Cisco Catalyst 2960 Series, Configuration Manual

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Aggressive Mode
In aggressive mode, UDLD detects a unidirectional link by using the previous detection methods. UDLD in
aggressive mode can also detect a unidirectional link on a point-to-point link on which no failure between the
two devices is allowed. It can also detect a unidirectional link when one of these problems exists:
• On fiber-optic or twisted-pair links, one of the ports cannot send or receive traffic.
• On fiber-optic or twisted-pair links, one of the ports is down while the other is up.
• One of the fiber strands in the cable is disconnected.
In these cases, UDLD disables the affected port.
In a point-to-point link, UDLD hello packets can be considered as a heart beat whose presence guarantees the
health of the link. Conversely, the loss of the heart beat means that the link must be shut down if it is not
possible to reestablish a bidirectional link.
If both fiber strands in a cable are working normally from a Layer 1 perspective, UDLD in aggressive mode
detects whether those fiber strands are connected correctly and whether traffic is flowing bidirectionally
between the correct neighbors. This check cannot be performed by autonegotiation because autonegotiation
operates at Layer 1.
Related Topics
Enabling UDLD Globally , on page 399
Enabling UDLD on an Interface , on page 400
Methods to Detect Unidirectional Links
UDLD operates by using two methods:
• Neighbor database maintenance
• Event-driven detection and echoing
Related Topics
Enabling UDLD Globally , on page 399
Enabling UDLD on an Interface , on page 400
Neighbor Database Maintenance
UDLD learns about other UDLD-capable neighbors by periodically sending a hello packet (also called an
advertisement or probe) on every active port to keep each device informed about its neighbors.
When the switch receives a hello message, it caches the information until the age time (hold time or time-to-live)
expires. If the switch receives a new hello message before an older cache entry ages, the switch replaces the
older entry with the new one.
Whenever a port is disabled and UDLD is running, whenever UDLD is disabled on a port, or whenever the
switch is reset, UDLD clears all existing cache entries for the ports affected by the configuration change.
UDLD sends at least one message to inform the neighbors to flush the part of their caches affected by the
status change. The message is intended to keep the caches synchronized.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About UDLD