Chapter 31 Configuring Udld; Udld Aggressive Mode - Cisco 7609 Configuration Manual

Understanding How UDLD Works
A unidirectional link occurs whenever traffic transmitted by the local device over a link is received by
the neighbor but traffic transmitted from the neighbor is not received by the local device. If one of the
fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up. In
this case, the logical link is undetermined, and UDLD does not take any action. If both fibers are working
normally at Layer 1, then UDLD at Layer 2 determines whether those fibers 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.
The Cisco 7600 series router periodically transmits UDLD packets to neighbor devices on LAN ports
with UDLD enabled. If the packets are echoed back within a specific time frame and they are lacking a
specific acknowledgment (echo), the link is flagged as unidirectional and the LAN port is shut down.
Devices on both ends of the link must support UDLD in order for the protocol to successfully identify
and disable unidirectional links.
Note By default, UDLD is locally disabled on copper LAN ports to avoid sending unnecessary control traffic
on this type of media since it is often used for access ports.
Figure 31-1
from Switch A on the port. However, Switch A does not receive traffic from Switch B on the same port.
UDLD detects the problem and disables the port.
Figure 31-1 Unidirectional Link
TX
TX

UDLD Aggressive Mode

Release 12.1(3a)E and later releases support UDLD aggressive mode. UDLD aggressive mode is
disabled by default. Configure UDLD aggressive mode only on point-to-point links between network
devices that support UDLD aggressive mode. With UDLD aggressive mode enabled, when a port on a
bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD packets,
UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.
To prevent spanning tree loops, nonaggressive UDLD with the default interval of 15 seconds is fast
enough to shut down a unidirectional link before a blocking port transitions to the forwarding state (with
default spanning tree parameters).
When you enable UDLD aggressive mode, you receive additional benefits in the following situations:
•
•
In these cases, UDLD aggressive mode disables one of the ports on the link, which prevents traffic from
being discarding.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
31-2
shows an example of a unidirectional link condition. Switch B successfully receives traffic
Switch A
RX
RX
Switch B
One side of a link has a port stuck (both Tx and Rx)
One side of a link remains up while the other side of the link has gone down
Chapter 31 Configuring UDLD
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