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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 13 Configuring Optional Spanning-Tree Fea tures Configuring Optional Spanning-Tree Features

Understanding Loop Guard

You ca n use l oo p gu ard to p revent altern ate o r root po rts f rom b ec omi ng desi gnated po rts b eca use of a
failur e that leads to a u nidirectional l ink. This feature is m ost effe ctive when it is conf igured on th e entire
switched network.
If your switch is running PVST or MSTP, you c an enable thi s feature by using the spanning-tree
loopguard default global configurat ion comm and. The MSTP i s available only i f you have the EI
installe d on your switc h.
When the switch is operating in PVST mode, loop guard prev ents alternate and root ports from becoming
designat ed po rts, a nd spa nnin g t ree d oes n ot send BPD Us o n r oot or alte rn ate p ort s.
When the sw itch i s op er ating in MST mod e, BPD Us are no t sent on nonb oun dary po rts on ly if the p ort
is blocked by loop guard in all MST instances. On a boundary port, loop guard blocks the port in all MST
instances.

Configuring Optio nal Spanning-Tre e Feature s

These sec tions de scri be how to c on figure o pti onal span ni ng-t ree fea ture s:
Default Opt iona l Sp ann ing- Tree Configuration , page 1 3-1 4
Enab ling Po rt Fast, pa ge 13 -14
Enab lin g B PDU Gu ar d, pa ge 13- 15
Enab ling BPDU Filtering , page 13 -16
Enab ling Upl ink Fast for Use wi th R ed unda nt Link s, pa ge 13 -1 7
Enabling Cross-Stack UplinkFast, page 13-18
Enab ling B ackb oneFast, page 1 3-1 9
Enab ling Root Guard, pa ge 13-1 9
Enab lin g L oo p Gua r d, page 1 3-2 0