Spanning-Tree Interface States; Figure 6-1 Spanning-Tree Topology - Cisco ONS 15454 Software Feature And Configuration Manual

Chapter 6 Configuring STP and RSTP
Figure 6-1
DP
DP
RP = root port
DP = designated port
When the spanning-tree topology is calculated based on default parameters, the path between source and
destination end stations in a switched network might not be ideal. For instance, connecting higher-speed
links to an interface that has a higher number than the root port can cause a root-port change. The goal
is to make the fastest link the root port.

Spanning-Tree Interface States

Propagation delays can occur when protocol information passes through a switched LAN. As a result,
topology changes can take place at different times and at different places in a switched network. When
an interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding
state, it can create temporary data loops. Interfaces must wait for new topology information to propagate
through the switched LAN before starting to forward frames. They must allow the frame lifetime to
expire for forwarded frames that have used the old topology.
Each Layer 2 interface on a switch using spanning tree exists in one of these states:
•
•
•
•
•
An interface moves through these states:
1.
2.
3.
4.
5.
Cisco ONS 15454 SONET/SDH ML-Series Multilayer Ethernet Card Software Feature and Configuration Guide, R4.0
January 2004
Spanning-Tree Topology
A
DP
RP DP
DP
RP DP
RP
B
Blocking—The interface does not participate in frame forwarding.
Listening—The first transitional state after the blocking state when the spanning tree determines
that the interface should participate in frame forwarding.
Learning—The interface prepares to participate in frame forwarding.
Forwarding—The interface forwards frames.
Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on
the port, or no spanning-tree instance running on the port.
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
D
DP
C

Spanning-Tree Interface States

6-5