loopback addresses

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Jun 15th, 2006
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when i use a loopback interface, do i need to advertise the address in the routing protocol ? what is the normal way of setting this up, say over a serial , do people always use loopback interfaces, or is this mainly an ospf thing ?

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Overall Rating: 4.5 (2 ratings)
vladrac-ccna Thu, 06/15/2006 - 05:17
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is always good to have loopback address.

Not only for OSPF.

"You can specify a software-only interface called a loopback interface to emulate a physical interface. A loopback interface is a virtual interface that is always up and allows Border Gateway Protocol (BGP) sessions, for example, to stay up even if the outbound interface is down.

You can use the loopback interface as the termination address for open shortest path first (OSPF) or BGP sessions. In applications where other routers or access servers attempt to reach this loopback interface, you should configure a routing protocol to distribute the subnet assigned to the loopback address.

Packets routed to the loopback interface are rerouted back to the router or access server and processed locally. IP packets routed out the loopback interface but not destined to the loopback interface are dropped. Under these two conditions, the loopback interface can behave like a null interface.


Imagine you have 10 interfaces on your device and you need to test if this device is up and running, would it be best to ping the loopback or a single interface?

loopback will always be reachable if you have at least 1 interface up.


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carl_townshend Thu, 06/15/2006 - 05:24
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so is there instances whereas people use the loopback address for there local gateway address ?

vladrac-ccna Thu, 06/15/2006 - 05:32
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Not really.

As you cant put (without bridging) 2 interfaces on the same subnet.


carl_townshend Thu, 06/15/2006 - 06:17
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but couldnt you use ip unnumbered loopback 0 in the ethernet address to achieve this ?

vladrac-ccna Thu, 06/15/2006 - 06:22
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good point,

Read the last post by Rick

And you'll see that you cant use ip unnumbered under ethernet interfaces



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