How A Topology Is Created; Bridge Id; Bridge Priority Value - Cisco Nexus 7000 Series Configuration Manual

Configuring Rapid PVST+ Using Cisco NX-OS
state. The STP port priority value is the efficiency with which that location allows the port to pass traffic. The
STP port path-cost value is derived from the media speed.

How a Topology is Created

All devices in a LAN that participate in a spanning tree gather information about other switches in the network
by exchanging BPDUs. This exchange of BPDUs results in the following actions:
• The system elects a unique root switch for the spanning tree network topology.
• The system elects a designated switch for each LAN segment.
• The system eliminates any loops in the switched network by placing redundant switch ports in a backup
state; all paths that are not needed to reach the root device from anywhere in the switched network are
placed in an STP-blocked state.
The topology on an active switched network is determined by the following:
• The unique device identifier Media Access Control (MAC) address of the device that is associated with
each device
• The path cost to the root that is associated with each switch port
• The port identifier that is associated with each switch port
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.
The mac-address bpdu source version 2 command enables STP to use the new Cisco MAC address
Note
(00:26:0b:xx:xx:xx) as the source address of BPDUs generated on vPC ports.
To apply this command, you must have identical configurations for both vPC peer switches or peers.
We strongly recommend that you disable EtherChannel Guard on the edge devices before using this
command to minimize traffic disruption from STP inconsistencies. Reenable the EtherChannel Guard
after updating on both peers.

Bridge ID

Each VLAN on each network device has a unique 64-bit bridge ID that consists of a bridge priority value, an
extended system ID (IEEE 802.1t), and an STP MAC address allocation.

Bridge Priority Value

The bridge priority is a 4-bit value when the extended system ID is enabled.
You can only specify a device bridge ID (used by the spanning tree algorithm to determine the identity of the
root bridge; the lowest number is preferred) as a multiple of 4096.
Note In this device, the extended system ID is always enabled; you cannot disable the extended system ID.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
STP
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