BGP update-source command

Unanswered Question
Oct 28th, 2008

Is it needed on both IBGP routers?

In my lab, I only set it on one IBGP router. On the other one, I only set the neighbor command. And I got a normal BGP peering establishment.

I have this problem too.
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Jon Marshall Tue, 10/28/2008 - 14:23

It's not needed at all by default. You only use it when you want your router to use a specific IP address when exchanging BGP updates with another router.

A common configuration is to have a loopback interface on your router and then to use under your BGP config

router bgp 100

neighbor x.x.x.x loopback 10

Jon

ccnpninja Tue, 10/28/2008 - 14:31

Jon,

This is what I meant:

Suppose we have 2 routers A and B within the same AS. We want them to become IBGP peers.

Both A and B reach each other through an IGP. Both of them have a loopback (lo1 and lo2).

There are two paths between A and B. And that justifies the use of update-source in the neighbor command.

My question is: do I need the "neighbor x.x.x.x update-source lo{number}" command on both A and B?

Jon Marshall Tue, 10/28/2008 - 14:40

"There are two paths between A and B. And that justifies the use of update-source in the neighbor command"

Not sure exactly what you mean by this. You need the update source command when you don't want your router to peer on it's directly connected interface so

(loopback 10) A (192.168.1.1) -> (192.168.1.2) B (loopback 10)

As giuseppe says it can be any combination ie. A uses loopback, B uses physical interface, both use physicals, both use loopbacks etc. It's up to you. Only if the interface is not directly connected do you need the "update source ..." command.

Apologies if i have still misunderstood.

Jon

ccnpninja Wed, 10/29/2008 - 13:53

Jon,

"There are two paths between A and B. And that justifies the use of update-source in the neighbor command"

Here's an excerpt from Cisco Student Guide.

Giuseppe Larosa Tue, 10/28/2008 - 14:25

Hello Wassim,

it depends : it is needed on the side that doesn't use the directly connected interface as its source ip address for BGP packets.

It is needed on both sides if you use loopback addresses that are logical

But you can have an iBGP between a loopback on R1 and the physical interface on R2.

Hope to help

Giuseppe

ccnpninja Tue, 10/28/2008 - 14:56

Giuseppe,

"But you can have an iBGP between a loopback on R1 and the physical interface on R2. "

Is it the case when R1 and R2 are directly connected?

Harold Ritter Tue, 10/28/2008 - 18:35

Wassim,

If both routers are configured to peer to the other router loopback address and that you configure the "neighbor update-source lo0" only on one side, the session will come up but only when the side configured with the update-source actively establishes the session. This is because only this side will try to establish the session using the proper source address. When the other side try to actively open the session using the physical interface address as its source, it fails as the source will not match the peering address configured on the other router.

Regards

sdoremus33 Wed, 10/29/2008 - 22:52

In the excerpt you posted, I believe what they are mentioning "if you are using multiple paths to IBGP neighbors" is because the loopback is obv a logical interface so in the event a physical to physical or phys to Logical (Loopback0)there is a possibility the phys int can experience HW issues.

Thus breaking the IBGP peering session.

As HRitter mentioned is only one side is configured with Update source loopback# that side will establish peering session, so obv both ends (if both peering endpoints are configured as loopback meaning the interface announcing the prefix

Ex:

router bgp xxx xxx= as number

neighbor "neighbor peer on loopback#)

neighbor update source loopback#

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