Understanding The Bridge Id - Cisco AJ732A - MDS 9134 Fabric Switch Configuration Manual
Cisco nexus 5000 series switch cli software configuration guide, nx-os 4.0(1a)n1 (ol-16597-01, january 2009)
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Chapter 8
Configuring Rapid PVST+
S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
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In a switched network, the root switch is the logical center of the spanning tree topology. STP uses
BPDUs to elect the root switch and root port for the switched network, as well as the root port and
designated port for each switched segment.
Understanding the Bridge ID
Each VLAN on each switch has a unique 64-bit bridge ID consisting of a bridge priority value, an
extended system ID (IEEE 802.1t), and an STP MAC address allocation.
This section contains the following topics:
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Bridge Priority Value
The bridge priority is a 4-bit value when the extended system ID is enabled (see
PVST+ Bridge Priority of a VLAN" section on page
In Cisco NX-OS, the extended system ID is always enabled; you cannot be disable the extended system
Note
ID.
Extended System ID
A 12-bit extended system ID field is part of the bridge ID (see
Figure 8-1
Bridge Priority
The switches always use the 12-bit extended system ID.
Combined with the bridge ID, the system ID extension functions as the unique identifier for a VLAN
(see
Table 8-1
Bridge Priority Value and Extended System ID with the Extended System ID Enabled
Bridge Priority Value
Bit 16
Bit 15
Bit 14
32768
16384
8192
OL-16597-01
The unique switch identifier Media Access Control (MAC) address of the switch that is associated
with each switch
The path cost to the root that is associated with each interface
The port identifier that is associated with each interface
Bridge Priority Value, page 8-3
Extended System ID, page 8-3
STP MAC Address Allocation, page 8-4
Bridge ID with Extended System ID
Bridge ID Priority
System ID Ext.
4 bits
12 bits
Table
8-1).
Extended System ID (Set Equal to the VLAN ID)
Bit 13
Bit 12
Bit 11
4096
2048
1024
8-22).
MAC Address
6 bytes
Bit 10
Bit 9
Bit 8
Bit 7
512
256
128
64
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
Information About Rapid PVST+
"Configuring the Rapid
Figure
8-1).
Bit 6
Bit 5
Bit 4
Bit 3
32
16
8
4
Bit 2
Bit 1
2
1
8-3
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