MSTP compatibility with RSTP or STP

IEEE 802.1s MSTP includes RSTP functionality and is designed to be compatible with both IEEE 802.1D and 802.1w spanning tree protocols. Using the default configuration values, your switches will interoperate effectively with RSTP and STP devices. MSTP automatically detects when the switch ports are connected to non-MSTP devices in the spanning tree and communicates with those devices using 802.1D or 802.1w STP BPDU packets, as appropriate.

To enable effective interoperation with STP (802.1D) configured devices, however, you may need to adjust the default configuration values. Here are two such examples:
  • The rapid state transitions employed by MSTP may result in an increase in the rates of frame duplication and misordering in the switched LAN. To allow the switch to support applications and protocols that may be sensitive to frame duplication and misordering, you can disable rapid transitions by setting the Force Protocol Version parameter to STP-compatible. The value of this parameter applies to all ports on the switch.

  • One of the benefits of MSTP is the implementation of a larger range of port path costs, which accommodates higher network speeds. However, this can create some incompatibility between devices running the older 802.1D STP. You can adjust to this incompatibility by implementing the global spanning tree legacy-path cost command.

RSTP and MSTP implement a greater range of path costs than 802.1D STP, and use different default path cost values to account for higher network speeds. These values are shown in the following table.

Port type

802.1D STP path cost

RSTP and MSTP path cost

10 Mbps


2 000 000

100 Mbps


200 000

1 Gbps


20 000

Because the maximum value for the path cost allowed by 802.1D STP is 65535, devices running that version of spanning tree cannot be configured to match the values defined by MSTP, at least for 10 Mbps and 100 Mbps ports. In LANs where there is a mix of devices running 802.1D STP, RSTP, and MSTPs, you should reconfigure the devices so the path costs match for ports with the same network speeds.