Users Guide

Table Of Contents
1240 Layer-2 and Layer-3 Relay Features
What Is the IP Helper Feature?
The IP Helper feature provides the ability for a router to unicast-forward
configured UDP broadcast packets to a particular IP address (including
DHCP packets). This allows applications to reach servers on non-local
subnets. This is possible even when the application is designed to assume a
server is always on a local subnet or when the application uses broadcast
packets to reach the server (with the limited broadcast address
255.255.255.255, or a network directed broadcast address). When configured
for DHCP, the IP Helper performs the function of a DHCP L3 Relay agent.
Relay entries may be configured globally and on routing interfaces. Each relay
entry maps an ingress interface and destination UDP port number to a single
IPv4 address (the helper address). Multiple relay entries may be configured
for the same interface and UDP port, in which case the relay agent relays
matching packets to each server address. Interface configuration takes priority
over global configuration. If the destination UDP port for a packet matches
any entry on the ingress interface, the packet is handled according to the
interface configuration. If the packet does not match any entry on the ingress
interface, the packet is handled according to the global IP helper
configuration.
Discard relay entries may also be configured. Discard entries are used to
discard packets received on a specific interface when those packets would
otherwise be relayed according to a global relay entry. Discard relay entries
may be configured on interfaces, but are not configured globally.
Additionally, the administrator can configure which UDP ports are forwarded.
Certain UDP port numbers can be selected from the web interface or
specified by name in the CLI, but a relay entry can also be configured with
any UDP port number. It is possible to configure relay entries that do not
specify a destination UDP port. The relay agent assumes that these entries
match packets with the UDP destination ports listed in Table 33-1 (the list of
default ports).