Inverse ARP

[jaighobahi]

Folks,

Please help me out here.  Where does the Inverse ARP run - in the frame-relay switch or in the attached router?

Also, when static mapping is configured, does it simply turn off the Inverse ARP or does it just overwrite the dynamic mapping?

Thanks

[John Blakley · Accepted solution]

First, the router sends the inverse arp when it has an address and a dlci to go out on. When there is a static mapping for a certain address/dlci combo, inverse arp is disabled on the dlci. Here's a good explanation of the other ways this can be configured:

http://astorinonetworks.com/2011/06/15/understanding-frame-relay-inverse-arp/

HTH,
John

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[jaighobahi]

Thanks a lot.  The picture is clearer now.

[Peter Paluch]

Hello John,

When there is a static mapping for a certain address/dlci combo, inverse arp is disabled on the dlci

How do you personally understand this information? To my best knowledge, the DTE that has a static mapping on a DLCI does not send InverseARP queries on that DLCI - however, it if receives an InverseARP query itself, it will respond with an InverseARP response. This is the same way point-to-point subinterfaces work, by the way.

Can you please confirm my understanding?

Best regards,

Peter

[John Blakley]

Peter,

I should have been more specific. My apologies. I don't believe you can disable inverse arp replies at all, can you? Under normal circumstances though I believe, unless you can confirm, that once a static mapping is done for a dlci/address combo, there's no need to send a query. Likewise if the other side of that PVC (assuming only 2 in the network) also mapped their dlci to the other side, you would effectively disable arp queries and the need for a reply would be moot if only these two in the network needed to talk to each other.

I'm going to lab this up tonight :)

John

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[Peter Paluch]

Hi John,

No apologies necessary whatsoever.

Yes, you are right - I've made a quick dynamips lab right now and it seems that you cannot prevent InverseARP Reply messages from being sent. You can prevent Requests but not Replies, not even with the no frame-relay inverse-arp command. Thanks for pointing this fact out!

Best regards,

Peter

[John Blakley]

I'm honored to get a rating from you Peter! Thank you :)

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[jaighobahi]

Looks like i have another question on this Inverse ARP discussion.

I configured static mapping on one side of a PVC and left the other side to use Inverse ARP.  The router interface on the static side started going up and down.  Any comments?

[Peter Paluch]

Hello,

This is strange.

In Frame Relay, the line protocol up/down state is determined by the router exchanging LMI messages with the nearest Frame Relay switch. The InverseARP should not affect the operation of LMI whatsoever. How exactly did you configure the static mappings? Did you perhaps modify something that relates to the LMI? Did you modify the keepalive setting?

Best regards,

Peter

[jaighobahi]

I did not configure anything on the LMI.  I just issued the command such as:

RTA(config-if)#frame-relay map ip 192.168.1.2 200 broadcast cisco.

When the interface started going up and down, I simply removed the static mapping and it was alright again. I did enable EIGRP on the interface.  With static mapping, the Hello packet was timing out and it was loosing and re-establishing neighbor relationship.  This did not happen when I configured static mapping on both sides of the PVC.

[Peter Paluch · Accepted solution]

Hello,

I see. I believe that what you experienced was not an interface but rather EIGRP flapping. The most probable cause is that you have actually forgotten the broadcast keyword in the mapping. This would cause a problem where the router with the static mapping was unable to deliver its multicast EIGRP packets to the other router, resulting in EIGRP retransmission timeouts and adjacency drops/rebuilding.

Can yuo try it again, making sure that the broadcast keyword is specified? Also, unless required, you do not need to specify the cisco keyword - the Cisco-style encapsulation is the default.

best regards,

Peter

[jaighobahi]

Thanks. You are right.  I did not include the broadcast keyword.