It is always the good practice to manually configure the root, by using the root command so that the root switch automatically reduces the bridge priroty if it sees the same or lower pri. bpdu.
here are some descritpion about the 802.1T
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/1218ea1/3550scg/swstp.htm#xtocid4
The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which determines the selection of the root switch. Because each VLAN is considered as a different logical bridge with PVST+, the same switch must have as many different bridge IDs as VLANs configured on it. Each VLAN on the switch has a unique 8-byte bridge ID; the two most-significant bytes are used for the switch priority, and the remaining six bytes are derived from the switch MAC address.
In Release 12.1(8)EA1 and later, Catalyst 3550 switches support the 802.1T spanning-tree extensions, and some of the bits previously used for the switch priority are now used as the VLAN identifier. The result is that fewer MAC addresses are reserved for the switch, and a larger range of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID. As shown in Table 10-1, the two bytes previously used for the switch priority are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID. In earlier releases, the switch priority is a 16-bit value.
Before Release 12.1(8)EA1, entering the spanning-tree vlan vlan-id root global configuration command on a a Catalyst 3550 switch (no extended system ID) causes it to set its own switch priority for the specified VLAN to 8192 if this value causes this switch to become the root for the specified VLAN. If any root switch for the specified VLAN has a switch priority lower than 8192, the switch sets its own priority for the specified VLAN to 1 less than the lowest switch priority.
These examples show the effect of the spanning-tree vlan vlan-id root command with and without the extended system ID support:
For Catalyst 3550 switch with the extended system ID (Release 12.1(8)EA1 and later), if all network devices in VLAN 20 have the default priority of 32768, entering the spanning-tree vlan 20 root primary command on the switch sets the switch priority to 24576, which causes this switch to become the switch bridge for VLAN 20.
For Catalyst 3550 switches without the extended system ID (software before Release 12.1(8)EA1), if all network devices in VLAN 100 have the default priority of 32768, entering the spanning-tree vlan 100 root primary command on the switch sets the switch priority for VLAN 100 to 8192, which causes this switch to become the root switch for VLAN 100.
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Note If your network consists of Catalyst 3550 switches that do not support the extended system ID and Catalyst 3550 switches that do support it, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software.
. The cat6000 and cat4000 both supports this command in 7.X. software
