Cisco Systems WSC4500X16SFP Manual De Usuario
13-2
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 13 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.
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.
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.
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
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
).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
). See the
.
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
support a 12-bit extended system ID field as part of the bridge ID (see
). 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
addresses, you can enable use of the extended system ID. STP uses the VLAN ID as the extended system
ID. See the
Table 13-1 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 13-2 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