Cisco NM-8B-U - HW ROUTERS L-M User Manual page 13

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
Bridge Protocol Data Units
The stable active spanning tree topology of a switched network is determined by the following:
The unique bridge ID (bridge priority and MAC address) associated with each VLAN on each switch
•
The spanning tree path cost to the root bridge
•
The port identifier (port priority and MAC address) associated with each Layer 2 interface
•
The Bridge Protocol Data Units ( BPDU) are transmitted in one direction from the root switch, and each
switch sends configuration BPDUs to communicate and compute the spanning tree topology. Each
configuration BPDU contains the following minimal information:
• The unique bridge ID of the switch that the transmitting switch believes to be the root switch
The spanning tree path cost to the root
•
The bridge ID of the transmitting bridge
•
Message age
•
The identifier of the transmitting port
•
Values for the hello, forward delay, and max-age protocol timers
•
When a switch transmits a bridge packet data unit (BPDU) frame, all switches connected to the LAN on
which the frame is transmitted receive the BPDU. When a switch receives a BPDU, it does not forward
the frame but instead uses the information in the frame to calculate a BPDU, and, if the topology
changes, initiate a BPDU transmission.
A BPDU exchange results in the following:
One switch is elected as the root switch.
•
The shortest distance to the root switch is calculated for each switch based on the path cost.
•
A designated bridge for each LAN segment is selected. This is the switch closest to the root bridge
•
through which frames is forwarded to the root.
A root port is selected. This is the port providing the best path from the bridge to the root bridge.
•
Ports included in the spanning tree are selected.
•
Election of the Root Bridge.
•
For each VLAN, the switch with the highest bridge priority (the lowest numerical priority value) is
elected as the root switch. If all switches are configured with the default priority (32768), the switch with
the lowest MAC address in the VLAN becomes the root switch.
The spanning tree root switch is the logical center of the spanning tree topology in a switched network.
All paths that are not needed to reach the root switch from anywhere in the switched network are placed
in spanning tree blocking mode.
BPDUs contain information about the transmitting bridge and its ports, including bridge and MAC
addresses, bridge priority, port priority, and path cost. Spanning tree uses this information to elect the
root bridge and root port for the switched network, as well as the root port and designated port for each
switched segment.
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
Feature Overview
13