Loopback, unnumbered and two ifs.

We're currently running a configuration that uses an IP address on the FastEthernet port, ip unnumbered on the Serial pointing to FE0/0, and OSPF. It works well but I am aware of the problems involved if we lose the FE connection so I am planning to move to a loopback interface.

Which should be simple enough, but I'm not positive of what I can get away with. The obvious setup I came up with was: R1: FE0/0: 10.1.1.254/24 Loopback0: 10.10.1.1/30 Serial: ip unnumbered Loopback0 R2: FE0/0: 10.1.2.254/24 Loopback0: 10.10.1.5/30 Serial: ip unnumbered Loopback0

But the question that came up was whether, instead, I could use

10.10.1.1/30 and 10.10.1.2/30 on Loopback0, or to put it another way, can the two be on the same subnet?

Then I wondered, could I use a single address for the whole router, in other words: R1: Loopback0: 10.1.1.254/24 Serial: ip unnumbered Loopback0 FE0/0: ip unnumbered Loopback0 R2: Loopback0: 10.1.2.254/24 Serial: ip unnumbered Loopback0 FE0/0: ip unnumbered Loopback0

Or is that too simple?

Reply to
spam.sc
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No, because then it won't send traffic to the other machine's loopback IP over the network.

You should usually use /32 for loopback IPs, so you don't have to waste any addresses in that block.

You can't use the loopback IP for Ethernet, because ARP needs a specific IP to respond to. Also, all the devices on an Ethernet need to be in the same subnet, and the mask has to be part of the Ethernet configuration so they know what subnet it is.

Reply to
Barry Margolin

There is as far as I know no reason to use ip unnumbered other than to conserve IP addresses. Have you considered dispensing with them entirely?

Reply to
bod43

Not totally sure I follow you here.

After all, if I used two subnets on the two ends of the serial link, as I initially wrote up there, you're going to need a route on R1 to get to 10.1.2.0/24. So you'd have a route (dynamic or static) on R1 like

10.1.2.0/24 via 10.10.1.5, Serial0/0/0.

But the problem there is that it's not on the same subnet with your own serial port and therefore by IP rules you'd have to have a gateway. What I'd expect to work would be a route like

10.1.2.0/24 via 10.10.1.2, Serial0/0/0 10.10.1.2/32 is directly connected, Serial0/0/0

(With 10.10.1.2 being the loopback/serial address on R2)

Am I just completely confused? I think I'm overthinking.

Yeah, that was kind of dopey of me.

Reply to
S. Checker

That's why you should configure them with /32 masks, not /30. If you configure them as /30, your routing protocol will not create that second route on R1. It will receive the /30 route from R2, but not put it into the routing table because its directly connected route on Loopback0 takes precedence.

Reply to
Barry Margolin

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