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7-2 User Guide for the Avaya P580 and P882 Multiservice Switches, v6.1
Chapter 7
In STP, a port that is blocking must change to listening and then learning
before it can change to forwarding. Normally, a port remains in the listening
state for 15 seconds and in the learning state for 15 seconds. With these two
15-second delays, a port normally takes 30 seconds to change from
blocking to forwarding. This 30-second transition time results in a 30-
second loss of traffic, which is not acceptable in many of today’s networks.
Port States STP has four different port states: listening, learning, blocking, forwarding,
and disabled. RSTP has combined three of these states— listening,
blocking, and disabled—into a single port state: discarding.
Interoperability
with STP
RSTP is completely interoperable with STP. Switches running RSTP and
switches running STP can be on the same network and work together to
detect and break loops in the network.
Interoperability is achieved by the ability of RSTP to detect the presence of
bridges running STP and to operate in common-spanning-tree mode. When
an RSTP bridge is connected to an STP bridge and receives STP bridge
protocol data units (BPDUs), the RSTP bridge sends only STP BPDUs out
the port that is connected to the STP bridge. If a bridge running STP
receives RSTP BPDUs, the STP bridge drops the RSTP BPDUs.
You can also manually set the P580 or P882 to run common Spanning Tree.
Upgrading to
RSTP
To ensure that your network is not disrupted when you upgrade the switch
from v5.x application software to v6.0 or later, RSTP has the following
settings:
■ The default setting for Spanning Tree version is common Spanning
Tree.
■ Switch ports for which fast start was enabled are set as edge ports.
In v6.0 and later, the fast start feature is replaced with the ability to
set switch ports as edge ports. For information on setting switch
ports as edge ports, see “Configuring Spanning Tree Settings for
Switch Ports” later in this chapter.
■ All settings for port priorities and path costs are preserved.