Dell PowerConnect 8100 CLI Reference Guide - Page 1039

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 IEN-116 Name Service UDP Port Number 42 IP Helper/DHCP Relay Commands 1039