Industrial Managed
Ethernet Switch – EH9711
User Manual
Page
143
of
223
be bi-directional and the UDLD information times out while the link on the port is still up, UDLD tries to re-establish the state
of the port. If not successful, the port is put into the
errdisable
state.
Aging of UDLD information happens when the port that runs UDLD does not receive UDLD packets from the neighbor port
for duration of hold time. The hold time for the port is dictated by the remote port and depends on the message interval at the
remote side. The shorter the message interval, the shorter the hold time and the faster the detection. Recent implementations
of UDLD allow configuration of message interval.
UDLD information can age out due to the high error rate on the port caused by some physical issue or duplex mismatch. Such
packet drop does not mean that the link is unidirectional and UDLD in normal mode will not disable such link.
It is important to be able to choose the right message interval in order to ensure proper detection time. The message interval
should be fast enough to detect the unidirectional link before the forwarding loop is created, however, it should not overload
the switch CPU. The default message interval is 7 seconds, and is fast enough to detect the unidirectional link before the
forwarding loop is created with default STP timers. The detection time is approximately equal to three times the message
interval.
For example:
T
detection
~ message_interval x 3
This is 21 seconds for the default message interval of 7 seconds.
It takes T
reconvergence
=max_age + 2x forward_delay for the STP to reconverge in case of unidirectional link failure. With the
default timers, it takes 20+2x7=34 seconds.
It is recommended to keep T
detection
< T
reconvergence
by choosing an appropriate message interval.
In aggressive mode, once the information is aged, UDLD will attempt to re-establish the link state by sending packets every
second for eight seconds. If the link state is still not determined, the link is disabled.
Aggressive mode adds additional detection of these situations:
The port is stuck (on one side the port neither transmits nor receives, however, the link is up on both sides).
The link is up on one side and down on the other side. This is issue might be seen on fiber ports. When transmit fiber
is unplugged on the local port, the link remains up on the local side. However, it is down on the remote side.
Most recently, fiber FastEthernet hardware implementations have Far End Fault Indication (FEFI) functions in order to bring
the link down on both sides in these situations. On Gigabit Ethernet, a similar function is provided by link negotiation. Copper
ports are normally not susceptible to this type of issue, as they use Ethernet link pulses to monitor the link. It is important to
mention that, in both cases, no forwarding loop occurs because there is no connectivity between the ports. If the link is up on
one side and down on the other, however, blackholing of traffic might occur. Aggressive UDLD is designed to prevent this.
This UDLD webpage shown Figure 2.118 allows the user to inspect the current UDLD configurations, and possibly change
them as well. Table 2.106 provides the descriptions of UDLD Port Configuration.
