Command Reference Guide

IP Helper Commands
CLI Command Reference
September 2014 Page 546
HP Moonshot Switch Module CLI Command Reference
IP Helper Commands
This section describes the commands to configure and monitor the IP Helper agent. IP Helper relays DHCP and
other broadcast UDP packets from a local client to one or more servers which are not on the same network at
the client.
The IP Helper feature provides a mechanism that allows a router to forward certain configured UDP broadcast
packets to a particular IP address. This allows various applications to reach servers on non-local subnets, even
if the application was designed to assume a server is always on a local subnet and uses broadcast packets (with
either the limited broadcast address 255.255.255.255, or a network directed broadcast address) to reach the
server.
The network administrator can configure relay entries both 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). The
network administrator may configure multiple relay entries 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. That is, if a packet’s destination UDP port 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.
The network administrator can configure discard relay entries, which direct the system to discard matching
packets. 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.
In addition to configuring the server addresses, the network administrator also configures which UDP ports are
forwarded. Certain UDP port numbers can be specified by name in the UI as a convenience, but the network
administrator can configure a relay entry with any UDP port number. The network administrator may configure
relay entries that do not specify a destination UDP port. The relay agent relays assumes these entries match
packets with the UDP destination ports listed in Table 10. This is the list of default ports.
The system limits the number of relay entries to four times the maximum number of routing interfaces. The
network administrator can allocate the relay entries as he likes. There is no limit to the number of relay entries
on an individual interface, and no limit to the number of servers for a given
{interface, UDP port} pair.
Table 10: Default Ports - UDP Port Numbers Implied by Wildcard
Protocol UDP Port Number
IEN-116 Name Service 42
DNS 53
NetBIOS Name Server 137
NetBIOS Datagram Server 138
TACACS Server 49
Time Service 37
DHCP 67
Trivial File Transfer Protocol (TFTP) 69