Cisco WS-C2960-24LC-S Software Guide - Page 306
Bridge ID, Switch Priority, and Extended System ID, Spanning-Tree Interface States
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Understanding Spanning-Tree Features Chapter 15 Configuring STP 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 controls the selection of the root switch. Because each VLAN is considered as a different logical bridge with PVST+ and rapid PVST+, the same switch must have a different bridge IDs for each configured VLAN. Each VLAN on the switch has a unique 8-byte bridge ID. The 2 most-significant bytes are used for the switch priority, and the remaining 6 bytes are derived from the switch MAC address. The switch supports the IEEE 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 15-1, the 2 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. Table 15-1 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. 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 example, when you change the switch priority value, you change the probability that the switch will be elected as the root switch. Configuring a higher value decreases the probability; a lower value increases the probability. For more information, see the "Configuring the Root Switch" section on page 15-14, the "Configuring a Secondary Root Switch" section on page 15-16, and the "Configuring the Switch Priority of a VLAN" section on page 15-19. Spanning-Tree Interface States Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When an interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding state, it can create temporary data loops. Interfaces must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for forwarded frames that have used the old topology. Each Layer 2 interface on a switch using spanning tree exists in one of these states: • Blocking-The interface does not participate in frame forwarding. • Listening-The first transitional state after the blocking state when the spanning tree decides that the interface should participate in frame forwarding. • Learning-The interface prepares to participate in frame forwarding. • Forwarding-The interface forwards frames. • Disabled-The interface is not participating in spanning tree because of a shutdown port, no link on the port, or no spanning-tree instance running on the port. 15-4 Catalyst 2960 Switch Software Configuration Guide OL-8603-04