Dell POWEREDGE M1000E - Manual (Page 897)

IP Helper/DHCP Relay Commands 897
47
IP Helper/DHCP Relay Commands
The IP Helper feature provides the ability for a router to forward configured
UDP broadcast packets to a particular IP address. 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).
Network administrators can configure relay entries 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.
Network administrators can configure discard relay entries. 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, administrators can configure which UDP ports are forwarded.
Certain UDP port numbers can be specified by name in the UI, but network
administrators can configure a relay entry with any UDP port number.
Administrators may 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 47-1.
Table 47-1. UDP Destination Ports
Protocol UDP Port Number
IEN-116 Name Service 42
2CSPC4.XModular-SWUM200.book Page 897 Thursday, March 10, 2011 11:18 AM