted solution]

Hello,

The default behavior is to use the exit interface for sourcing updates. To change this you need to use the "update-source" knob.

neighbor x.x.x.x update-source Loopbackx

Remember also that the loopback is not directly connected so you will need to be able to reach the loopback, static route most likely, and then use multihop.

Regards,

-Rob

[acameron12]

Hello,

Thanks for the prompt response.

When you state exit interface, you mean

the loopback address on the local router,

correct ?

[pkhatri]

Hi,

Suppose you have two links between your routers, A and B. Call them link1 and link2. Link 1 has the IP of 10.1.1.1 on the router A side and IP of 10.1.1.2 on the router B side. Link 2 has the IP of 192.168.1.1 on the router A side and IP of 192.168.1.2 on the router B side.

Now, if router A wants to send a packet to address 10.1.1.2, it will send it out of Link1, since that is the shortest way to get to it. Since Link1 has an IP of 10.1.1.1, the source address in the packet will be 10.1.1.1 and link1 is the "exit" interface. That will be the case if you are setting up a BGP session on A with a peer-address of 10.1.1.2.

Continuing the example above, if you configure loopback0 on A with IP 172.16.1.1/32 and loopback0 on B with IP 172.16.1.2/32, this is how the bgp setup would look:

Using IBGP:

A --

router bgp 100

neighbor 172.16.1.2 remote-as 100

neighbor 172.16.1.2 update-source loopback0

B --

router bgp 100

neighbor 172.16.1.1 remote-as 100

neighbor 172.16.1.1 update-source loopback0

Using EBGP:

A --

router bgp 100

neighbor 172.16.1.2 remote-as 200

neighbor 172.16.1.2 update-source loopback0

neighbor 172.16.1.2 ebgp-multihop

B --

router bgp 200

neighbor 172.16.1.1 remote-as 100

neighbor 172.16.1.1 update-source loopback0

neighbor 172.16.1.1 ebgp-multihop

Hope that helps - pls rate the post if it does.

Paresh

[acameron12]

Hello, This is very helpful.

[acameron12]

I have one more question, on the

neighbor 172.16.1.2 ebgp-multihop statement,

I have a value of 255 appended.

What is the significance of this ?

[dwyerr]

No, it is sourced via the interface the update leaves. So if you are peering on int serial x, the update is sourced using that address. This behaivor can be changed as described earlier.

-Rob

[acameron12]

Got it. I have the two routers working as you prescribed earlier. Once I added the static route, the updates flowed and connectivity resumed. Because I have two serial links, I used weighted static routes pointing to the remote loopback addresses as you stated.

Is more than one static route necessary ?

[dwyerr]

Routes should look something like the following, assuming the remote loopback ip is 1.1.1.1.

ip route 1.1.1.1 255.255.255.255 serial1

ip route 1.1.1.1 255.255.255.255 serial2

This will balance traffic over both links.

I think it's wise to point the routes to the interface so if the link goes down the route is withdrawn. Otherwise you can get into a situation where one of the interface addresses is learned via bgp and when the link fails your route is not withdrawn.

-Rob