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1 MSTP Configuration
When configuring MSTP, go to these sections for information you are interested in:
z MSTP Overview
z Configuration Task List
z Configuring the Root Bridge
z Configuring Leaf Nodes
z Performing mCheck
z Configuring Digest Snooping
z Configuring No Agreement Check
z Configuring Protection Functions
z Displaying and Maintaining MSTP
z MSTP Configuration Example
MSTP Overview
Introduction to STP
Why STP?
The Spanning Tree Protocol (STP) was developed based on the 802.1d standard of IEEE to eliminate
loops at the data link layer in a local area network (LAN). Devices running this protocol detect loops in
the network by exchanging information with one another and eliminate loops by selectively blocking
certain ports to prune the loop structure into a loop-free tree structure. This avoids proliferation and
infinite cycling of packets that would occur in a loop network and prevents decreased performance of
network devices caused by duplicate packets received.
In the narrow sense, STP refers to IEEE 802.1d STP; in the broad sense, STP refers to the IEEE 802.1d
STP and various enhanced spanning tree protocols derived from that protocol.
Protocol Packets of STP
STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol
packets.
STP-enabled network devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient
information for the network devices to complete spanning tree calculation.
In STP, BPDUs come in two types:
z Configuration BPDUs, used for calculating a spanning tree and maintaining the spanning tree
topology.
z Topology change notification (TCN) BPDUs, used for notifying the concerned devices of network
topology changes, if any.
Basic concepts in STP
1) Root bridge
A tree network must have a root; hence the concept of root bridge was introduced in STP.
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