Cisco Systems 4500 Operation Modes, Default States for UDLD

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 28 Configuring UDLD About UDLD
Figure 28-2 Fast UDLD Topology
Note For Fast UDLD, Catalyst 4900M, Catalyst 4948E, Catalyst 4948E-F, Supervisor Engine 6-E, and
Supervisor 6L-E support up to 32 ports. For all other supervisor engines, the limit is 16 ports.
Operation Modes
UDLD and Fast UDLD support the following operation modes:
• Normal—A UDLD-capable port (A) periodically sends a UDLD probe to a second port (B). If B is
not UDLD capable, no unidirectional link detection occurs. If both devices are UDLD capable and
bidirectional connectivity exists, probe messages travel in both directions at the rate of the
configured message time interval. When the UDLD protocol receives the probe, it attempts to
synchronize the devices by sending echo messages to the peer port and waiting for an answer during
the detection window. If unidirectional traffic is detected when the port link is still up (B longer
sends traffic to A), B enters errdisable mode, and A is marked undetermined but does not enter
errdisable mode. It continues to operate under its current STP status because this mode is
informational only; it is potentially less disruptive although it does not prevent STP loops.
Note Bidirectional link failures cannot be detected using normal mode.
• Aggressive—If a port (A) loses its neighbor connectivity, it actively attempts to reestablish the
relationship by sending a probe to a second port (B). If port B does not respond, the link is
considered unidirectional and port A enters an errdisable state to avoid silent drop traffic.
Note Both unidirectional and bidirectional link failures can be detected in aggressive mode.
UDLD aggressive mode can interoperate with UDLD normal mode. When a unidirectional condition is
detected, only the aggressive mode link shuts down.
Default States for UDLD
The following are the defaults for UDLD:
• UDLD is locally disabled on copper LAN ports to avoid sending unnecessary control traffic (BPDU
control packets). This protocol is commonly used for access ports.
• UDLD is enabled on a fiber port if global UDLD is activated.
• Fast UDLD is disabled on all ports.
Switch A Switch B
EtherChannel consisting
of two interfaces
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