User's Manual

Configuration Guide of XGS-5240-Series
110
Fig 4-2 One End of Each Fiber Not Connected
This kind of problem often appears in the following situations: GBIC (Giga Bitrate Interface
Converter) or interfaces have problems, software problems, hardware becomes unavailable or
operates abnormally. Unidirectional link will cause a series of problems, such as spinning tree
topological loop, broadcast black hole.
ULDP (Unidirectional Link Detection Protocol) can help avoid disasters that could happen
in the situations mentioned above. In a switch connected via fibers or copper Ethernet line (like
ultra five-kind twisted pair), ULDP can monitor the link state of physical links. Whenever a
unidirectional link is discovered, it will send warnings to users and can disable the port
automatically or manually according to users’ configuration.
The ULDP of switches recognizes remote devices and check the correctness of link
connections via interacting ULDP messages. When ULDP is enabled on a port, protocol state
machine will be started, which means different types of messages will be sent at different
states of the state machine to check the connection state of the link by exchanging information
with remote devices. ULDP can dynamically study the interval at which the remote device
sends notification messages and adjust the local TTL (time to live) according to that interval.
Besides, ULDP provides the reset mechanism, when the port is disabled by ULDP, it can
check again through reset mechanism. The time intervals of notification messages and reset in
ULDP can be configured by users, so that ULDP can respond faster to connection errors in
different network environments.
The premise of ULDP working normally is that link works in duplex mode, which means
ULDP is enabled on both ends of the link, using the same method of authentication and
password.
12.2 ULDP Configuration Task Sequence
1. Enable ULDP function globally
g1/0/1
Switch B
Switch C
g1/0/2
g1/0/3
Switch A