CHAPTER
19-1
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
19

Configuring UDLD

This c hapt er d escri b es how to c onfigure the Un iDi rec tiona l Li n k De tect ion (UD LD) pr otoc ol o n y our
switch.
Note For comple te syntax and us age inform ation for the co mmands used in this chapter, refer to th e command
refere nce fo r thi s r ele ase.
This chapter consists of these sections:
•Understa ndin g UD L D, p age 19- 1
•Configuring U DLD , pa ge 19- 3
•Displaying UDLD Status, page 19-6

Understandin g UDLD

UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet
cables to monitor the physical configuration of the cables and detect when a unidirectional link exists.
All conne cted devices must suppor t UDLD for the protocol to successfull y identi fy and disab le
unidirec tional link s. When UDL D detect s a unidir ectional link, it admi nistratively shuts down the
affected port and alerts you . Unidi rection al links ca n cause a variety of problems, inc ludin g
spannin g-tr ee top ology loop s.
UDLD wo rks with the L ayer 1 mec hani sm s to de ter mine the physi cal status of a l ink. At La yer 1,
autonegoti ation t akes c are o f physi cal s igna lin g and fau lt det ectio n. UD LD per forms t asks t h at
autonegotiation cannot perf orm, such as detecting the identities of neighbo rs and shutting down
misconn ect ed i nte rface s. When yo u e nabl e b oth aut onegot iati on a nd UDL D, La yer 1 a nd Lay er 2
detecti ons work together t o prevent physical and logical unid irecti onal conne ctions and the
malfunct ionin g of other pro tocols.
A unidirectional link occurs whenever traffic sent by the local dev ice is received by the neighbor but
traffic from the neighb or is not rece ived by the local device. If one of th e fiber-optic stra nds in a pai r is
disconnecte d, as long as a utonegotiation is acti ve, the link does not sta y up. In th is c ase, the lo gica l lin k
is unde ter mine d, a nd UD LD d oe s n ot ta ke any ac ti on. I f both fiber-optic stra nds a re wor king no rm all y
from a L ayer 1 persp ec tive, UDLD at La yer 2 d ete rmi nes w het her t hose fiber-optic stra nds a re
connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This
check ca nn ot b e pe rf ormed by a utonego tia tio n be ca use a uton egotia tion ope rat es a t L ay er 1.