Brocade Network OS Administrator’s Guide v4.1.1 User Manual
Page 434
FIGURE 50 Four-switch example for UDLD
In the figure above, STP detects that the port on switch D that is connected to switch C should be put
into a blocked state. Therefore, no data traffic gets transmitted or received on this port. Data traffic
remains blocked as long as switch D receives bridge protocol data units (BPDUs) from both
switches C and B.
If the link between switch C and switch D becomes unidirectional (for reasons such as hardware
failure or incorrect cabling) in the direction from D to C, switch D ages out the status that it was
receiving BPDUs from switch C. This eventually causes STP to put the port in a forwarding state, thus
allowing all data traffic. This creates a loop for all BUM traffic that enters the network. BUM traffic can
go from switch B to switch D to switch C to switch A, and then back to switch B.
To prevent this loop from forming, UDLD can be used to detect that the link between switch C and
switch D has become unidirectional.
The UDLD protocol is disabled by default. To use the UDLD protocol, you must first enable the
protocol globally and then enable UDLD on each desired individual physical port. For a configuration
example, refer to
UDLD determines that a link has become unidirectional if the UDLD daemon stops receiving UDLD
PDUs from the other end of the link. The UDLD daemon then blocks the physical link. The physical
link remains up but the line protocol goes down. During this time, the link continues to transmit and
receive UDLD PDUs.
NOTE
In a VCS environment, the UDLD protocol is applicable only to the edge ports in the VCS.
A configuration command to enable the UDLD protocol on a logical port or a non-edge port will be
rejected.
Configuring UDLD
434
Network OS Administrator’s Guide
53-1003225-04