Specifications

13-2
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-07
Chapter 13 Configuring RSTP and 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:
• Spanning-Tree Instances Using RSTP, page 13-2
• Port Roles and the Active Topology, page 13-2
• Rapid Convergence, page 13-3
• Synchronization of Port Roles, page 13-4
• Bridge Protocol Data Unit Format and Processing, page 13-5
For configuration information, see the “Configuring RSTP and MSTP Features” section on page 13-11.
Spanning-Tree Instances Using RSTP
The switch supports the PVRST and a maximum of 64 spanning-tree instances. When PVRST is
enabled, the switch uses RSTP instead of STP to provide faster convergence. For information about the
PVST, see the “Supported Spanning-Tree Instances” section on page 12-2. For information about how
spanning tree interoperates with the VLAN Trunking Protocol (VTP), see the “STP Configuration
Guidelines” section on page 12-11.
When a network contains switches running PVRST and switches running PVST, we recommend that the
PVRST switches and PVST switches be in different spanning-tree instances. In the PVRST
spanning-tree instances, the root switch must be a PVRST switch. In the PVST instances, the root switch
must be a PVST switch. The PVST switches should be at the edge of the network.
Port Roles and the Active Topology
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by determining
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 “Election of the Root
Switch” section on page 12-3. 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 designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected together 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 port with the alternate or
backup port role is excluded from the active topology.