DELL 8132 ユーザーズマニュアル

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 

. 

IEN-116 Name Service

42

2CSPC4.X8100-SWUM102.book  Page 1039  Friday, March 15, 2013  8:56 AM