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Cisco IE 3010 Switch Software Configuration Guide
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Chapter 18 Configuring MSTP
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 IEEE 802.1D STP to select the switch with the highest switch
priority (lowest numerical priority value) as the root switch as described in the “Spanning-Tree Topology
and BPDUs” section on page 17-3. Then the RSTP assigns one of these port roles to individual ports:
• Root port—Provides the best path (lowest cost) when the switch forwards packets to the root switch.
• Designated port—Connects to the designated switch, which incurs the lowest path cost when
forwarding packets from that LAN to the root switch. The port through which the designated switch
is attached to the LAN is called the designated port.
• Alternate port—Offers an alternate path toward the root switch to that provided by the current root
port.
• Backup port—Acts as a backup for the path provided by a design ated 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 switch 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 p ort with the alternate
or backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the n etwork, the RSTP ensures that every root
port and designated port immediately transition to the forwar ding state while all alternate and backup
ports are always in the discarding state (equivalent to blocking in IEEE 802.1D). The port state controls
the operation of the forwarding and learning processes. Ta ble 18-2 provides a comp arison of
IEEE 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.
Rapid Convergence
The RSTP provides for rapid recovery of connectivity following the failure of a switch, a switch port,
or a LAN. It provides rapid convergence for edge ports, new root ports, and port s connected through
point-to-point links as follows:
• Edge ports—If you configure a port as an edge port on an RSTP switch by using the spanning-tree
portfast interface configuration command, the edge port immediately transitions to the forwarding
state. An edge port is the same as a Port Fast-enabled port, and you should enable it only on ports
that connect to a single end station.
Tab l e 18-2 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