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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 11 Configuring STP
Unders tanding Spanni ng-Tree Fea tures
BPDUs conta in informa tion about the sending switch and i ts ports, inclu ding switch a nd MAC
addresses, switch priority, port priority, and path cost. Spanning tree uses this information to elect the
root sw itch and ro ot po rt for the sw itc hed net work an d the roo t port and de signa te d port for e ach
switched segment.

Bridge ID, Switch Priority, and Extended System ID

The IEEE 80 2.1D 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)EA 1 and later, Catalyst 2950 switch es support the 802.1T spann ing-tree extensions,
and some of the bits previously used for the switch priority are now used as the VLAN identifier. The
result is t hat fewer MAC addresses are reser ved for the sw it ch, an d a l arger ra nge of V LAN ID s can b e
supported, all while maintaining the uniqueness of the bridge ID. As shown in Table 11-1, the two b ytes
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.
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. W ith ea rlier releases, spanning tree used one MA C
address pe r VLAN to make the bridg e ID uniqu e for each VLAN.
Support f or the extend ed sy stem ID affects how you m anu all y con figure th e root swi tch, the second ar y
root switch, and the switch priority of a VLAN. For more information, see the “Configurin g th e Roo t
Switch” section on pag e 11-12 , “Co nfiguring a Secon dary Root Swi tch” section on page 11-14 , and
“Configuring the Switch Pri ori ty o f a VLAN ” sec tion on page 11-1 8.

Spannin g- Tr ee Time rs

Table 11-2 describes the timers that affect the entire spanning-tree performance.
Table 11-1 Switch Priority Value and Extended System ID
Switch Priority Value Extended System ID (Set Equal to th e 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
Table 11-2 Spanni ng-Tree Timers
Variable Description
Hello timer Determines how often the switch broadcasts hello messages to other switches.
Forward-de lay time r Deter mines how long eac h of the li stening and learni ng states last before the inter face begins
forwarding.
Maxi mum-ag e tim er De ter mines the am ount of time th e switch st ores pro tocol informat ion rec eived on an interface .