Specifications

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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide

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Chapter 13 Configuring RSTP and MSTP

Interoperability with 802.1D STP

A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate

with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration BPDU (a BPDU with

the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MSTP switch can also detect

that a port is at the boundary of a region when it receives a legacy BPDU, an MSTP BPDU (version 3)

associated with a different region, or an RSTP BPDU (version 2).

However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D

BPDUs because it cannot determine whether the legacy switch has been removed from the link unless

the legacy switch is the designated switch. Also, a switch might continue to assign a boundary role to a

port when the switch to which this switch is connected has joined the region. To restart the protocol

migration process (force the renegotiation with neighboring switches), you can use the clear

spanning-tree detected-protocols privileged EXEC command.

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.

Configuring RSTP and MSTP Features

These sections describe how to configure basic RSTP and MSTP features:

• Default RSTP and MSTP Configuration, page 13-12

• RSTP and MSTP Configuration Guidelines, page 13-12

• Specifying the MST Region Configuration and Enabling MSTP, page 13-13 (required)

• Configuring the Root Switch, page 13-14 (optional)

• Configuring a Secondary Root Switch, page 13-16 (optional)

• Configuring the Port Priority, page 13-17 (optional)

• Configuring the Path Cost, page 13-18 (optional)

• Configuring the Switch Priority, page 13-19 (optional)

• Configuring the Hello Time, page 13-19 (optional)

• Configuring the Forwarding-Delay Time, page 13-20 (optional)

• Configuring the Maximum-Aging Time, page 13-21 (optional)

• Configuring the Maximum-Hop Count, page 13-21 (optional)

• Specifying the Link Type to Ensure Rapid Transitions, page 13-22 (optional)

• Restarting the Protocol Migration Process, page 13-22 (optional)