Ip Helper-Address - Enterasys SECURESTACK C3 Configuration Manual

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