Frame-relay map question - how to get the IP addr at the other end?

Unanswered Question
Mar 11th, 2008


I have a frame-relay interface which has this config:

interface Serial0/1/0:0.500 point-to-point

ip address

ip access-group 100 in

frame-relay interface-dlci 500

crypto map CRX0

do you know what command will give you the other end's IP addr? (ie, how do you see the results of inverse arp?)

show frame-relay map doesn't show it

Serial0/1/0:0.500 (up): point-to-point dlci, dlci 500(0x1F4,0x7C40), broadcast

status defined, active

Thx LisaG

I have this problem too.
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mheusing Tue, 03/11/2008 - 07:42

Hi LisaG,

On a point-to-point link with a /30 the IP address of the other end is "known". Inverse ARP is only needed on multipoint interfaces. As such the "show frame-relay map" command will not reveal the IP of the other end.

The easiest way to get the other ends IP address in case of two Cisco devices is to turn on CDP.

To clarify, what the output looks like I configured a p2p and a multipoint PVC:

interface Serial1/0.100 point-to-point

ip address

frame-relay interface-dlci 100


interface Serial1/0.500 multipoint

ip address

frame-relay interface-dlci 500

R1#show frame-relay map

Serial1/0.100 (up): point-to-point dlci, dlci 100(0x64,0x1840), broadcast

Serial1/0.500 (up): ip dlci 500(0x1F4,0x7C40), dynamic,


As you can see, there is no Inverse ARP on P2P.

Hope this helps! Please use the rating system.

Regards, Martin

vaisharm Tue, 03/11/2008 - 07:51

Also, you wont be able to retrieve the IP from the other end via CDP in case of frame-relay as there would be a frame-relay switch in between.



Richard Burts Tue, 03/11/2008 - 08:53


What does the frame relay switch have to do with CDP. CDP is the payload in a frame/packet that is transported over the Frame Relay network. The switch does not know nor care that the payload is CDP.

Perhaps you are thinking of the situation where you have routers connected by an Ethernet switch. If the Ethernet switch is a Cisco switch then you would be correct that the routers would see the switch as a neighbor but not the other router.

But that is not the case with a Frame Relay switch. I have lots of Frame Relay point to point connections and CDP works very well over the Frame Relay point to point and I can see the IP address of the neighbor via CDP over Frame Relay.



Kevin Dorrell Tue, 03/11/2008 - 09:01

Just to add to what Rick says here ... I should point out that on a FR multipoint or physical interface, CDP is disabled by default. But you can easily re-enable it with cdp enable. In that case, the CDP gets passed to each and every one of the frame map entries. The switch doesn't enter into it at all.

By that is beside the point. In response to the original posting, the p2p interface does not care about the IP address at the other end of the link. Any packet with a destination in the subnet will be sent down the link.

So if a p2p link were configured with a /28 instead of a /30 ... e.g. ... then any address like,,, etc would be sent down the link on that DLCI. The fact that the actual address at the other end of the link is, say, does not matter to it. (Except obviously you would not get a response to a ping.)

Kevin Dorrell


Hi There

Also as was pointed out in the post above, as this is a point to point link and is using a /30 mask, you can work out the far end IP address just by working out the subnet information.

Your serial 0 interface =


Subnet Number is

Local interface IP:

Subnet Broadcast IP:

Which leaves



Best Regards,


vaisharm Tue, 03/11/2008 - 11:06

Rick, Kevin,

Thanks for correcting me. I had tried this earlier but it didn't work for me then. Probably as Kevin pointed CDP was disabled on the FR interface. Just tried it again, enabled CDP and I see the neighbor.

Thanks again!


Richard Burts Tue, 03/11/2008 - 11:36


no problem. It is one of the really good things about the forum that we have so many good people reading the questions and responses and willing to share their experience and insight. The collective wisdom helps keep us all straight.



lgontarsk Tue, 03/11/2008 - 11:41

thank you all! i have been really amazed at the fantastic responses & quality here....

thanks again!



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