Core switch DHCP setup

Cisco IOS routers and layer 3 switches (eg, 3650) can be configured as a DHCP server. This is useful when deploying IP phones!

To establish if your core switch is providing DHCP, login to it and enter: sh run | s dhcp

Example with two pools for two TR's. Gateway (default-router) and CUCM (option 150) addresses are sent to clients:

rs1#sh run | s dhcp

ip dhcp excluded-address 10.1.0.124 10.1.0.126          .126 is TR1 gateway (from/to range)
ip dhcp excluded-address 10.1.0.188 10.1.0.190          .190 is TR2 gateway (from/to range)

ip dhcp pool TR1-VOIP
 network 10.1.0.0 255.255.255.128                       /25
 default-router 10.1.0.126                              gateway, VLAN IP on core switch
 option 150 ip 10.1.1.1                                 option 150 is the call manager

ip dhcp pool TR2-VOIP
 network 10.1.0.128 255.255.255.192                     /26
 default-router 10.1.0.190                              gateway, VLAN IP on core switch
 option 66 ascii tftp:// 10.1.1.2                       options 66, 120, 42 for phones
 option 120 ascii 10.1.1.2
 option 42 ip 10.1.0.190                                NTP server address
 option 150 ip 10.1.1.18 10.1.1.1                       CM secondary / publisher

 domain-name cucm.pbxbook.com
 ip dhcp snooping trust
rs1#

With the core switch providing DHCP, if a closet TR switch reboots for some reason, all connected phones are going to reboot and request a DHCP address at pretty much the same time. This might not go well, especially as the previous leases won't have been released. If you suspect this scenario, check and clear it with:

• show ip dhcp conflict
     - if you see conflicts, clear them with:

• clear ip dhcp conflict *
     - clears all conflicts, note the * (wildcard)

• show ip dhcp binding
     - lists all addresses assigned by the routers DHCP service

• clear ip dhcp binding *
     - clears all automatic DHCP bindings, note the * (wildcard)

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