14-2
Software Configuration Guide—Release 12.2(25)EW
OL-6696-01
Chapter 14 Understanding and Configuring STP
Overview of STP
A spanning tree defines a tree with a root switch and a loop-free path from the root to all switches in the
Layer 2 network. A spanning tree forces redundant data paths into a standby (blocked) state. If a network
segment in the spanning tree fails and a redundant path exists, the spanning tree algorithm recalculates
the spanning tree topology and activates the standby path.
When two ports on a switch are part of a loop, the spanning tree port priority and port path cost setting
determine which port is put in the forwarding state and which port is put in the blocking state. The
spanning tree port priority value represents the location of an interface in the network topology and how
well located it is to pass traffic. The spanning tree port path cost value represents media speed.
Understanding the Bridge ID
Each VLAN on each network device has a unique 64-bit bridge ID consisting of a bridge priority value,
an extended system ID, and an STP MAC address allocation.
Bridge Priority Value
The bridge priority value determines whether a given redundant link will be given priority and
considered part of a given span in a spanning tree. Preference is given to lower values, and if you want
to manually configure a preference, assign a lower bridge priority value to a link than to its redundant
possibility. With releases prior to 12.1(12c)EW, the bridge priority is a 16-bit value (see
Table 14-1).With Release 12.1(12c)EW and later releases, the bridge priority is a 4-bit value when the
extended system ID is enabled (see Table 14-2). See the “Configuring the Bridge Priority of a VLAN”
section on page 14-16.
Extended System ID
Extended system IDs are VLAN IDs between 1025 and 4096. Releases 12.1(12c)EW and later releases
support a 12-bit extended system ID field as part of the bridge ID (see Table 14-2). 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 on page 14-8.
Table 14-1 Bridge Priority Value with the Extended System ID Disabled
Bridge Priority Value
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 14-2 Bridge Priority Value and Extended System ID with the Extended System ID Enabled
Bridge 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 VLAN ID
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