Understanding The Bridge Id - Cisco Catalyst 4500 Series Software Configuration Manual

About STP
Beginning in Cisco IOS Release 15.2(4)E and Cisco IOS Release 3.8.0E, Cisco Catalyst 4500 series,
Cisco Catalyst 4900M, Cisco Catalyst 4948E and Cisco Catalyst 4948F switches., by default, use STP
(the IEEE 802.1w RSTP) on all VLANs. By default, a single spanning tree runs on each configured
VLAN (provided you do not manually disable the spanning tree). You can enable and disable a spanning
tree on a per-VLAN basis.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The spanning tree algorithm calculates the best loop-free path throughout a switched Layer 2
network. Switches send and receive spanning tree frames at regular intervals. The switches do not
forward these frames, but use the frames to construct a loop-free path.
Multiple active paths between end stations cause loops in the network. If a loop exists in the network,
end stations might receive duplicate messages and switches might learn end station MAC addresses on
multiple Layer 2 interfaces. These conditions result in an unstable network.
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.
These sections describe STP:
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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 is 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 Cisco IOS releases prior to 12.1(12c)EW, the bridge priority is a 16-bit value (see
Cisco IOS Release 12.1(12c)EW and later releases, the bridge priority is a 4-bit value when the extended
system ID is enabled (see
page
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
23-2
Understanding the Bridge ID, page 23-2
Bridge Protocol Data Units, page 23-3
Election of the Root Bridge, page 23-4
STP Timers, page 23-4
Creating the STP Topology, page 23-5
STP Port States, page 23-5
MAC Address Allocation, page 23-6
STP and IEEE 802.1Q Trunks, page 23-6
Per-VLAN Rapid Spanning Tree, page 23-6
Table
23-17.
23-2). See the "Configuring the Bridge Priority of a VLAN" section on
Chapter 23 Configuring STP and MST
Table 23-1).With