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New Member

Central DHCP Server for IP phones?

We have many remote sies in our IPT system with central callmanager. I want to design a centralized DHCP server for all IP phones from different voice VLAN from diferent sites for easy management. What I am worried about is that during SRST, the remote site IP phones will also lose connection with central DHCP server. If there is a reboot or lease expiration, the phone will be dead. I am wondering if any new model IP phone can remember assigned IP address after reboot even it cannot reach DHCP server? Or if there is better solution for DHCP backup?

6 REPLIES
Blue

Re: Central DHCP Server for IP phones?

what you will do is create a DHCP server on the SRST router that can be used during SRST operation.

please see the following link for IOS dhcp configuration:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800ca6b5.html

New Member

Re: Central DHCP Server for IP phones?

But we will have about 100 remote sites with IP phones subnets. They are joining internal EIGRP. It will be a management nightmare if we are using distributed DHCP control that way. I am looking for a specific feature sounds like DHCP address sticky, so when the ip phone cannot reach DHCP server, it will keep using the previous assigned address.

Blue

Re: Central DHCP Server for IP phones?

the cisco recommended solution for dhcp when in SRST mode is IOS dhcp configured on the SRST router.

please see the following link for more SRST dhcp info:

http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_configuration_guide_chapter09186a0080557e3a.html#wp1331670

the administration should not be that much of an issue, as you only have to configure the dhcp on the SRST router once.

you should never need to touch those dhcp configs again unless you need to make changes.

from above document:

"rebooting a cisco ip phone when there is no dhcp server available after the lease has expired will not reactivate the phones."

you may be able to use dhcp relay, where you can relay the dhcp requests to another device that hands out dhcp.

(of course, if your WAN is out, and the other dhcp server is on the other side, you still have the same problem; at this point, i again recommend you use ciscos recommendation of providing SRST dhcp on the router)

Cisco Employee

Re: Central DHCP Server for IP phones?

It's a standards-based DHCP client on the phones so they will attempt to renew their address 1/2 way through the lease and if they cannot renew by the end will go into reboot until they can pull an address. That's why the recommendation is to have local DHCP servers. If you don't want to do this you can set long lease times for the voice IP addresses and expect (hope?) that the WAN doesn't stay down longer than 1/2 the lease time. Of course you run into the situation where previously bound but now unused addresses take a long time to age out.

Don

Super Bronze

Re: Central DHCP Server for IP phones?

Also if you have any form of backup (even just BRI or a VPN) you can block voice control traffic over that so that phones go into SRST, but allow DHCP to maintain that service.

Regards

Aaron

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Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
New Member

Re: Central DHCP Server for IP phones?

You could use the SRST support under CME. http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_configuration_guide_chapter09186a00806a7fe8.html

Using the local DHCP, you would need to have 2 option 150s - one for the CCM, then one for the local CCME. If it was unable to boot the image, then it will boot locally into CCME. This would at least give connectivity if the phone rebooted, although it would not necessarily give you the right extension #.

If you wanted to be really smart, you could do an automated SQL extract from the CCM database to load the mac addresses onto the CCME router, so that the phone would always boot onto the right extension.

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