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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter15 Configuring MSTP
Understanding RSTP
Understanding RSTP
The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree.
Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the
default settings in the 802.1D spanning tree), which is critical for networks carrying delay-sensitive
traffic such as voice and video.
These section describes how the RSTP works:
•Port Roles and the Active Topology, page 15-6
•Rapid Convergence, page 15-7
•Synchronization of Port Roles, page 15-8
•Bridge Protocol Data Unit Format and Processing, page 15-9
For configuration information, see the “Configuring MSTP Features” section on page 15-11.

Port Roles and the Active Topology

The RSTP provides rapid convergence of the spanning tree by assigning port roles and by de termi ning
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 14-3. 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 th rough 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 designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected toge ther in a loopback by
a point-to-point link or when a switch has two or more co nne ctio ns t o a sh ar ed LAN segme nt.
•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 net work, the RSTP ensures that every root
port and designated port immediately transition to the forwar ding stat e w hile a ll a ltern at e a nd bac kup
ports are always in the discarding state (equivalent to blocking in 80 2.1D) . The por t st at e cont ro ls t he
operation of the forwarding and learning processes. Ta ble 15 -1 provides a comparison of 802.1D and
RSTP port states.