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Home > Archive > CCNP > August 2002 > OSPF Router-ID
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| Hey everyone...
I know this is probably going to be a dumb question and I think its been asked before I just couldn't find it, but is your loopback IP always used as the router-ID in OSPF? Or do you have to specifically give it a higher IP than your other interfaces?
For example, if you have a router with two Ethernet interfaces with IP's of 172.17.12.1/24 (E0) and 172.18.12.1/24 (E1) and you configure your loopback interface to be 172.16.1.1/30, will E1 be the router-ID or the loopback address?? 
From what I've read - it should be the loopback interface regardless of its IP and if there are more than one loopbacks, it should be the one with the highest IP. Am I wrong??
Thanks in advance!!  | |
| SurfinUSA 2002-08-14, 9:51 am |
| I believe it will use the loopback address only if its set. So, if you dont set an ip address for the loopback interface, then obviously it wont. But if you do, then it will automatically use it as the router ID. | |
| Hippo 2002-08-14, 10:02 am |
| The answer is:
If a loopback interface is configured, then its IP address will be used; if more than one loopback address is used, then the HIGHEST loopback IP address is used, regardless of the loopback interface number.
Here's the link to the same question originally posted by sukuvi:
http://examnotes.net/forums/showthr...?threadid=48692
Hippo | |
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| Thanks for your help!
And thanks for the thread link, Hippo - obviously I didn't look hard enough!  | |
| SureshHomepage 2002-08-16, 1:05 am |
| Hi,
I just had a chance to refer Karl Solie of this issue. I was part of the mail posting pointed above so I am rejoiner now.
The Router chooses the highest IP address from any operational interface. If a lookback s configured on the router, the router chooses that address. If multiple loopbacks are configured on the router, it chooses the higest IP address assigned. Not necessarily this loopback has to be a ping-able one. This router ID can also be 'hardcoded' with the OSPF router command "router-id ip address xx.xx.xx.xx". This takes the overrides all the above said theory. |
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