H3C Technologies H3C S5560 Series Switches - Manual (Page 94)

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Configuring spanning tree protocols
Spanning tree protocols eliminate loops in a physical link-redundant network by selectively blocking
redundant links and putting them in a standby state.
The recent versions of STP include the Rapid Spanning Tree Protocol (RSTP), the 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 running STP detect loops in the network by exchanging information with one another.
They 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.
In a narrow sense, STP refers to IEEE 802.1d STP. In a 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. This chapter uses BPDUs to represent all types of spanning tree 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 by the network devices to calculate a spanning tree and maintain the
spanning tree topology.
• Topology change notification (TCN) BPDUs—Notify network devices of network topology changes.
Configuration BPDUs contain sufficient information for the network devices to complete spanning tree
calculation. Important fields in a configuration BPDU include the following:
• Root bridge ID—Consisting of the priority and MAC address of the root bridge.
• Root path cost—Cost of the path to the root bridge denoted by the root identifier from the
transmitting bridge.
• Designated bridge ID—Consisting of the priority and MAC address of the designated bridge.
• Designated port ID—Consisting of the priority and global port number of the designated port.
• Message age—Age of the configuration BPDU while it propagates in the network.
• Max age—Maximum age of the configuration BPDU stored on the switch.
• Hello time—Configuration BPDU transmission interval.
• Forward delay—Delay that STP bridges use to transit port state.