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26 UDP Forwarding

UDP is a connectionless transport protocol that is used for applications that do not require the
establishment of a session and end-to-end error checking, such as email and file transfer. This
chapter describes the UDP relay function in the switch, which allows UDP broadcast packets
to be forwarded across groups and VLANs that have IP routing enabled. The UDP relay allows
you to use nonroutable protocols in a routing environment. (For information about IP rout-
ing, see Chapter 25, “IP Routing.”)
♦ Note ♦
BOOTP/DHCP relay has previously been available on
the switch. It is now part of an expanded feature that
includes relays for NetBIOS and generic services.
The relay may be configured for the following services:
• Bootstrap Protocol (BOOTP)/Dynamic Host Configuration Protocol (DHCP)
•NetBIOS Name Server (NBNS)
•NetBIOS Datagram Distribution Server (NBDD)
• Generic applications, such as Trivial File Transfer Protocol (TFTP)
The UDP services, their corresponding well-known port numbers, and configurable options
on the switch are listed here.

UDP Relay and RIF Stripping

Routing Information Field (RIF) stripping is required for transparent bridge ports in source
route environments and may also be useful in non-source route environments.
In a source route environment, where RIF stripping is enabled for transparent bridging to
Ethernet, UDP relay clients should not be more than one switch away from the DHCP server.
(In RIF stripping, 2 bytes are stripped from the RIF and each bridge adds 2 bytes to the RIF.
Packets with a RIF greater than 2 bytes are discarded.)
Service UDP Port No. Configurable Options
BOOTP/DHCP 67/68 Next-hop address (up to 8)
Forward delay
Maximum hops
NBNS 137 Next-hop address (up to 8)
Forwarding VLANs (up to 32)
NBDD 138 Next-hop address (up to 8)
Forwarding VLANs (up to 32)
Generic user-configured Next-hop address (up to 8)
Forwarding VLANs (up to 32)