DHCP and Vlan 1

Unanswered Question
Dec 2nd, 2008
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I have a server acting as a dhcp server connected to a 4506 core switch. I have closet switches (3750s) connected to it via fiber (trunked). I would like to give those switches ip addresses via dhcp. So I setup a dhcp reservation with the MAC address of what I think Vlan 1 is on the closet switch. I get an IP address but not the reserved one. Is there something special I need to do to get that switch to get the reserved IP address?

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Overall Rating: 4 (1 ratings)
don-sullivan Tue, 12/02/2008 - 08:42
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Does the mac address match between what is reserved and what has been issued?

jgadbois Tue, 12/02/2008 - 10:19
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Yes. I took the mac from "sh int vlan1" and posted it in the reservation.

Jon Marshall Tue, 12/02/2008 - 10:27
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If it has handed out a different address what mac-address is associated with that IP address - does the DHCP server record this information ?


jgadbois Tue, 12/02/2008 - 12:36
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Jon, thanks for your reply. All of the other dhcp entries show a 12 digit number which looks like mac addresses. But my switch shows a 48 digit number. At first, I thought it might be an IPV6 address but those are 128 bit addresses. If I arp the IP address that is given out, it shows the vlan1 mac that I put in the reservation.

jgadbois Tue, 12/02/2008 - 14:49
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Debug to the rescue. After Jon's note (thanks Jon) I deceided to use debug to see what was going on. It turns out that that 48 digit number is actually the client-id in hexidecimal format. The switch makes up the client-id by using the word cisco followed by the mac address and then the abbreviated interface name like VL1 or such.

And of course you can set the client-id to anything you want (almost) by using the "ip dhcp client client-id" command. When i used this with the mac of the interface everything worked although I did have to drop the leading zeros in the reservation on the server. I guess the switch really wasn't sending them. Anyway, thanks for everyones help.


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