PVST+ Operation
In a Cisco PVST+ environment, you can tune the spanning-tree parameters so that half of
the VLANs forward on each uplink trunk. To easily achieve this, you con?¬?gure one switch
to be elected the root bridge for half of the total number of VLANs in the network and a
second switch to be elected the root bridge for the other half of the VLANs. Providing
different STP root switches per VLAN creates a more redundant network.
Spanning-tree operation requires that each switch has a unique BID. In the original 802.1D
standard, the BID was composed of the bridge priority and the MAC address of the switch,
and all VLANs were represented by a CST. Because PVST+ requires that a separate
instance of spanning tree runs for each VLAN, the BID ?¬?eld is required to carry VID
information. This is accomplished by reusing a portion of the Priority ?¬?eld as the extended
system ID to carry a VID. Figure 2-29 shows how modifying the bridge priority offers this
support.
Root for VLAN 1 Root for VLAN 2
Forwarding Port for VLAN 2
Blocking Port for VLAN 1
Forwarding Port for VLAN 1
Blocking Port for VLAN 2
58 Chapter 2: Medium-Sized Switched Network Construction
Figure 2-29 PVST+ VLAN ID
To accommodate the extended system ID, the original 802.
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