Releases 12.1(8a)E and later support a 12-bit extended system ID field as part of the bridge ID (see Table 11-2). 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

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HTH

Narayan