Router DHCP Server Reservations Being Overridden

jrector · ‎12-12-2012

I am running into a DHCP problem on a Cisco 891 running 15.0(1)M2. I am running DHCP on the router and want to reserve an address for a printer. I have done this hundreds of times on hundreds of routers. Per Cisco recommendation, I let the printer lease an address to see whether it uses it's "hardware-address" or "client-id". This printer uses the "hardware-address" format. The printer has leased 10.0.0.102 right off the bat. I want it at 10.0.0.50.

I have a DHCP pool, like this:

ip dhcp pool CLIENT

network 10.0.0.0 255.255.255.0

domain-name chsinc.ds

default-router 10.0.0.1

dns-server 10.0.0.1

lease 2

I create a "reservation" like so:

ip dhcp pool HP_LaserJet_Printer

host 10.0.0.50 255.255.255.0

hardware abcd.ef12.3456

Now I do a "show running-config" to see what I've put in and it looks just like above. Now we reset the network settings or change any network setting on the printer to force it to renew it's lease. What should happen and what usually happens is that the printer should request an IP, be offered 10.0.0.50, accept that IP and come up on that IP address. What is happening with this particular printer is that the printer requests a specific IP (the same IP that it had initially = 10.0.0.102), the router says OK and does not enforce the reservation. Then the router actually deletes the hardware-address config line right out of the running config. A "show running-config" shows that the reservation config now looks like this:

ip dhcp pool HP_LaserJet_Printer

host 10.0.0.50 255.255.255.0

See the logs below. Notice the ones in bold.

000226: Dec 12 17:34:01.382: DHCPD: Seeing if there is an internally specified pool class:

000227: Dec 12 17:34:01.382: DHCPD: htype 1 chaddr 101f.74b0.575c

000228: Dec 12 17:34:01.382: DHCPD: remote id 020a00000ae1e10100000001

000229: Dec 12 17:34:01.382: DHCPD: circuit id 00000000

000230: Dec 12 17:34:05.922: DHCPD: Sending notification of DISCOVER:

000231: Dec 12 17:34:05.922: DHCPD: htype 1 chaddr 101f.74b0.575c

000232: Dec 12 17:34:05.922: DHCPD: remote id 020a00000ae1e10100000001

000233: Dec 12 17:34:05.922: DHCPD: circuit id 00000000

000234: Dec 12 17:34:05.922: DHCPD: Seeing if there is an internally specified pool class:

000235: Dec 12 17:34:05.922: DHCPD: htype 1 chaddr 101f.74b0.575c

000236: Dec 12 17:34:05.922: DHCPD: remote id 020a00000ae1e10100000001

000237: Dec 12 17:34:05.922: DHCPD: circuit id 00000000

000238: Dec 12 17:34:05.922: DHCPD: Found previous server binding

000239: Dec 12 17:34:05.922: DHCPD: client requests 10.0.0.102.

000240: Dec 12 17:34:05.922: DHCPD: Removing previous server binding

000241: Dec 12 17:34:05.922: DHCPD: Sending notification of TERMINATION:

000242: Dec 12 17:34:05.922: DHCPD: address 10.0.0.52 mask 255.255.255.0

000243: Dec 12 17:34:05.922: DHCPD: reason flags: noalloc

000244: Dec 12 17:34:05.922: DHCPD: lease time remaining (secs) = 4294967295

000245: Dec 12 17:34:05.922: DHCPD: Adding binding to radix tree (10.0.0.102)

000246: Dec 12 17:34:05.922: DHCPD: Adding binding to hash tree

000247: Dec 12 17:34:05.922: DHCPD: assigned IP address 10.0.0.102 to client 101f.74b0.575c.

000248: Dec 12 17:34:07.922: DHCPD: Sending notification of DISCOVER:

000249: Dec 12 17:34:07.922: DHCPD: htype 1 chaddr 101f.74b0.575c

000250: Dec 12 17:34:07.922: DHCPD: remote id 020a00000ae1e10100000001

000251: Dec 12 17:34:07.922: DHCPD: circuit id 00000000

000252: Dec 12 17:34:07.922: DHCPD: Seeing if there is an internally specified pool class:

000253: Dec 12 17:34:07.922: DHCPD: htype 1 chaddr 101f.74b0.575c

000254: Dec 12 17:34:07.922: DHCPD: remote id 020a00000ae1e10100000001

000255: Dec 12 17:34:07.922: DHCPD: circuit id 00000000

000256: Dec 12 17:34:07.922: DHCPD: Found previous server binding

000257: Dec 12 17:34:07.922: DHCPD: requested address 10.0.0.102 has already been assigned.

000258: Dec 12 17:34:07.922: DHCPD: Sending notification of ASSIGNMENT:

000259: Dec 12 17:34:07.922: DHCPD: address 10.0.0.102 mask 255.255.255.0

000260: Dec 12 17:34:07.922: DHCPD: htype 1 chaddr 101f.74b0.575c

000261: Dec 12 17:34:07.922: DHCPD: lease time remaining (secs) = 172800

How/Why is this happening? Is there a way to make the router enforce the reservation and not let clients just bring their own IP and actually make changes to the running-config of the router?

John Blakley · ‎12-12-2012

Have you tried clearing the existing binding to see if that would help before releasing/renewing the address?

Sent from Cisco Technical Support iPhone App

HTH, John *** Please rate all useful posts ***

jrector · ‎12-12-2012

You can't make a reservation unless you've already cleared the original binding. The router will throw an error about a binding already existing if you try. You must clear any existing DHCP binding for a hardware-address or client-id before you can make a reservation.

John Blakley · ‎12-13-2012

Okay. The only other thing that I could think of is do you have excluded addresses configured where you could possibly excluding the one that you want to assign? I'd have to lab that up though, but if I remember right I don't believe you can exclude an address and have it assigned as a manual binding.

HTH,
John

*** Please rate all useful posts ***

HTH, John *** Please rate all useful posts ***

cadet alain · ‎12-13-2012

Hi John,

I think the OP answered its problem, he must clear the dynamic binding before configuring the manual binding.

you're right you can't exclude an IP address and at the same time give it out as a manual binding.

Regards.

Alain

Don't forget to rate helpful posts.

jrector · ‎12-13-2012

I think you misunderstood my response. I was saying that in order to make the reservation in the first place I had to clear the existing binding in the router. That is a given since you can't even make the reservation I made above without clearing the binding because it is not allowed. I did that before any of the other configuration above.

jrector · ‎12-13-2012

I know how to force the reservation. All you need to do is temporarily exclude the originally leased IP (in this case 10.0.0.102). Then the printer tries to renew, is denied that IP by the router and then accepts the reserved IP (10.0.0.50).

What I really want to know is why is this happening? This is the first DHCP server implementation I have ever seen where a client is allowed to talk the server out of a reservation, keep its previous IP and actually modify the config (the hardware-address line of the router config actually disappears from the reservation when this happens).