IP Unnumbered Explained

(Rene Molenaar) #1

This topic is to discuss the following lesson:

(system) #2

Hhhm, so the routers don’t care if they are not on the same subnet?

(system) #3

tnx alot man

(Rene Molenaar) #4

Hi Timothy,

With IP unnumbered they don’t, it’s kinda a weird concept but it works.


(system) #5

Thanks Rene,

That was Amazing …


learn something new everday. wow.

(Angel M) #7

Rene, you are the man. Thanks!

(Rene Molenaar) #8

Thanks Angel!

(francesco r) #9

Hello Rene, Is this the only way 2 Router get adjagency ?
I want to mean…till now we learn that 2 router make up adj if they share the same subnet…so if the two router have an ip interface with ip addrress in that subnet…
Now I’m discovering that the ip could be “broken” …in the way that 2 router get adj also if they don’t share a subnet??? I’m fantasizing??? :slight_smile:

(Rene Molenaar) #10

Hi Francesco,

It’s strange but that’s correct. In my example above it works with EIGRP, it also works with OSPF.

You have to use ip unnumbered on both sides though, otherwise you won’t get an adjacency.


(leo d) #11

you don’t need to use IP unnumbered on both ends. The one without IP unnumbered will still check for valid subnet. IP unnumbered just disables it.

(Rupesh D) #12

Hi Rene,
My question is
If R1 Serial Interface is configured for ip unumbered and R2 Serial interface is not configured for ip unumbered but instead it is configured for some different subnet, will that work??

(Rene Molenaar) #13

Hi Rupesh,

This unfortunately won’t work. The side that doesn’t have IP unnumbered will ignore any IP packets that are not on the subnet of the IP address that you configured on the interface. For example, if you try to run EIGRP like I did in the lesson then R1 will accept the EIGRP packets from R2 (since it has IP unnumbered) but R2 will show “not on common subnet” errors.


(William K) #14

How does IP un-numbered work from an L2 perspective? Each interface on the serial will answer ARP?

(Lazaros Agapides) #15

Hello William

First of all, the example used by Rene includes the use of a serial interface, which is a very common use of the IP unnumbered feature. For a serial interface, ARP would not be a problem, because ARP is not used on serial interfaces. If it’s a point to point interface, the encapsulation from Layer 3 (IP) to Layer 2 (HDLC or PPP for example) doesn’t require a Layer 2 destination address. If you’re employing a point to multi-point serial connection such as Frame Relay, then reverse ARP is used to determine the IP address from the DLCI. In this case, the reverse ARP determines the IP address (indeed the borrowed address) from the DLCI.

Because the IP unnumbered feature can also be applied to Ethernet ports, your question is indeed a valid one. In this case, ARP would function as it does when sending an ARP request for any other IP address. The only restriction, according to Cisco, is that you cannot configure static ARP for this particular interface.

I hope this has been helpful!