User Manual Managed Switches
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Bridge A has been selected as the Root Bridge, since it was determined to have the lowest
Bridge Identifier on the network.
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Since Bridge A is the Root Bridge, it is also the Designated Bridge for LAN segment 1. Port 1 on
Bridge A is selected as the Designated Bridge Port for LAN Segment 1.
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Ports 1 of Bridges B, C, X, and Y are all Root Ports since they are nearest to the Root Bridge, and
therefore have the most efficient path.
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Bridges B and X offer the same Root Path Cost for LAN segment 2. However, Bridge B was
selected as the Designated Bridge for that segment since it has a lower Bridge Identifier. Port 2
on Bridge B is selected as the Designated Bridge Port for LAN Segment 2.
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Bridge C is the Designated Bridge for LAN segment 3, because it has the lowest Root Path Cost
for LAN Segment 3:
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The route through bridges C and B costs 200 (C to B=100, B to A=100)
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The route through bridges Y and B costs 300 (Y to B=200, B to A=100)
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The Designated Bridge Port for LAN Segment 3 is port 2 on bridge C.
Differences between STP and RSTP
RSTP is similar to STP, but includes additional information in the BPDUs that allow each bridge to
confirm that it has taken action to prevent loops from forming when it decides to enable a link to a
neighboring bridge. Adjacent bridges connected via point-to-point links will be able to enable a link
without waiting to ensure that all other bridges in the network have had time to react to the change.
The main benefit of RSTP is that the configuration decision is made locally rather than network-wide,
allowing RSTP to carry out automatic configuration and restore a link faster than STP.
The MSTP concept
Multiple Spanning Tree Protocol (MSTP) is a standard protocol based on IEEE 802.1S. It defines an
extension to RSTP to further develop the usefulness of virtual LANs (VLANs). The calculations of
