In a network running CST, these statements are true:
100Base-T
10Base-T
Designated
Port (F)
Designated
Port (F)
Root Bridge
Default Priority 32768
MAC 0c00.1111.0000
Port 1
Designated
Port (F)
Port 1
Nondesignated
Port (BLK)
Root Port (F)
Port 0
Default Priority 32768
MAC 0c00.1111.2222
Default Priority 32768
MAC 0c00.1111.1111
Root Port (F)
Port 0
Switch Z
Switch X Switch Y
Port 1 Port 0
Improving Performance with Spanning Tree 57
?– No load sharing is possible; one uplink must block for all VLANs.
?– The CPU is spared; only one instance of spanning tree must be computed.
Per VLAN Spanning Tree Plus (PVST+) de?¬?nes a spanning-tree protocol that has several
spanning-tree instances running for the network, one instance of STP per VLAN. Figure 2-28
shows an example of how you can use PVST+ to forward traf?¬?c on all network segments
but still maintain STP integrity.
Figure 2-28 Per VLAN Spanning Tree+
In a network running several spanning-tree instances, these statements are true:
?– Optimum load sharing can result.
?– One spanning-tree instance for each VLAN maintained can mean a considerable waste
of CPU cycles for all the switches in the network (in addition to the bandwidth used
for each instance to send its own BPDUs).
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