Cisco 2950G 24 Software Configuration Guide - Page 272

Understanding Spanning-Tree Features Chapter 11 Configuring STP BPDUs contain information about the sending switch and its ports, including switch and MAC addresses, switch priority, port priority, and path cost. Spanning tree uses this information to elect the root switch and root port for the switched network and the root port and designated port for each switched segment. Bridge ID, Switch Priority, and Extended System ID 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(9)EA1 and later, Catalyst 2950 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 11-1, 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. Table 11-1 Switch Priority Value and Extended System ID Switch 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 Spanning tree uses the extended system ID, the switch priority, and the allocated spanning-tree MAC address to make the bridge ID unique for each VLAN. With earlier releases, spanning tree used one MAC address per VLAN to make the bridge ID unique for each VLAN. Support for the extended system ID affects how you manually configure the root switch, the secondary root switch, and the switch priority of a VLAN. For more information, see the "Configuring the Root Switch" section on page 11-12, "Configuring a Secondary Root Switch" section on page 11-14, and "Configuring the Switch Priority of a VLAN" section on page 11-18. Spanning-Tree Timers Table 11-2 describes the timers that affect the entire spanning-tree performance. Table 11-2 Spanning-Tree Timers Variable Hello timer Forward-delay timer Maximum-age timer Description Determines how often the switch broadcasts hello messages to other switches. Determines how long each of the listening and learning states last before the interface begins forwarding. Determines the amount of time the switch stores protocol information received on an interface. 11-4 Catalyst 2950 Desktop Switch Software Configuration Guide 78-14982-01