When the max_age timer on switch Y
expires before a new BPDU has been received from switch Z, a spanning-tree recalculation
is initiated. Switch Y transitions its blocking port (port 1) from the blocking state to the
listening state to the learning state, and then ?¬?nally to the forwarding state.
Figure 2-27 Spanning-Tree Recalculation
After all the switch and bridge ports have transitioned to either a forwarding state or a
blocking state, switch X becomes the root bridge and forwards traf?¬?c between the
segments.
STP Convergence
Convergence in STP is a state in which all the switch and bridge ports have transitioned to
either the forwarding or the blocking state. Convergence is necessary for normal network
operations. For a switched or bridged network, a key issue is the time required for
convergence when the network topology changes.
Fast convergence is a desirable network feature because it reduces the time that bridge and
switch ports are in transitional states and not sending user traf?¬?c. The normal convergence
time is 30 to 50 seconds for 802.1D STP.
Per VLAN Spanning Tree+
The 802.1D standard de?¬?nes a Common Spanning Tree (CST) that assumes only one
spanning-tree instance for the entire switched network, regardless of the number of
VLANs.
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