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Community Member

EBGP and IBGP and update-source loopback use

Cisco curriculum CCNP1 V3.0 "9.3.3 EBGP and IBGP configuration example" says:

(((Because EBGP sessions are typically point-to-point, there is no need to use update-source loopback command with EBGP))).

Is this acceptable ? because we know we can use update-source loopback with EBGP,,,Don't we ?

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Re: EBGP and IBGP and update-source loopback use

It does not say you cant use the loopback but just that as these are point-to-point links you can use the links as source/dst rather than a loopback address. It is perfectly acceptable to use loopbacks.

Community Member

Re: EBGP and IBGP and update-source loopback use

(((It does not say you cant use the loopback---etc)))

Yes I agree with you it does not say explicitly that ,,,,it is a normal conclusion (yes it wrong coclusion, because we have seen another cisco document using update-source for EBGP) for that,,,, if you look to another paragrph for the same curriculum it says: (((This command is typically used in all IBGP configurations))).

Re: EBGP and IBGP and update-source loopback use

For IBGP peerings Loopback's are normally used, for EBGP peerings the interface IP address is usually used since the connections are usually point-to-point and rely on the physical link. In fact to use Loopback's for EBGP peerings you have to add additional commands (ebgp multihop).

HTH

Andy

Community Member

Re: EBGP and IBGP and update-source loopback use

((for EBGP peerings the interface IP address is usually used since the connections are usually point-to-point and rely on the physical link)).

So if it is point-to-point,,,what is the concept behind that ?

Re: EBGP and IBGP and update-source loopback use

I think you are missing the point of what BGP is designed to provide - it is an inter-domain routing protocol, designed to link autonomous systems. It runs on the BORDERS between Autonomous Systems (EBGP) so is generally reliant on a Physical Link (Point-to-Point) whether this is a WAN circuit or an Ethernet link it doesn't matter. The point is it runs on the Border router of you Autonomous System to a neighbors Autonomous System.

IBGP is run within your Autonomous System and is a means to ensure that your EBGP routers advertise the correct routes.

HTH

Andy

Community Member

Re: EBGP and IBGP and update-source loopback use

Thanks Andy,,,I am familiar with all that you have mentioned above,,,,but may be I did not put my last query in such way to reflect my exact meaning.

What I meant by my last query is this, Cisco says:

(((Because EBGP sessions are typically point-to-point, there is no need to use update-source loopback command with EBGP))).

Why there is no need for this command for point-to-point link ? why we do need to use this with IBGP

Re: EBGP and IBGP and update-source loopback use

It is becasue of the nature of where EBGP is used as opposed to IBGP.

IBGP is generally ran within your AS, along with an IGP such as OSPF, ISIS or EIGRP. Your routers generally have multiple interfaces and multiple paths to each IBGP neighbor. Therefore the use of a loopback interface as a source is the best option (it doesn't goe down etc).

With EBGP you normally only have a single path to a Neighbor and you don't run other routing protocols to that neighbor. They are generally directly connected by a physical link. If the physical link goes down you generally want the EBGP peering to break as well so it is normal to use the physical interface IP address as the source for the neighbor relationship.

Does that explain it any better?

Andy

Community Member

Re: EBGP and IBGP and update-source loopback use

(((If the physical link goes down you generally want the EBGP peering to break as well ))).

Here I did not get you well.

My understanding is :

Since we have point-to-point, that means we have one and only one exit ponit to a neighbor,,,,Am I right ?

If this link goes down for any reason , I am not going to reach a physical interafe for a neighbor nor its loopback.

But it will be different in IBGP since we can have multiple paths to a neighbor (say A), if any one of these paths goes down for any reason, still I can reach a neighbor (A) form other way, here I can see the implication of update-source,which is independant from these multiple paths (i.e.there is no possibility for a loopback to go down.)

Re: EBGP and IBGP and update-source loopback use

One exit point to an EBGP neighbor - yes. If you used a loopback interface there is the posibility that the EBGP neighbor relationship will be re-formed over a less preferable path.

With IBGP you have control of your AS so peering with loopbacks is preferable.

Andy

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