HP FlexNetwork NJ5000, User Manual

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Configuring MSTP
Overview
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) and the Multiple
Spanning Tree Protocol (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 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.
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. 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 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.
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 a device.
•
Hello time
—Configuration BPDU transmission interval.
•
Forward delay
—Delay that STP bridges use to transit port state.
The descriptions and examples in this chapter only use the following fields in the configuration
BPDUs:
•
Root bridge ID (represented by device priority)