H3C Technologies H3C S12500 Series Switches - Manual (Page 67)

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Configuring spanning tree protocols
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 include the Rapid Spanning Tree Protocol (RSTP), the Multiple Spanning Tree
Protocol (MSTP), and the Per VLAN Spanning Tree Protocol (PVST).
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. Throughout this document, BPDUs refer to STP BPDUs.
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, which notify network devices of network topology
changes.
A configuration BPDU contains the following information for network devices to complete spanning tree
calculation:
• Root bridge ID—Comprises the priority and MAC address of the root bridge.
• Root path cost—The cost of the path to the root bridge.
• Designated bridge ID—Comprises the priority and MAC address of the designated bridge.
• Designated port ID—Comprises the port priority and global port number.
• Message age—The times that the configuration BPDU has been forwarded on the network.
• Max age—The maximum age of the configuration BPDU.
• Hello time—The transmission interval of the configuration BPDU.
• Forward delay—The delay before a port transits to the forwarding state.