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IP Unnumbered

I have a general question on IP unnumbered. I have a WAN network that has multiple "stub" networks ("stub" in quotes because I haven't selected a dynamic network protocol at this point so please don't think OSPF when you see "stub"). The "stubs" have two interfaces to the outside world - one will be running across and 802.11b wireless connection and connectivity will come and go on this link. The second connection to the outside world will run on a serial link that uses IP unnumbered. Connectivity through this ip unnumbered link will also come and go - that is, the route via this link will be dependent on some external factors beyond my control. Let's say I decide to run RIPv2 as my dynamic routing protocol. I want my "stub" router to advertise routes to his directly connected LAN networks via the serial and the wireless links whenever those links are in the up/up state. My question is this: will the IP unnumbered interface propagate those routes and how does it know exactly who to send those routes to since the interface at both ends has no IP number? It's a serial interface, not something like Ethernet that can broadcast.

Thanks.

3 REPLIES
New Member

Re: IP Unnumbered

Don't your IP unnumbered statements reference some IP on each device? Unnumbered isn't truly unnumbered. It's more like IP borrowed.

http://www.cisco.com/warp/public/701/20.html

New Member

Re: IP Unnumbered

Yes, it's more like IP borrowed, as you point out. I'm going to use loopback interfaces so my "borrowed" addresses can never go down. I haven't done this before and I want to make sure that the route advertisements happen as I expect, that's my biggest issue. I won't have any default routes configured and will have to depend entirely on the dynamic routing protocol and its updates to make this work

New Member

Re: IP Unnumbered

From the link above, this explains how routers and dynamic protocols deal with the issue:

"A router receiving a routing update installs the source address of the update as the next hop in its routing table. Normally, the next hop is a directly connected network node. This is no longer the case if we use IP unnumbered because each serial interface "borrowed" their IP address from a different LAN interface, each in a different subnet and possibly in a different major network. When IP unnumbered is configured, routes learned through the IP unnumbered interface have the interface as the next hop instead of the source address of the routing update. Thus we avoid an invalid next hop address problem due to the source of the routing update coming from a next hop that is not directly connected."

Regards.

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