15-3
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
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Chapter 15 Configuring STP and IEEE 802.1s MST
Understanding How STP Works
Understanding the Bridge ID
Each VLAN on each network device has a unique 64-bit bridge ID consisting of a bridge priority value,
an extended system ID, and an STP MAC address allocation.
This section contains these topics:
• Bridge Priority Value, page 15-3
• Extended System ID, page 15-3
• STP MAC Address Allocation, page 15-3

Bridge Priority Value

With Release 12.1(8a)E and later releases, the bridge priority is a 4-bit value when the extended system
ID is enabled (see Table 15-2 on page 15-3). With earlier releases, the bridge priority is a 16-bit value
(see Table 15-1 on page 15-3). See the “Configuring the Bridge Priority of a VLAN” section on
page 15-30.

Extended System ID

Release 12.1(8a)E and later releases support a 12-bit extended system ID field as part of the bridge ID
(see Table 15-2 on page 15-3). Chassis that support only 64 MAC addresses always use the 12-bit
extended system ID. On chassis that support 1024 MAC addresses, you can enable use of the extended
system ID. STP uses the VLAN ID as the extended system ID. See the “Enabling the Extended System
ID” section on page 15-24.

STP MAC Address Allocation

Catalyst 6500 series switch chassis have either 64 or 1024 MAC addresses available to support software
features such as STP. To view the MAC address range on your chassis, enter the show catalyst6000
chassis-mac-address command.
Release 12.1(8a)E and later releases support chassis with 64 or 1024 MAC addresses. For chassis with
64 MAC addresses, STP uses the extended system ID plus a MAC address to make the bridge ID unique
for each VLAN.
Earlier releases support chassis with 1024 MAC addresses. With earlier releases, STP uses one MAC
address per VLAN to make the bridge ID unique for each VLAN.
Table 15-1 Bridge Priority Value with the Extended System ID Disabled
Bridge Priority Value
Bit 16 Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1
32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1
Table 15-2 Bridge Priority Value and Extended System ID with the Extended System ID Enabled
Bridge Priority Value Extended System ID (Set Equal to the VLAN ID)
Bit 16 Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1
32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1