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Loopback inteface in OSPF

Unanswered Question
Sep 9th, 2006
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"If no loopback interface defined, the highest local active IP address of the router's physical interfaces will be used. It is considered good practice to use loopback interfaces for all routers running OSPF. This loopback interface should be configured with an address using a 32-bit subnet mask of 255.255.255.255." Why these consider good practice? If there is no active interface running on a router at all, why would we need to keep OSPF enable with loopback? If there is no active interface running, will the router still function as an OSPF router? What will it do in that case? Is router ID unique? If yes, how? What if there are some routers in the network use the same loopback address? Is it still unique?

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vijayasankar Sat, 09/09/2006 - 23:51
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Hi,



Per RFC 2328, a Router ID is a A 32-bit number assigned to each router running the OSPF

protocol. This number uniquely identifies the router within an Autonomous System.


The OSPF Router id is the highest IP address on the box or the loopback interface, calculated at boot time or whenever the OSPF process is restarted.


This RID is serving the purpose for uniquely identifying a Router, and that's why the IP address is choosed to be the value for Router ID.


The reason for preferring loopback IP address is due to the fact that a loopback interface will never go down ( as it is a logical interface in the router). Hence there wont be any disturbance in OSPF neighbor relation ship and adjancecies due to interface link flaps. Thats why it is a best practice to use Loopback interfaces in OSPF.


If no loopback interface is configured in the router, the highest ip address available in the router will be choosed as a Router ID.


In no network, we can have a duplicate IP Address. If two routers uses the same ip address for loopback interface, then it is a mistake.



Hope this helps


-VJ




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