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声明：转自 Cisco.com

ip helper-address

To enable the forwarding of User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address command in interface configuration mode. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.

ip helper-address [vrf name | global] address [redundancy vrg-name]

no ip helper-address [vrf name | global] address [redundancy vrg-name]

Usage Guidelines

Combined with the ip forward-protocol command, the ip helper-address command allows you to control which broadcast packets and which protocols are forwarded.

One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP), which is defined in RFC 1531. To enable BOOTP or DHCP broadcast forwarding for a set of clients, configure a helper address on the router interface connected to the client. The helper address should specify the address of the BOOTP or DHCP server. If you have multiple servers, you can configure one helper address for each server.

All of the following conditions must be met in order for a UDP or IP packet to be helpered by the ip helper-address command:

•The MAC address of the received frame must be all-ones broadcast address (ffff.ffff.ffff).

•The IP destination address must be one of the following: all-ones broadcast (255.255.255.255), subnet broadcast for the receiving interface, or major-net broadcast for the receiving interface if the no ip classless command is also configured.

•The IP time-to-live (TTL) value must be at least 2.

•The IP protocol must be UDP (17).

•The UDP destination port must be for TFTP, Domain Name System (DNS), Time, NetBIOS, ND, BOOTP or DHCP packet, or a UDP port specified by the ip forward-protocol udp command in global configuration mode.

If the DHCP server resides in a Virtual Private Network (VPN) or global space that is different from the interface VPN, then the vrf name or global option allows you to specify the name of the VRF or global space in which the DHCP server resides.

The ip helper-address vrf name address option uses the address associated with the VRF name regardless of the VRF of the incoming interface. If the ip helper-address vrf name address command is configured and later the vrf is deleted from the configuration, then all IP helper addresses associated with that VRF name will be removed from the interface configuration.

If the ip helper-address address command is already configured on an interface with no VRF name configured, and later the interface is configured with the ip helper-address vrf name address command, then the previously configured ip helper-address address is considered to be global.

Note The ip helper-address command does not work on an X.25 interface on a destination router because the router cannot determine if the packet was intended as a physical broadcast.

Examples

The following example defines an address that acts as a helper address:

interface ethernet 1 

 ip helper-address 10.24.43.2 

The following example defines an address that acts as a helper address and is associated with the VRF named host1:

interface ethernet 1/0 

 ip helper-address vrf host1 10.25.44.2 

The following example defines an address that acts as a helper address and is associated with the VRG named group1:

interface ethernet 1/0 

 ip helper-address 10.25.45.2 redundancy group1 

Related Commands