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Configuring the spanning tree
As a Layer 2 management protocol, the Spanning Tree Protocol (STP) eliminates Layer 2 loops by
selectively blocking redundant links in a network, and in the mean time, allows for link redundancy.
STP evolves as the network grows. The later versions of STP are the Rapid Spanning Tree Protocol (RSTP)
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
local area network (LAN). Routers running this protocol 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 performance of network devices caused by duplicate
packets received.
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.
In STP, BPDUs have the following types:
Configuration BPDUs—Used for calculating a spanning tree and maintaining the spanning tree
•
topology.
Topology change notification (TCN) BPDUs—Used for notifying the involved routers of network
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topology changes, if any.
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—Cost of the path to the root bridge.
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•
Designated bridge ID—Comprises the priority and MAC address of the designated bridge.
Designated port ID—Comprises the port priority and global port number.
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Message age—Times that the configuration BPDU has been forwarded on the network.
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Max age—Maximum age of the configuration BPDU.
Hello time—Transmission interval of the configuration BPDU.
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Forward delay—Delay before a port transitions to the forwarding state.
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