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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 13 Configuring Optional Spanning-Tree Features
Unders tanding Optional Spanning -Tree Features

Understanding BackboneFast

BackboneFast detects indirect failures in the core of the backbone. BackboneFast is a complementary
technol ogy to the Uplin kFast featu re, which respo nds to failure s on links dir ectly con nected to access
switc hes . Back boneFa st opt imize s th e maximu m-ag e tim er, which det ermine s th e amou nt of tim e th e
switch stores protocol information received on an interface. When a switch receives an inferior BPDU
from t he de sign ate d port of a nothe r switc h, t he BPD U is a s ig nal t hat the oth er sw itch m ight have lost
its pa th to t he root , a nd Ba ckbon eFas t tri es t o find an a lter nat e p ath t o the ro ot.
Backbon eFas t, w h ich is e na ble d by u sin g the spanning- tree backbonefast globa l configu ratio n
comman d, star ts when a ro ot port or bloc ked port on a switc h receives inferior BP DUs from its
designat ed bridge. A n inferior BPDU iden tifies one switch as both the root brid ge and the desi gnated
bridge. Wh en a switch receiv es an infer ior BPDU, it means that a l ink to which the sw itch is not direc tly
connected (an indirect link) has failed (that is, the designated bridge has lost its connection to the root
switch) . Under spanni ng-tre e rules, the swit ch ignores i nferio r BPDUs for the co nfigured maxi mum
aging time specified by the spanning-tree max-age global configuration command. The BackboneFast
feature is supported onl y when the switc h is running PVST.
The switch tries to determine if it has an alternate path to the root switch. If the inferior BPDU arrives
on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root
switch. (Self-looped ports are not considered alternate paths to the root switch.) If the inferior BPDU
arri v es on the ro ot port, all b lock ed ports become a lternate paths t o the r oot swit ch. If the inferi or BPDU
arrives on the root port and there are no bloc ked ports, th e switch as sumes that it has lost conne ctivity
to th e roo t swi tch, c a uses t he ma ximu m ag ing ti m e on t he ro ot port to expi re, and be come s the ro ot
switch according to normal spanning-tree rules.
If the switch has alternate paths to the root switch, it uses these alternate paths to send a root link query
(RLQ) request. The switch sends the RLQ request on all alternate paths to the root switch a nd wa its for
an RLQ reply from other switches in the network. If the switch determines that it still has an alternate
path to the root, it expires the maximum aging time on the port that received the inferior BPDU. If all
the alte rnat e p aths to the root switch in dicate th at t he switc h has lo st co nne ctivity to the r oot sw itch, th e
switch expires the maximum aging time on the port that received the RLQ reply . If one or more alternate
paths can still connect to the root switch, the switch makes all ports on which it received an inferior
BPDU it s designated ports and moves them fr om the blocking state (if they w ere in the blocki ng state),
through the listening and learning states, and into the forwarding state.
Figure 13-7 shows an example topology with no link failures. Swi tch A, the root switch, co nnects
directly to Switch B over link L1 and to Switch C over link L2. The Layer 2 interface on Switch C that
connec ts directl y to Switch B is in the bloc king state.
Figure 13-7 BackboneFast Example Before Indirect Link Failure
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Blocked port
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