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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 13 Configuring Optional Spanning-Tree Fea tures Understanding Optional Spanning-Tree Features
If lin k L 1 fail s as s hown in Figure 13-8, Switch C cannot detect this failure because it is not connected
directly to link L1. Ho wever , because Switch B is directly connected to the root switch ov er L1, it detects
the failure, elects itself the root, and begins sending BPDUs to Switch C, identifying itself as the root.
When Switch C receives the inferior BPDUs from Switch B, Switch C assumes that an indirect failure
has occurred. At that point, Backbone Fast a llows the blo cke d port on Switch C to move immediatel y to
the listening state without waiting for the maximum aging time for the port to e xpire. BackboneFast then
transition s th e La yer 2 in ter f a ce on Switc h C to the for w ar di ng stat e, p r oviding a p ath fro m Switch B to
Switch A. This switchover takes approximately 30 seconds, twice the Forward Delay time if the default
Forward Delay t ime of 1 5 se cond s is set . Figure 13-8 shows how BackboneFast reco nfigures the
topology to account for the failure of link L1.
Figure 13-8 BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure 13-9, BackboneFast is
not ac tivated beca use t h e i nferi or B PDUs did not co me f rom t he r e cog niz ed de signa te d b ridg e
(Switch B). The new switch begins sending infe ri or BPDUs that indica te it is the roo t switch . However ,
the oth er swi tches ignor e the se inf erior BPDUs, a nd t he n ew switch learns that Switch B is the
designated bridge to Switch A, the root switch.
Figure 13-9 Adding a Switch in a Shared-Medium Topology
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Link failure
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BackboneFast transitions port
through listening and learning
states to forwarding state.
Switch A
(Root)
Switch C Switch B
(Designated bridge)
Added switch
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Blocked port