33-2
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 33 Configuring UDLD
About UDLD
This section includes these topics:
• UDLD Topology, page 33-2
• Fast UDLD Topology, page 33-2
• Operation Modes, page 33-3
• Default States for UDLD, page 33-3
UDLD Topology
Each switch can send packets to a neighbor switch but cannot receive packets from the switch it is
sending packets to. UDLD detects and disables these one-way connections. Figure 33-1 illustrates a
unidirectional link condition.
Figure 33-1 Unidirectional Link Topology
Fast UDLD Topology
Figure 33-2 illustrates a typical Fast UDLD topology. Switch A and B are connected through a 2-port
EtherChannel, and Fast UDLD is enabled on the individual ports. If one of the links becomes
unidirectional, Fast UDLD detects this situation faster than regular UDLD and errdisables the link.
Traffic is switched over to the second link by EtherChannel. Because this occurs very quickly, traffic loss
is minimized.
Figure 33-2 Fast UDLD Topology
Note For Fast UDLD, Catalyst 4900M, Catalyst 4948E, Supervisor Engine 6-E, Supervisor 6L-E, Supervisor
7-E, and Supervisor Engine 7L-E support up to 32 ports.
Switch X
Switch Y
Switch Z
Tx
Rx
Tx
Rx
Rx
Tx
144608
Switch A Switch B
EtherChannel consisting
of two interfaces
277892
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