DHCP Design Considerations

While conducting a review of considerations when it comes to voice infrastructure, I've compiled the following key points related to DHCP.

1. Any DHCP server may be used to provide address for your IP communications network as long as they are RFC 2131 compliant.
2. Voice devices should be addressed via their own unique scope that does not service normal data clients.  This provides segmentation and security to the design.
3. DHCP is critical to a voice network, suggest deploying at least two DHCP servers capable of servicing the necessary voice infrastructure independently to avoid loss of service in the event of a DHCP server failure.  Instead of duplicating address ranges, instead, configure the scopes to handle half of the assigned IP range.  For example, DHCP-A uses 10.1.1.0/25 and DHCP-B uses 10.1.1.128/25.
4. DHCP Option 150 is used to provide the address of the TFTP server to the phones.  Up to two TFTP servers can be listed.  If the primary TFTP server is available but is not able to grant the requested file to the phone (for example, because the requesting phone is not configured on that cluster), the phone will not attempt to contact the secondary TFTP server.  It is recommended that a direct IP address be used instead of DNS name to avoid reliance on another service whenever possible.
5. If an IP phone cannot reach a DNS server during power-up, it will re-use the previously obtained DHCP information.
6. Adjust the DHCP lease times to match the environment, shorter times for environments with many mobile devices or where users are nomadic (1 day). Use long lease times when the environment is very static and does not change often (1 week).  This will minimize network traffic used by DHCP renewals.  Cisco IP phones will begin trying to renew their lease at half the lease duration configured on the DHCP server.
7. If the design uses a centralized DHCP server that is reached via WAN, consider using a DHCP lease time that is long enough to survive a WAN outage of at least half the lease duration.  For example, if a DHCP lease of 1 day is used, a WAN outage of 12 hours could result in IP phones becoming unregistered due to the lease becoming expired (as it cannot reach the DHCP server to renew).
8. If greater than 1000 devices are registering to the cluster, a dedicated DHCP server is required instead of the co-resident DHCP server on the CUCM server

Cisco IOS DHCP Server Configuration Commands

! Activate DHCP Service on the IOS Device

service dhcp

! Specify any IP Address or IP Address Range to be excluded from the DHCP pool

ip dhcp excluded-address |
! Specify the name of this specific DHCP pool, the subnet and mask for this
! pool, the default gateway and up to four TFTP
ip dhcp pool
network
default-router
option 150 ip ...
 
Note: IP phones use only the first two addresses supplied in the option 150 field even if more than two are configured.