loopback interface with EIGPR

Answered Question
Apr 22nd, 2008

I want one of my routers to participate in the EIGRP routing. The problem is that I want to only

redistribute static route via EIGRP on that new router. I don't want to advertise the connected network via EIGRP.

However if I don't make any connected interfaces part of the EIGRP, the router doesn't advertise

and it doesn't see any neighbors. The question is how can I connect that router to the EIGRP 10 AS?

Can I use loopback interface to establish the EIRGP advertisements? I am not sure how the lookback

interfaces work so I need some kind of direction. Thanks as always!!!!

I have this problem too.
0 votes
Correct Answer by Richard Burts about 8 years 7 months ago

Bart

I can understand wanting to test with something that will not disrupt a production network. As I said if you have the router that you want to redistribute on and if you configure a single network statement for a physical intereface that will allow the router to form EIGRP neighbor relationship. And if you think about it, the router that you are using to test will not advertise anything at that point. (The router it connects to will advertise that physical subnet, but since that subnet does exist in the physical network will it have any negative impact to advertise that single physical subnet?)

After you have established the single EIGRP neighbor to the test router then the test router can redistribute the test static route. And that would be the only thing that it advertised.

You ask what loopback interfaces are used for in EIGRP and that is not a simple question. EIGRP will prefer to use a loopback interface address as its routerID. And that is the single real use of loopback that I can think of in EIGRP. Beyond its specific use in EIGRP loopback interfaces can be used for many things. A loopback interface gives you indipendence from physical interfaces, so if there is some address that you want to be always available (perhaps as the end point of a tunnel, or as the address used for network management, for telnet/SSH, for AAA authentication, as the source of log messages, etc) then it is helpful to use loopback interfaces addresses for things like this.

HTH

Rick

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francisco_1 Tue, 04/22/2008 - 09:09

yes you can use loopback interface to establish neighbour relationships and exchange hello packets. Once the neighbour relationship is established between your routers via the loopback interface, then you can advertised your available subnets on your physical interfaces.

Firstly, create your loopback interafce and assign it an ip address.

Secondly, under EIGRP, Advertise your loopback interface on both routers.

Francisco.

bsudol79p Tue, 04/22/2008 - 09:49

Do I have to have loopback interfaces on both of the routers to establish adjacency? Do the loopback interface addresses have to be in the same subnet as the networks that I want to advertise? Thanks

Richard Burts Tue, 04/22/2008 - 09:57

Bart

I think that the assertion that you can establish EIGRP neighbor relationships over loopback interfaces is pretty remarkable - and I disagree with it. You can only establish EIGRP neighbor relations over physical interfaces that can send and receive EIGRP hello messages.

I am very puzzled about what you are trying to accomplish. I think I understand the part about wanting to redistribute static routes. But I do not understand the problem with advertising a physical interface subnet. If you put a network statement for 1 physical interface which connects to another router to run EIGRP that router will know the subnet anyway, since it is connected. Why does it matter whether you advertise it or not?

And if you really do not want to advertise the subnets of physical interfaces then I suggest that you think about configuring a distribute list to permit advertisement of the static routes and to deny advertisement of the physical subnets.

HTH

Rick

bsudol79p Tue, 04/22/2008 - 10:17

OK, thanks for the recommendation Rick. If I cannot use the loopback interfaces to establish adjacency then what are the loopback interfaces used for? As for your question why I want to avoid using the physical interfaces? I do not want to avoid using physical interfaces. I just wanted to test the static route redistribution first before advertising the phycial networks. The reason behind this is that I want to redistribute a "test network" that doesn't have any effect on the production. Thanks Rick you helped me so much over the time!!!!

Correct Answer
Richard Burts Tue, 04/22/2008 - 10:30

Bart

I can understand wanting to test with something that will not disrupt a production network. As I said if you have the router that you want to redistribute on and if you configure a single network statement for a physical intereface that will allow the router to form EIGRP neighbor relationship. And if you think about it, the router that you are using to test will not advertise anything at that point. (The router it connects to will advertise that physical subnet, but since that subnet does exist in the physical network will it have any negative impact to advertise that single physical subnet?)

After you have established the single EIGRP neighbor to the test router then the test router can redistribute the test static route. And that would be the only thing that it advertised.

You ask what loopback interfaces are used for in EIGRP and that is not a simple question. EIGRP will prefer to use a loopback interface address as its routerID. And that is the single real use of loopback that I can think of in EIGRP. Beyond its specific use in EIGRP loopback interfaces can be used for many things. A loopback interface gives you indipendence from physical interfaces, so if there is some address that you want to be always available (perhaps as the end point of a tunnel, or as the address used for network management, for telnet/SSH, for AAA authentication, as the source of log messages, etc) then it is helpful to use loopback interfaces addresses for things like this.

HTH

Rick

Richard Burts Tue, 04/22/2008 - 11:15

Bart

I am glad that my responses were able to help resolve your questions. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that they will read how the question was resolved.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick

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