Operation Modes - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 33 Configuring UDLD

Operation Modes

UDLD and Fast UDLD support the following operation modes:
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Note
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Note
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:
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Default UDLD Configuration
Table 33-1
Table 33-1 UDLD Default Configuration
Feature
UDLD global enable state
UDLD per-interface enable state for fiber-optic media
UDLD per-interface enable state for twisted-pair (copper) media Disabled on all Ethernet 10/100 and 1000BASE-TX
Fast UDLD per-interface enable state.
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.
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.
Both unidirectional and bidirectional link failures can be detected in aggressive mode.
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.
shows the UDLD default configuration.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Default Status
Globally disabled.
Enabled on all Ethernet fiber-optic interfaces.
interfaces.
Disabled on all interfaces.
Default UDLD Configuration
33-3