V-273687

Discussion

In topologies where fiber optic interconnections are used, physical misconnections can occur that allow a link to appear to be up when there is a mismatched set of transmit/receive pairs. When such a physical misconfiguration occurs, protocols such as STP can cause network instability. UDLD is a layer 2 protocol that can detect these physical misconfigurations by verifying that traffic is flowing bidirectionally between neighbors. Ports with UDLD enabled periodically transmit packets to neighbor devices. If the packets are not echoed back within a specific time frame, the link is flagged as unidirectional and the interface is shut down.

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Review configuration for UDLD configuration ("link keep-alive"). 

Router# show link-keepalive
Total link-keepalive enabled ports: 4
Keepalive Retries: 3    Keepalive Interval: 1 Sec.
Port    Physical Link   Logical Link    State           Link-vlan
1/1/1   up              up              FORWARDING      3
1/1/2   up              up              FORWARDING
1/1/3   down            down            DISABLED
1/1/4   up              down            DISABLED

If UDLD is not configured to protect against one-way connections, this is a finding.