Unnumbered interfaces?

Just read something that I've never heard of before... and it doesn't make sense to me. Love it if one of you techies can break it down for me.

The BSCI study guide (CiscoPress) by Teare and Paquet, Chap 4 on OSPF, page190:

"Usually, the IP source address of an OSPF pcket is set to the address of the outgoing interface on the router. It is possible to use IP unnumbered interfaces with OSPF. On unnumbered interfaces, the IP source address is set to the IP address of another interface on the router."

So if I got that right, I could have a point-to-point link between two routers, have the link come up, but not assign IP's to the interfaces.

Router A could use the IP of another interface... say, 10.10.10.1/24 as it's source, while Router B might use the IP of one of its ints... say 192.168.100.1/24. And these two devices can share LSA's / LSU's????? Seriously?

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Comments

cisco_trooper

Yep. I even did a full mesh frame relay topology with 5 devices using all IP un-numbered interfaces because I just couldn't get it through my head. You should try it if you have doubts.

happy420golucky

mikearama wrote:

Just read something that I've never heard of before... and it doesn't make sense to me. Love it if one of you techies can break it down for me.

The BSCI study guide (CiscoPress) by Teare and Paquet, Chap 4 on OSPF, page190:

"Usually, the IP source address of an OSPF pcket is set to the address of the outgoing interface on the router. It is possible to use IP unnumbered interfaces with OSPF. On unnumbered interfaces, the IP source address is set to the IP address of another interface on the router."

So if I got that right, I could have a point-to-point link between two routers, have the link come up, but not assign IP's to the interfaces.

Router A could use the IP of another interface... say, 10.10.10.1/24 as it's source, while Router B might use the IP of one of its ints... say 192.168.100.1/24. And these two devices can share LSA's / LSU's????? Seriously?

Yeah what's even more crazy is that you can configure a point to point tunnel via ospf. Both of those use an IP unnumbered interface and then of all things have the loopback addresses in completely different subnets and still form an adjacency. Pretty crazy, cuz here at my shop I learned about what you are talking about and it had me mind boggled.