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New Member

DHCP and Vlan 1

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?

6 REPLIES
New Member

Re: DHCP and Vlan 1

Does the mac address match between what is reserved and what has been issued?

New Member

Re: DHCP and Vlan 1

Yes. I took the mac from "sh int vlan1" and posted it in the reservation.

Hall of Fame Super Blue

Re: DHCP and Vlan 1

James

If it has handed out a different address what mac-address is associated with that IP address - does the DHCP server record this information ?

Jon

Re: DHCP and Vlan 1

deleted

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

Re: DHCP and Vlan 1

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.

New Member

Re: DHCP and Vlan 1

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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