Loopback interface in OSPF?

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Lets say I have 5 routers all are connected on a broadcast ethernet network.The router to whom I want to make DR, I will configure loopback interface with higher IP address.Ok fine. But I am confused here that when the ethernet link of DR will go down, then loopback interface will also get down. So what is the benefit of using loopback interface. Please explain this in detail if someone can, that loopback interface will remain UP even when the physical interface connected with network is down?

I have this problem too.
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Danilo Dy Fri, 03/09/2007 - 00:24

Loopback interface will always be up even physical interface is down.

How did you know that loopback interface is down? Did you console to the router and check this? Note, if the router physical connection to network is down, the loopback interface ip address will not be reachable from the network since the path to it is gone.

Danilo Dy Fri, 03/09/2007 - 04:32

You have not answer my question....

But here is the info regarding DR and BDR elections

DR and BDR elections are accomplished via Hello protocol. Hello packets are exchanged via IP multicast packets on each segment.

However, only segments that are broadcast and non-broadcast multi-access networks (examples are Ethernet and Frame Relay) will perform DR and BDR elections. Poin-to-point links, like a serial WAN for example, will not have a DR election process.

On a broadcast or non-broadcast multi-access network, the router with the highest OSPF priority on a segment will become the DR for that segment. This priority is shown with the "show ip ospf interface" command.

The default priority for a router interface is one "1". If all routers have the default priority set, the router with the highest Router ID (RID) will win.

The RID is determined by the highest IP address on any inteface at the moment of OSPF startup. This can be overridde with a loopback (logical) interface.

If you set a router interface to a priority value of zero "0", that router won't participate in the DR or BDR election on that interface. The stat of the interface with priority zero will then be DROTHER.

mheusinger Fri, 03/09/2007 - 04:09

Hi,

could it be, that you talk about the OSPF router-ID, which is something different than the Loopback interface?

With "show ip ospf neighbors" you will see, that the OSPF router ID is usually chosen from a Loopback interface IP. With the same command you can see, that a neighbor (DR or not) is gone, once its interface is down (or keepalive timeout).

Router ID and Loopback are different things though. Have a look at this other posting explaining it in more detail:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1ddd9895/6#selected_message

So the Loopback is still up and potentially even reachable though a physical interface might be down and the OSPF adjacency dropped.

Hope this helps! Please use the rating system.

Regards, Martin

mheusinger Fri, 03/09/2007 - 04:41

Hi,

you could have a redundant setup and the second interface will give you connectivity, f.e.

E0/0 connected to switch1 - down

E0/1 connected to switch2 - up

All traffic is routed through E0/1 (with an OSPF adjacency) and Loopback remains reachable.

Regards, Martin

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