FUJITSU PSWITCH User’s Guide
140 December/2018
3.1.9.19. UDP Relay/IP Helper
By using the IP helper function, the router can forward a specific IP broadcast
packet to a specific IP address. This ensures applications can reach servers on
non-local subnets even if the application is designed to assume that the server is
always on a local subnet and uses broadcast packets (limited broadcast address
255.255.255.255, or a directed broadcast address for a specific network). IP helper
provides DHCP relay function.
Network administrators can configure relay entries both globally and on routing
interfaces. Each relay entry maps the input interface and the destination UDP port
number to a single IPv4 address (helper address). Network administrators can
configure multiple relay entries for the same interface and UDP port. In this case,
the relay agent relays the matching packet to each server address. Interface
configurations take precedence over global configuration. That is, if the destination
UDP port of the packet matches the entry on the ingress interface, that packet is
processed according to the interface configuration. If the packet does not match
the entry on the ingress interface, the packet is processed according to the global
IP helper configuration.
The network administrator can set a discard relay entry that instructs to discard the
packet that matched the condition. A discard entry is used to discard a packet
received on a particular interface when relaying it according to the global relay
entry. Discard relay entries are configured on interfaces and can not be configured
globally.
In addition to setting the server address, the network administrator configures UDP
ports to be transferred. A specific UDP port number can be specified by name for
convenience, but the network administrator can configure a relay entry with any
UDP port number. In addition, the network administrator can configure relay
entries that don‘t specify the destination UDP port. The relay agent relay assumes
that these entries match packets with the UDP destination port listed in the table
below. This is the default port list.
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