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). With earlier releases, the bridge priority is a 16-bit value (see Table 15-1). See the “Configuring the Bridge Priority of a VLAN” section.
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). 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.
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.
If you have a network device in your network with MAC address reduction enabled, you should also enable MAC address reduction on all other Layer-2 connected network devices to avoid undesirable root bridge election and spanning tree topology issues.
When MAC address reduction is enabled, the root bridge priority becomes a multiple of 4096 plus the VLAN ID. With MAC address reduction enabled, a switch bridge ID (used by the spanning-tree algorithm to determine the identity of the root bridge, the lowest being preferred) can only be specified as a multiple of 4096. Only the following values are possible: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440.
If another bridge in the same spanning-tree domain does not run the MAC address reduction feature, it could win root bridge ownership because of the finer granularity in the selection of its bridge ID.
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