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Chapter 4
Layer 2 Switching and the Spanning Tree Protocol (STP)
Selecting the Root Bridge
Switches or bridges running STP exchange information with what are called
Bridge Protocol Data Units (BPDUs). BPDUs are used to send configuration
messages using multicast frames. The bridge ID of each device is sent to other
devices using BPDUs.
The bridge ID is used to determine the root bridge in the network and to
determine the root port. The bridge ID is 8 bytes long and includes the pri-
ority and the MAC address of the device. The priority on all devices running
the IEEE STP version is 32768 by default.
To determine the root bridge, combine the priority of the bridge and the
MAC address. If two switches or bridges have the same priority value, then
the MAC address is used to determine which has the lowest ID.
For example, if two switches, A and B, both use the default priority of
32768, the MAC address will be used. If switch A's MAC address is
0000.0c00.1111.1111 and switch B's MAC address is
0000.0c00.2222.2222, switch A would become the root bridge.
The network analyzer output below shows a BPDU broadcasted on a net-
work. BPDUs are sent out every two seconds by default. That may seem like
a lot of overhead, but remember that this is only a layer 2 frame, with no
layer 3 information in the packet:
Flags: 0x80 802.3
Status: 0x00
Packet Length:64
Timestamp: 19:33:18.726314 02/28/2000
802.3 Header
Destination: 01:80:c2:00:00:00
Source: 00:b0:64:75:6b:c3
LLC Length: 38
802.2 Logical Link Control (LLC) Header
Dest. SAP: 0x42
802.1 Bridge Spanning Tree
Source SAP: 0x42
802.1 Bridge Spanning Tree
Command: 0x03
Unnumbered Information
802.1 - Bridge Spanning Tree
Protocol Identifier: 0
Protocol Version ID: 0
Message Type: 0
Configuration Message
Flags: %00000000
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