7-6
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Spanning Tree Protocols Work
Spanning-Tree Port States
Topology changes can take place in a switched network due to a link coming up or a link going down
(failing). When a switch port transitions directly from nonparticipation in the topology to the forwarding
state, it can create temporary data loops. Ports must wait for new topology information to propagate
through the switches in the LAN before they can start forwarding frames. Also, they must allow the
frame lifetime to expire for frames that have been forwarded using the old topology.
Note
With Cisco IOS Release 12.1.(1)E or later releases on the Multilayer Switch Feature Card (MSFC), the
Address Resolution Protocol (ARP) on the STP Topology Change Notification feature ensures that
excessive flooding does not occur when the MSFC receives a topology change notification (TCN) from
the supervisor engine. The feature causes the MSFC to send ARP requests for all the ARP entries
belonging to the VLAN interface where the TCN is received. When the ARP replies come back, the
Policy Feature Card (PFC) learns the MAC entries, which were lost as a result of the topology change.
Learning the entries immediately following a topology change prevents excessive flooding later. There
is no configuration required on the MSFC. This feature works with supervisor engine software
release 5.4(2) or later releases.
At any given time, each port on a switch using a spanning-tree protocol is in one of these states:
•
Blocking
•
Listening
•
Learning
•
Forwarding
•
Disabled
A port moves through these states as follows:
•
From initialization to blocking
•
From blocking to listening or to disabled
•
From listening to learning or to disabled
•
From learning to forwarding or to disabled
•
From forwarding to disabled
Figure 7-2
illustrates how a port moves through the states.
