To enable forwarding of User Datagram Protocol (UDP) broadcasts, including Bootstrap Protocol (BOOTP), received on an interface,
use the
ip
helper-address command in interface configuration mode. To disable forwarding of broadcast packets to specific addresses, use theno 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]}
Syntax Description
vrf
name
|
(Optional) Enables the VPN routing and forwarding (VRF) instance and the VRF name.
|
global
|
(Optional) Configures a global routing table.
|
address
|
Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address
per interface.
|
redundancy
vrg-name
|
(Optional) Defines the Virtual Router Group (VRG) name.
|
Command Default
UDP broadcasts are not forwarded.
Command Modes
Interface configuration (config-if)
Command History
Release
|
Modification
|
10.0
|
This command was introduced.
|
12.2(4)B
|
This command was modified. The
vrf
name keyword and argument pair and the
global keyword were added.
|
12.2(15)T
|
This command was modified. The
redundancy
vrg-name keyword and argument pair was added.
|
12.2(28)SB
|
This command was integrated into Cisco IOS Release 12.2(28)SB.
|
12.2(33)SRA
|
This command was integrated into Cisco IOS Release 12.2(33)SRA.
|
12.2SX
|
This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends
on your feature set, platform, and platform hardware.
|
Usage Guidelines
The
ip
forward-protocol command along with the
ip
helper-address command allows you to control broadcast packets and protocols that are forwarded.
One common application that requires helper addresses is 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
must specify the address of the BOOTP or DHCP server. If you have multiple servers, configure one helper address for each
server.
The following conditions must be met for a UDP or IP packet to be able to use 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 VPN or global space that is different from the interface VPN, then the
vrf
name or the
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 command 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.
|
The
service
dhcp command must be configured on the router to enable IP helper statements to work with DHCP. If the command is not configured,
the DHCP packets will not be relayed through the IP helper statements. The
service
dhcp command is configured by default.
Examples
The following example shows how to define an address that acts as a helper address:
Router(config)# interface ethernet 1
Router(config-if)# ip helper-address 10.24.43.2
The following example shows how to define an address that acts as a helper address and is associated with a VRF named host1:
Router(config)# interface ethernet 1/0
Router(config-if)# ip helper-address vrf host1 10.25.44.2
The following example shows how to define an address that acts as a helper address and is associated with a VRG named group1:
Router(config)# interface ethernet 1/0
Router(config-if)# ip helper-address 10.25.45.2 redundancy group1