CCNP - ip unnumbered

Author

ip unnumbered

SurfinUSA

2002-01-29, 2:28 pm

Guys, dealing with IP UNNUMBERED, how is it that when the serial int borrows an ip address from an ethernet int, it can still "talk" to the next router who is set up the same wah? I mean, if the two serial interfaces are "borrowing" the ip address from their respective E interfaces, then how, if they are not on the same subnet, can they talk? Is something placed in the routing table? And if its in the routing table, how does it KNOW that its the serial interface and not the ethernet interface? Any ideas???

The Reamer

2002-01-29, 5:20 pm

Routes in the routing table can be either subnet or major network entries. I would imagine that the engineers for this network would have worked an addressing scheme to make this possible via the "major" network. I also got the following from a site that may shed some light on the subject for you.

So how is the routing table updated? Normally, a router receiving an update will use the source address of the routing update as the next hop since we are normally guaranteed to be directly connected to networks sending the update (using the 1 interface, 1 network model).

However, since the remote serial line borrowed the address from a remote LAN, we are almost guaranteed that the source address will not be directly connected to the router receiving the update. So, instead of simply entering a next hop address based on the source address of the update, routes learned on the IP unnumbered interface name are entered into the routing table as interface routes, which means that the invalid next hop address is bypassed in favor of the interface name from which we received the update. Because of this, IP unnumbered only makes sense for point-to-point links.

This comes from this site: http://infoblast.comptek.ru:8100/kn...ip/ip_unnum.htm

HTH,

Reamer

SurfinUSA

2002-01-29, 11:35 pm

Thanks Reamer. I actually saw that on the Cisco site, except the wording was the same, but different, if that makes sense. http://www.cisco.com/warp/public/701/20.html
Im still having trouble understanding it, because I cant seem to get it out of my head that "to communicate with another device, you must be on the same subnet". I do believe it, but I just dont understand it. Maybe a couple of more times of reading it and a couple more drinks will help. Let me go see. Thanks again Reamer for making the effort.

Captain_Miller

2002-01-30, 8:12 am

Great link Surfin, it really cleared that up for me. I was having the same problem as you. If you look at the link you provided, there are a couple of parts to really look at that should really help you out with this. Pay very close attention to the "show ip route" entries in the example. These really illustrate what is really happening. Also, I'll point you to a quote from the page; "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."

I hope this helps!

Best regards,
Miller

Yankee

2002-02-02, 7:03 am

The ip unnumbered interface advertises its "assumed address" as a /32 network. I too was confused how the two ends could talk so I asked a TAC tech and he simply said because they are directly connected.

IP unnumbered generally works and I have used it for backup circuits, but if you can avoid it, please do. Look into using the ip negotiated command or physically addressing both ends (same subnet) if possible.

Yankee

haseeb_eng

2002-02-03, 7:50 am

static routes will also work out with this even both of the networks are not on the same subnet

SurfinUSA

2002-02-04, 8:21 am

I guess what I am having a hard time understanding is how the two ends can communicate even though they are directly connected. I mean, I have seen on networks before where there where two different subnets on a network. And those with the 255.255.255.0 scheme could only communicate with those on that scheme, and those on the 255.255.255.192 scheme could only communicate with those on its own scheme. Wether they are directly conncected or not, I thought they still had to have the same subnet. Now, I could be wrong, but it sounds like to me that the TAC support guy just didnt know the answer. Can anyone explain how two devices that are directly conncected can talk to each other (without a static route added to the routing table)?

Yankee

2002-02-05, 4:03 am

I agree the TAC tech didn't know. The answer, it finally dawns on me, is that as I said before the IP is a /32 network and gets added to the routing table that way. Therefore everyone nows how to find it.

Yankee

SurfinUSA

2002-02-05, 8:55 am

Yankee, my friend, I finally understand now. I dont know if I can explain it yet or not, but I believe I do understand. That last post you did was a breakthrough for me on this. Thanks friend.

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