Loopback interface in OSPF?

imran_mcse · ‎03-09-2007

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?

Danilo Dy · ‎03-09-2007

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.

imran_mcse · ‎03-09-2007

Thanks for your reply.

But what about the DR election? Other routers will still believe that the DR (which has loopback interface) is available? Can you please provide me more details regarding the OSPF relatioan with loopback interface.

Danilo Dy · ‎03-09-2007

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 · ‎03-09-2007

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

imran_mcse · ‎03-09-2007

Dear Martin I understand the whole story. I accept that loopback interface will be still up but here I have another question that how it can be reachable even the originating router's physical interface is down?

mheusinger · ‎03-09-2007

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