ip helper-address (Interface Config)

Use this command to configure the relay of certain UDP broadcast packets received on a specific interface or range of interfaces. This command can be invoked multiple times on a routing interface, either to specify multiple server addresses for a given port number or to specify multiple port numbers handled by a specific server.

Default No helper addresses are configured.
Format ip helper-address {server-address | discard} [dest-udp-port | dhcp | domain | isakmp | mobile ip | nameserver | netbios-dgm | netbios-ns | ntp | pim-auto-rp | rip | tacacs | tftp | time]
Mode Interface Config
Parameter Description
server-address The IPv4 unicast or directed broadcast address to which relayed UDP broadcast packets are sent. The server address cannot be in a subnet on the interface where the relay entry is configured, and cannot be an IP address configured on any interface of the local router.
discard Matching packets should be discarded rather than relayed, even if a global ip helper-address configuration matches the packet.
dest-udp-port A destination UDP port number from 0 to 65535.
port-name The destination UDP port may be optionally specified by its name. Whether a port is specified by its number or its name has no effect on behavior. The names recognized are as follows:
  • dhcp (port 67)
  • domain (port 53)
  • isakmp (port 500)
  • mobile-ip (port 434)
  • nameserver (port 42)
  • netbios-dgm (port 138)
  • netbios-ns (port 137)
  • ntp (port 123)
  • pim-auto-rp (port 496)
  • rip (port 520)
  • tacacs (port 49)
  • tftp (port 69)
  • time (port 37)

Other ports must be specified by number.

To relay DHCP (Dynamic Host Configuration Protocol) packets received on interface 1/0/2 to two DHCP servers, 192.168.10.1 and 192.168.20.1, use the following commands:

(Extreme 220) #config
(Extreme 220) (Config) #interface 1/0/2
(Extreme 220)(interface 1/0/2)#ip helper-address 192.168.10.1 dhcp
(Extreme 220)(interface 1/0/2)#ip helper-address 192.168.20.1 dhcp

To relay both DHCP and DNS packets to 192.168.30.1, use the following commands:

(Extreme 220) #config
(Extreme 220) (Config) #interface 1/0/2
(Extreme 220)(interface 1/0/2)#ip helper-address 192.168.30.1 dhcp
(Extreme 220)(interface 1/0/2)#ip helper-address 192.168.30.1 dns

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 1/0/2 and 1/0/17 to 192.168.40.1, relays DHCP and DNS packets received on 1/0/2 to 192.168.40.2, relays SNMP (Simple Network Management Protocol) traps (port 162) received on interface 1/0/17 to 192.168.23.1, and drops DHCP packets received on 1/0/17:

(Extreme 220) #config
(Extreme 220) (Config) #ip helper-address 192.168.40.1 dhcp
(Extreme 220) (Config) #interface 1/0/2
(Extreme 220)(interface 1/0/2)#ip helper-address 192.168.40.2 dhcp
(Extreme 220)(interface 1/0/2)#ip helper-address 192.168.40.2 domain
(Extreme 220)(interface 1/0/2)#exit
(Extreme 220) (Config) #interface 1/0/17
(Extreme 220)(interface 1/0/17)#ip helper-address 192.168.23.1 162
(Extreme 220)(interface 1/0/17)#ip helper-address discard dhcp