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Configuring MSTP
As a Layer 2 management protocol, the Spanning Tree Protocol (STP) eliminates Layer 2 loops by
selectively blocking redundant links in a network, and in the mean time, allows for link redundancy.
Like many other protocols, STP evolves as the network grows. The later versions of STP are Rapid
Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP). This chapter describes the
characteristics of STP, RSTP, and MSTP.
STP
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 the 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.
STP protocol packets
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 have the following types:
• Configuration BPDUs—Used for calculating a spanning tree and maintaining the spanning tree
topology.
• Topology change notification (TCN) BPDUs—Used for notifying the concerned devices of network
topology changes, if any.
Basic concepts in STP
Root bridge
A tree network must have a root bridge. The entire network contains only one root bridge, and all the
other bridges in the network are called "leaf nodes". The root bridge is not permanent, but can change
with changes of the network topology.
Upon initialization of a network, each device generates and periodically sends configuration BPDUs,
with itself as the root bridge. After network convergence, only the root bridge generates and periodically
sends configuration BPDUs. The other devices only forward the BPDUs.
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