Question about the output of "show ip dhcp binding"

snakayama · ‎08-01-2008

Hi everyone,

I have the question about the output of "show ip dhcp binding".

Cisco 2821 IOS 12.4(19) is running as IOS DHCP Server.

The following is the configuration of DHCP on the C2821 and the output of "show ip dhcp binding" (paste only the pertinent section).

----------

ip dhcp excluded-address 172.16.1.0 172.16.1.50

!

ip dhcp pool POOL1

network 172.16.1.0 255.255.255.0

default-router 172.16.1.1

domain-name test.co.jp

dns-server 172.16.10.10

netbios-name-server 172.16.10.10

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Router#sh ip dhcp binding

Bindings from all pools not associated with VRF:

IP address Client-ID/ Lease expiration Type

Hardware address/

User name

172.16.1.56 0100.1676.d6f3.53 Aug 02 2008 08:52 AM Automatic

172.16.1.57 0100.0fea.ff60.6a Aug 02 2008 09:46 AM Automatic

172.16.1.128 000f.eaff.606a Aug 02 2008 09:44 AM Automatic

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I think DHCP binding information for 172.16.1.56 is normal but for 172.16.1.57 and 172.16.1.128 look strange a little.

"Client-ID/Hardware address/User name" of 172.16.1.57 is 0100.0fea.ff60.6a

"Client-ID/Hardware address/User name" of 172.16.1.128 is 000f.eaff.606a

Because I think 0100.0fea.ff60.6a and 000f.eaff.606a indicate same one DHCP client that has MAC address 000f.eaff.606a not two different DHCP clients, that is

0100.0fea.ff60.6a indicates "Client Identifier" embeded in DHCP Request from 000f.eaff.606a

000f.eaff.606a indicates "Client Hardware Address" embeded in DHCP Request from 000f.eaff.606a

so both "Client Identifier" and "Client Hardware Address" mean one MAC address sent DHCP request.

However IOS DHCP Server creates two entries for one MAC address 000f.eaff.606a, in other words, IOS DHCP Server assigns different IP addresses 172.16.1.57 and 172.16.1.128 to one MAC address 000f.eaff.606a.

Is this symptom might be problem ?

Your information would be appreciated.

Best regards,

Shin

tdrais · ‎08-01-2008

I think your analysis of this is correct. He does appear to have assigned 2 addresses to a single server.

I think most the confusion comes in that cisco has a header that says hardware address when really this is a client id.

I suspect what happened is that the machine issued 2 DHCP requests. 1 with a clientid prefixed by 01 and the second without a client id so the router then used the requesters mac address.

Other than being wasteful this will not hurt since the DHCP table is not related to the ARP entries. Now which address the server decides to use is another story.

If you run debug on DHCP you should see exactly how this is happening. If it does assign 2 addresses on a single DHCP request then there is a bug.

snakayama · ‎08-03-2008

Thank you very much for your reply.

I'm going to get debug info if I can, because this symptom has happened on customer live network, so I need to get permission from customer to execute debug on C2821.

Best regards,