Enterasys Networks 9034313-07 manual Ip helper-address, Examples

ip helper-address

Examples

The following example globally disables IP forwarding for UDP port 69.

C3(su)->router(Config)#no ip forward-protocol udp 69

The following example disables IP forwarding for UDP port 69 on a specific interface.

C3(su)->router(Config)#interface vlan 10

C3(su)->router(Config-if(Vlan 10))#no ip forward-protocol udp 69

ip helper-address

Use this command to enable the DHCP/BOOTP relay agent on a SecureStack C3 routed interface and/or to forward broadcast traffic identified with the ip forward‐protocol command to a unicast address. Enabling the relay agent allows forwarding of client DHCP/BOOTP requests to a DHCP/ BOOTP server that does not reside on the same broadcast domain as the client. Up to 6 IP helper addresses may be configured per interface.

The no form of this command disables the forwarding of UDP datagrams to the specified address.

Syntax

ip helper-address address

no ip helper-address address

Parameters

address Address of the host where UDP broadcast packets should be forwarded.

Defaults

None.

Mode

Interface configuration: C3(su)‐>Router1(Config‐if(Vlan 1))#

Usage

Typically for DHCP/BootP, when a host requests an IP address, it sends out a DHCP broadcast packet. Normally, the router drops all broadcast packets. However, by executing this command, you enable the routed interface to pass DHCP broadcast frames through, sending them directly to the remote DHCP server’s IP address.

The DHCP/BOOTP relay agent will detect DHCP/BOOTP requests based on UDP source and destination ports. It will then make the necessary changes to the packet and send the packet to the DHCP server. The changes include:

•Replacing the destination IP address with the address of the DHCP server,

•Replacing the source IP address with its own address (that is, the IP address of the local routed interface), and

•Within the BOOTP part of the packet, changing the Relay Agent IP address from 0.0.0.0 to the address of the local routed interface.

The last change to the BootP packet “tells” the DHCP server that it needs to assign an IP address that is in the same subnet as the Relay Agent IP. When the response comes from the server, the DHCP/BOOTP relay agent sends it to the host.

19-18 IP Configuration