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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 19 Configuring Standard-Compliant IEEE MST
Understanding RSTP
Port Roles and the Active Topology
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the
active topology. The RSTP builds upon the 802.1D STP to select the router with the highest switch
priority (lowest numerical priority value) as the root bridge as described in the
“Election of the Root
Bridge” section on page 20-4
. The RSTP then assigns one of these port roles to individual ports:
•
Root port—Provides the best path (lowest cost) when the router forwards packets to the root bridge.
•
Designated port—Connects to the designated router, which incurs the lowest path cost when
forwarding packets from that LAN to the root bridge. The port through which the designated router
is attached to the LAN is called the designated port.
•
Alternate port—Offers an alternate path toward the root bridge to that provided by the current root
port.
•
Backup port—Acts as a backup for the path provided by a designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected in a loopback by a
point-to-point link or when a router has two or more connections to a shared LAN segment.
•
Disabled port—Has no role within the operation of the spanning tree.
A port with the root or a designated port role is included in the active topology. A port with the alternate
or backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root
port and designated port immediately transition to the forwarding state while all alternate and backup
ports are always in the discarding state (equivalent to blocking in 802.1D). The port state controls the
operation of the forwarding and learning processes.
Table 19-2
provides a comparison of 802.1D and
RSTP port states.
To be consistent with Cisco STP implementations, this guide defines the port state as
blocking
instead
of
discarding
. Designated ports start in the listening state.
Table 19-2 Port State Comparison
Operational Status
STP Port State
(IEEE 802.1D)
RSTP Port State
Is Port Included in the
Active Topology?
Enabled
Blocking
Discarding
No
Enabled
Listening
Discarding
No
Enabled
Learning
Learning
Yes
Enabled
Forwarding
Forwarding
Yes
Disabled
Disabled
Discarding
No
