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Foundry NetIron M2404C and M2404F Metro Access Switches
Election of the Root Bridge
The devices in the network exchange data messages, called Bridge Protocol Data Units (BPDUs)
to gather), for information about other switches in the network.
This exchange of messages results in the following actions:
•
The election of a unique root bridge for each spanning tree instance
•
The election of a designated bridge for every switched LAN segment
•
The removal of loops in the switched network by blocking ports connected to redundant links.
The bridge with the highest bridge priority (the lowest numerical priority value) is elected as the
root device. If all bridges are configured with the default priority (32768), the bridge with the
lowest MAC address in the VLAN becomes the root bridge. The bridge priority value occupies the
most significant bits of the bridge ID.
The user can change the probability that a bridge will be elected as the root device by configuring
the device priority value. Raising the priority value increases the probability; and lowering the
value decreases the probability.
The root bridge is the logical center of the STP topology in a switched network. All paths that are
not needed for reaching the root bridge from anywhere in the switched network are placed in STP
blocking mode.
Path Cost
Devices use an algorithm to determine how close they are to the root bridge. This metric is called
the Path Cost. The lower the cost, the closer the device is to the root. The idea is to traverse the
tree using the lowest costs. If two devices have identical path costs in the node of a tree, the device
with the lowest MAC address value is used for the tiebreaker.
Bridge Protocol Data Units (BPDUs)
BPDUs contain the information about the transmitting device and its ports, including the MAC
address, switch priority, port priority, and path cost. STP uses this information to elect the root
device and root port for the switched network and the root port and designated port for each
switched segment.
The BPDU contains information regarding:
•
Root Bridge ID - Which device is the root bridge
Designated Bridge ID – The transmitting bridge ID
•
•
Path cost - The distance between the root and sender
Designated Port ID - The port ID that identifies the port on the bridge from which the
•
configuration message was originated.
STP Parameters
STP Timers
Table 1
describes the STP timers that affect the entire spanning-tree performance. These three
timers are contained in each configuration BPDU.
Spanning Tree Protocol (STP)
© 2008 Foundry Networks, Inc.
Configuring STP (Rev. 03)
Page 4 of 38
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