How can you get an incomplete ARP?

Perhaps someone can put this into perspective for me. This is an age old question that has arisen many times throughout the years and my web searches have failed to answer this. The host is visible in arp and shows incomplete, the host cannot be pinged. Sometimes the prob is a cable, sometimes a host route setting, sometimes an intermediary device. (Let me know if there are other known things that cause this).

ca-santa-barbara-router#sh arp | i 70.169.191 Internet 0 Incomplete ARPA Internet - 0018.731f.407d ARPA Ethernet1

My question is, how is it, that we get the IP address from the directly connected host into the ARP cache, but not the MAC address? We arp out for that IP, and the connected host is smart enough to reply that it has that IP, but it isn't smart enough to send it MAC address?

Thanks, crzzy1

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No, this just means the cisco device has sent an ARP for the IP in the list ( and it has not received a reply.

So the host is down or not connected.

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RPA =A0 Ethernet1

That is not correct. I forgot to mention that this is a /24, and only the host is in fact connected. I can ping the broadcast address, and only this IP comes into the ARP table. In this case there was a problem with the routing on the host. So in other words, my router sees the host, the host in some way replies that it has this IP, but is unable to say what its MAC is. I have seen this numerous times, but never have figured out what really takes place to let my router know that that host is there, but not what is the MAC?


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crzzy1 - the nearest I can get to an identification!

Interesting. You have managed to work out how to have some contact with an interface on a LAN - albeit a not very productive one given that it relies on a response to a LAN broadcast - *without* getting a complete entry into the ARP cache.

I believe when an IP node detects that the destination IP address corresponds to the broadcast address for an address range assigned to an attached LAN (as determined by the subnet mask), it has no need of ARP support since it can use the broadcast MAC address on the LAN. I hope someone who is a practical specialist in these matters rather than only a theoretical amateur like myself can confirm or deny this assertion - and, if deny, perhaps provide an alternative explanation.

I think you need to read and understand the ARP RFC which is quite short, 10 pages, and has a very helpful schematic of the ARP logic.

It may be best to access the RFC, 826, via the Wikipedia article:

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I see that the article references a 3-page PDF of the logic over which you can pour.

ly connected host into the ARP cache, but not the MAC address?

A quick scan of the RFC indicates that it is not required that the node sending an ARP request places the requested IP address in the ARP cache at this time *without*, obviously, the MAC address. However I can see that some implementations might do this. Actually I could be wrong - and, if so, I hope for correction.

ly that it has that IP, but it isn't smart enough to send it MAC address?

According to the RFC, this is an impossibility. How are you certain that the ARP request was sent? If an ARP reply is received from the interface of another node on the LAN - even when the intended recipient is not the receiving node - a *complete* ARP entry should be built for the sending node.

e list ( and it has not

received a reply.

Rob's response indicates that the presumed Cisco implementation logic bothers to create incomplete ARP entries. I guess this may have the benefit - assuming it truly is optional - of revealing possible problems.

It's not clear what you may mean here. If it is a reference to ARP processing, you will see that, in effect, all ARP processing is "/32", that is, it concerns only the IP address of individual interfaces rather than address ranges. As I suggested above, if the destination IP address happens to correspond to the broadcast IP address of the address range assigned to the LAN to which the interface about to send a packet is attached, a broadcast MAC address can be used.

Having been told that the address range is determined by 24 contiguous bits, I can suppose that the broadcast address you used in the successful PING command was

If there is only one interface other than the sending interface, IP address I guess, connected to the LAN, you should receive responses only from that node - but based on having used the broadcast MAC address at the Ethernet level.

I think I see at what you have been getting all this time! Is this one of those famous Cisco extensions to the common interpretation of RFCs? Could it be that the Cisco implementation of ARP involves the ARP logic being informed of the IP addresses of interfaces known to fit the address range assigned to a LAN to which a local interface is attached in case broadcast requests were used to discover them? It would appear that this happens at a level in the logic where the MAC address associated with the IP address has been lost.

I wonder if this mightn't be some way of operating dynamic discovery of nodes within the network as extensively as possible. I guess one can speculate endlessly ...

Chris Mason

=A0ARPA =A0 Ethernet1

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Chris Mason


A host typically creates a temporarey entry in the ARP cache, sends the ARP request (broadcast) and when/if a reply is received, it then updates the record in the arp cache with the received ethernet address.

Generally, this is quick enough that you don't see it happening.

Reply to
JF Mezei

That is what I said, but he does not believe it.

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Here is an idea.

"I can ping the broadcast address, and only this IP comes into the ARP table. "

ca-santa-barbara-router#sh arp | i 70.169.191 Internet 0 Incomplete ARPA Internet - 0018.731f.407d ARPA Ethernet1

The broadcast ping from 218 just goes out with no ARPing. In order to reply (with a unicast) the "target" host must ARP for the ping sender

The router receives the ARP request from 209:- It replies to it and also does gratuitous ARP or ARP snooping processing creating the Incomplete entry. For some reason the entry cannot be completed. Perhaps after the snooping or gratuiting:) the router does a real arp to complete the process and the 209 host does not reply.

I am not sure of the details of gratuitous arp or snooping so I am not sure as to the plausibility of this hypothesis.

Have you checked that you do not have a subnet mask mismatch? Well, looking at the addresses I don't suppose that is possible without a discontiguous mask which is no longer permitted. Worth a check anyway.

Otherwise, maybe the ARP entry is nothing to do with your ping and the 209 host is sending some other traffic. Windows hosts for example are very chatty.

To investigate further you could try:- - Packet capture with some external device say with wireshark - hub or SPAN port needed. - debug arp - If you have very recent IOS, packet capture on router. - Packet capture on target (209). - Check you have no ACLs that could block the ARP.

You might post the router config.

By the way, most people mangle IP addresses in usenet messages so as to preclude identification. e.g change the first octet. Maybe you did that already:?)

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Thanks for the reply. So are you saying that I am getting an arp reply that doesn't have a MAC address in it? I ask this because I am only getting an IP and not an "ethernet address" that you assert that I am getting in your answer. My question is how does it get a reply with only an IP and no MAC? Or if there is no reply, then how does it know that only that single host out of 254 possibility's is out there?

Thanks, CJ

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Thank you for your well worded response. I did mangle my IP like you mentioned,, yes it would be a breach of protocol not to. I think you are correct in your assertion of the gratuitous arp. This issue was fixed by having the customer concentrate on his routing on his host side. (I have no visibility into his side, so I couldn't use wireshark. but I would like to try the packet capture on the router side next time one of these crop up. Again though, a really excellent answer.

Thanks, Crzzy1

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No, I think you are getting no ARP reply at all.

The "incomplete" entry with only IP in it is created when the router wants to send something to that IP. The system does not need to exist for that.

I think it doesn't.

Maybe it has heard traffic from that address.

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Typically the reverse. It means you have sent an Arp request, but not gotten any response.

"will IP address X please stand up and reply to me with their ethernet address ?

BUT, nobody responded.

If this is aon a cisco box, you can:

clear arp

This will remove the incomplete arp entry.

you can show arp to confirm.

You can then try to ping that host and you will ether get an incomplete arp entry or a completed one.

(instead of "ping" you can telnet or any other IP level command that would try to send an IP packet to that host.

If sends an ARP to ask about, then will implicetely know's ethernet address and add that record to its arp table (allowing it to reply to's requests from now on). will then send a response to, and will then get the ethernet address corresponding to and complete the incomplete arp entry.

Note that you could theoretically have a misbehaving machine on your LAN which uses a blank or otherwise invalid ethernet address so that when it responds to arp requests, the responses are considered illegal and not added to the arp tables (leaving those incomplete entries).

Reply to
JF Mezei


If host1 thinks host2 is part of the same subnet, it will send an arp asking for host2's ethernet address.

That broadcast arp will specify host2's ip address.

So at the low level, when host2 gets the ethernet broadcast, it will see "this is something that concerns me" and pass it upwards for processing.

If the ethernet broadcast contained an arp request for an ip address not used by host2, it would not be passed upwards for processing.

So, by the time the "upwards" layer gets the arp request, does it blindly respond to the ethernet address of the sender of the broadcast ?

Or does the arp response behave as an IP packet and gets routed to a router if the requestor is not in the same subnet ?

(at which point the arp request would never get to the requestor, leaving a incomplete arp record at the requestor's arp database)

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