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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 19 Co nfiguring UDLD
Unders tanding UDLD
UDLD operates by using two mechanisms:
•Neighbor database maintenance
UDLD l ear ns ab out othe r UDL D-c apabl e n ei ghbor s by p er iodi cal ly sen ding a hel lo p acket (al so
called an advertisement or probe) on every active interface to keep each device informed about its
neighbors.
When the switch r ecei v es a hello message, i t caches th e informat ion until the a ge time (ho ld 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 an interfac e is disab led and UDL D is runn ing, whenever UDLD i s disabled on an
interface, or whenever the switch is reset, UDLD clears all existing cache entries for the interfaces
affected by the configuration change. UDLD se nds at l east one message t o inform the neighbor s to
flush the part of their caches affected by the status change. The message is intended to keep the
caches synchronized.
•Event-driven detection and echoing
UDLD re lies on ech o ing as its detec tio n mech an ism . When ev er a UDLD device lear ns ab ou t a new
neighb or or receives a resynchro nizati on request from an out-of-sy nc neighb or, it resta rts the
detecti on win dow on its si de of the con ne ction and se nds echo m essages in rep ly. Because this
behavior is the same on all UDLD neighbors, the sender of the echoes expects to receiv e an echo in
reply. If the d etect ion w indow e nds an d no valid repl y messa ge i s re ceived, the li nk i s c ons ider ed
unidirec tional , and the i nterface is shut down.
Figure 19-1 shows an example of a unidirec tional link co nditi on.
Figure 19-1 UDLD Detection of a Unidirectional Link
TX
TX
RX
RX
Switch A
Switch B
Switch B successfully
receives traffic from
Switch A on this port. However, Switch A does not receive traffic
from Switch B on the same port. UDLD
detects the problem and disables the port.
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