Configuring The Spanning Tree; Using Stp Protocol Packets; Outlining The Basic Concepts In Stp - HP A5830 Configuration Manual

Configuring the spanning tree

Networks often have redundant links as backups in case of failures, but loops are a very serious problem.
STP, a Layer 2 management protocol, eliminates loops in a LAN's data link layer by putting redundant
links in a standby state that still allows for link redundancy.
Devices that run STP detect loops 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 network loop, 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 enhanced spanning tree protocols derived from that protocol.
STP versions include RSTP, PVST, and MSTP.

Using STP protocol packets

STP uses 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.
STP uses the following types of BPDUs:
Configuration BPDUs, used by network devices to calculate a spanning tree and maintain the

spanning tree topology
 TCN BPDUs, which notify network devices of the 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 transition port state.

Outlining the basic concepts in STP

Root bridge
A tree network must have a root bridge. The entire network contains only one root bridge. The root bridge
is not permanent, but can change with changes of the network topology.
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