10-14
Bridging—Transmitting Non-IP Traffic or Merging Two Networks
Configuring Spanning Tree
When a change in network topology makes STP determine that a new port
must become active, the port first passes through the listening and learning
states. (When STP is initially enabled and devices exchange configuration
BPDUs, all ports move through the listening and learning states until STP
determines whether they should become blocked or forwarding ports.)
In the listening state, the port processes BPDUs to determine whether it is
indeed the best connection to the root. If within 15 seconds it does not receive
a BPDU advertising a better connection, the port enters the learning state.
In the learning state, the port begins to transmit BPDUs as well as receive
them. This notifies other active ports of its presence, and the learning port
becomes part of the network topology. The port also listens for frames to build
up its address database. After 15 seconds, it enters the forwarding state and
begins to forward traffic. (If the port receives a better BPDU than it can
transmit during this interval, it returns to blocking.)
As you can see, the process of a port moving from blocked to forwarding can
be quite lengthy. A network running STP usually takes a minute to converge
after a link failure, and the network outage during this delay is not acceptable
for many environments.
RSTP Improvements
RSTP can reduce convergence time to less than 1 second.
RSTP does not always force ports to go through the listening and learning
states and removes the distinction between blocked and disabled ports.
RSTP speeds convergence by:
■ defining new roles for certain ports:
•edge ports
• backup ports
• alternate ports
• ports on a point-to-point connection
■ using sync to activate point-to-point ports
■ immediately purging old information
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