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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 15 Configuring MSTP
Understanding RSTP
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are
RSTP BPDUs. Therefore, MSTP switches send either a Version 0 configuration and TCN BPDUs or
Version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch
of which is either a single spanning-tree switch or a switch with a different MST configuration.
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 IEEE 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 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 14-3. Then the RSTP assigns one of these port roles to individual ports.
Note On the Cisco ME switch, only NNIs are RSTP ports. UNIs do not participate in STP.
• 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.
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 15-1 provides a comparison of IEEE 802.1D
and RSTP port states.
