H3C Technologies H3C SecPath F1000-E - Manual (Page 81)

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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 also allows for link redundancy.
Recent 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.
Introduction to 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). Networks often have redundant links as backups in case of failures, but loops
are a very serious problem. Devices running STP 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 device performance caused by receiving duplicate packets.
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.
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.
STP uses the following types of BPDUs:
• Configuration BPDUs—Used for calculating a spanning tree and maintaining the spanning tree
topology.
• Topology change notification (TCN) BPDUs—Notify network devices of network topology changes.
Basic concepts in STP
Root bridge
A tree network must have a root bridge.
There is only one root bridge in the entire network. 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 out configuration BPDUs
with itself as the root bridge. After network convergence, only the root bridge generates and periodically
sends out configuration BPDUs. The other devices only forward the BPDUs.
Root port
On a non-root bridge, the port nearest to the root bridge is the root port. The root port communicates with
the root bridge. Each non-root bridge has only one root port. The root bridge has no root port.