Ip Helper - Dell PowerConnect M6348 Configuration Manual

IP Helper

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 4-1
(the list of default ports).
Table 4-1. Default Ports - UDP Port Numbers Implied By Wildcard
Protocol
IEN-116 Name Service
DNS
NetBIOS Name Server
NetBIOS Datagram Server
TACACS Server
Time Service
DHCP
Trivial File Transfer Protocol
The system limits the number of relay entries to four times the maximum number of routing interfaces
(512 relay entries). 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.
NOTE:
DHCP relay cannot be enabled and disabled globally. IP helper can be enabled or disabled globally.
Enabling IP helper enables DHCP relay.
92
Routing Configuration
UDP Port Number
42
53
137
138
49
37
67
69