are we able to use loop back interface to form ospf neighborship between routers ?

[jaishankar1]

I know we can form ospf neighborship between routers using Physical and VLAN interfaces but would like to know if we are able to form neighbor ship using Loop back interfaces. how can we find neighbor routers from the router that I use in an environment if it is possible to make neighbor ship using loopback interfaces?

as far as I am concerned not able to form ospf neighborship between routers using loop back interfaces as it doesn't meet any of ospf neighbor ship requirements but would like to have your comments on this and also share documents if any

[Richard Burts · Accepted solution]

As a starting point for an answer let us remember that a loopback interface is a virtual interface. A virtual interface is not able to physically transmit any data or physically receive any data. If the loopback can not send OSPF hello messages or receive OSPF hello messages then it is not possible to establish any neighbor relationship using loopback interfaces.

You may very well want to have network statements in OSPF for loopback interfaces so that OSPF runs on the interface and the loopback interfaces are advertised. But you can not use any loopback interface to establish OSPF neighbor relationship.

HTH

Rick

[Joseph W. Doherty · Accepted solution]

In addition to what Rick describes, I recall (?) that OSPF requires "physical/logical" adjacency to allow a neighborship to form. Don't see how that's possible with loopback interfaces.

[Patrick.Williams · Accepted solution]

As @Richard Burts stated OSPF has to be formed between physical interfaces, this is mainly facilitated by hellos between both physical connected interfaces of which cannot be accomlished by Loopback interfaces.  You can however assign a Loopback address as your router-id for your ospf process, this will assist you in identifying you neighbor relationship much easier rather depending on the IP Address of an interface within the router.

Based on the fact that your main concern is being able to identify a neighboring router you can impliment the following.

SAMPLE

R1

interface loopback 0

ip add 172.1.1.1 255.255.255.255

router ospf 1
router-id 172.17.1.1

R2

interface loopback 0

ip add 172.1.1.2 255.255.255.255

router ospf 1
 router-id 172.17.1.2

From Router 4 perspective you will be seeing all other routers based on their perspective loopback interface address.

R1 could be labeled as location1 for simplicity.

 

[Peter Paluch · Accepted solution]

My friends,

Please allow me to join.

Fully agree with all comments shared by Rick, Joe, and Patrick. I would also like to add a couple of observations.

Forming an OSPF adjacency through loopbacks, if taking this requirement literally, is truly not possible. OSPF speakers can only create an adjacency over a pair of interfaces that connect them together. Surely, a Loopback interface as a software-based construct without a physical substance cannot interconnect two routers.

However, with a grain of salt, the original question can be reinterpreted as: Can two OSPF routers form an adjacency using Loopback addresses? With this interpretation, the answer would be YES under two circumstances:

1. The routers are connected together with a point-to-point link configured as an unnumbered link borrowing its IP address from a loopback interface
2. The routers are configured for a sham link (obviously makes sense only between PE routers in MPLS L3VPN scenarios)

I believe it is now up to Jai to choose the answer :)

Best regards,
Peter

[jaishankar1]

My question was to know if we are able to form ospf neighborship in normal way like how we do form using physical links.

but here I have got brief knowledge about it with your help. thanks everyone for your responses

[Deepak Kumar · Accepted solution]

Hi,

I will vote to @Peter Paluch and you can go with IP Unnumbered. Again If you are looking for a solution same as BGP then it's not possible. 

Regards,

Deepak Kumar

[Richard Burts]

Peter

I am glad to see you join this conversation (as I am glad every time that I see you posting in the community. thank you for continuing to be active with us). You present a very interesting alternative interpretation of the question in the original post. And certainly a point to point link with ip unnumbered would allow successful formation of OSPF neighbor relationship using the IP address of a loopback interface. So now Jai does have a choice of which version of the question did he mean.

HTH

Rick