Administrator Guide
Layer 3 Routing Commands 1480
User Guidelines
This command can be invoked multiple times on routing interface, either to
specify multiple server addresses for a given port number or to specify
multiple port numbers handled by a specific server. Broadcast packets other
than DHCP require configuration of a destination UDP port number for IP
helper if not listed in Table 7-1. The switch is able to supply the appropriate
UDP destination port numbers by examining the DHCP message, so
configuration of a UDP destination port is not required for unicast forwarding
of DHCP.
The command no ip helper-address with no arguments clears all helper
addresses on the interface.
Example
To relay DHCP packets received on vlan 5 to two DHCP servers, 192.168.10.1
and 192.168.20.1, use the following commands:
console#config
console(config)#interface vlan 5
console(config-if-vlan5)#ip helper-address 192.168.10.1 dhcp
console(config-if-vlan5)#ip helper-address 192.168.20.1 dhcp
To relay both DHCP and DNS packets to 192.168.30.1, use the following
commands:
console#config
console(config)#interface vlan 5
console(config-if-vlan5)#ip helper-address 192.168.30.1 dhcp
console(config-if-vlan5)#ip helper-address 192.168.30.1 domain
This command takes precedence over an ip helper-address command given in
global configuration mode. With the following configuration, the relay agent
relays DHCP packets received on any interface other than VLAN 5 and VLAN
6 to 192.168.40.1, relays DHCP and DNS packets received on VLAN 5 to
192.168.40.2, relays SNMP traps (port 162) received on interface VLAN 6 to
192.168.23.1, and drops DHCP packets received on VLAN 6:
console#config
console(config)#ip helper-address 192.168.40.1 dhcp
console(config)#interface vlan 5
console(config-if-vlan5)#ip helper-address 192.168.40.2 dhcp
console(config-if-vlan5)#ip helper-address 192.168.40.2 domain
console(config-if-vlan5)#exit
console(config)#interface vlan 6
console(config-if-vlan6)#ip helper-address 192.168.23.1 162
console(config-if-vlan6)#ip helper-address discard dhcp