H3C Technologies H3C WX5500E Series Access Controllers - Manual (Page 65)

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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, putting them in a standby state, which still allows for
link redundancy.
Recent versions of STP include the Rapid Spanning Tree Protocol (RSTP), Per VLAN Spanning Tree (PVST),
and the Multiple Spanning Tree Protocol (MSTP).
STP
STP was developed based on the 802.1d standard of IEEE to eliminate loops at the data link layer in a
LAN. Networks often have redundant links as backups in case of failures, but loops are a very serious
problem. Devices that run 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 received duplicate packets from decreasing the performance of network devices.
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 improved 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. This chapter uses BPDUs to represent all types of spanning tree protocol packet.
STP-enabled network devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient
information for network devices to complete spanning tree calculations.
STP uses the following types of BPDUs:
• Configuration BPDUs—Used by network devices to calculate a spanning tree and maintain the
spanning tree topology.
• Topology change notification (TCN) BPDUs—Used to notify network devices of the network
topology changes.
Configuration BPDUs contain sufficient information for network devices to complete spanning tree
calculations. Important fields in a configuration BPDU include the following:
• 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 transitions to the forwarding state.