Overview - HP ProCurve Secure 7000dl Series Basic Management And Configuration Manual

Bridging—Transmitting Non-IP Traffic or Merging Two Networks
Configuring Spanning Tree
10-12
The overview provides a brief background in STP and RSTP for those who
want to learn more about how the protocols function.

Overview

Network devices in a Data Link Layer network, such as bridges and switches,
run STP or RSTP. Bridged interfaces on the ProCurve Secure Router also
participate in the spanning tree protocol. The protocol helps devices to
generate a loopless topology.
Unlike routers, switches do not time out messages. Loops in a network topol-
ogy can lead to duplicated messages and broadcast storms, which can bring
a network down. However, the redundant links that cause loops can also be
desirable: they protect against loss of connectivity when a connection fails.
STP allows network devices to generate a shared loopless topology, blocking
all redundant links. However, if active connections fail, redundant links can
become active for as long as the original path is down.
RSTP is now the spanning tree standard. It improves convergence time to less
than one second and is the recommended implementation.
STP BPDUs
Devices running STP send and listen for configuration bridge protocol data
units (BPDUs) to determine the spanning tree topology. Each BPDU includes:
the identifier (priority plus MAC address) of the source port
the identifier of the root device
the cost between the source port and root device
Using these BPDUs, each device can determine:
Which device is root—The root is the device from which the tree topology
originates. All ports on the root must remain active. When STP is originally
implemented, each device believes that it is the root. In the initial
exchange of BPDUs, the device with the lowest identifier is elected root.
You can ensure that a router interface is elected by lowering its priority
number.
Which switch provides the local device the best connection to the root—
This switch becomes the device's designated switch.
Which port provides the best connection to the designated switch—This
becomes the root port.