Compatibility of 802.1W with 802.1D

802.1W-enabled bridges are backward compatible with IEEE 802.1D bridges. This compatibility is managed on a per-port basis by the Port Migration state machine. However, intermixing the two types of bridges in the network topology is not advisable if you want to take advantage of the rapid convergence feature.

Compatibility with 802.1D means that an 802.1W-enabled port can send BPDUs in the STP or 802.1D format when one of the following events occur:

  • The port receives a legacy BPDU. A legacy BPDU is an STP BPDU or a BPDU in an 802.1D format. The port that receives the legacy BPDU automatically configures itself to behave like a legacy port. It sends and receives legacy BPDUs only.
  • The entire bridge is configured to operate in an 802.1D mode when an administrator sets the bridge parameter to zero at the CLI, forcing all ports on the bridge to send legacy BPDUs only.

Once a port operates in the 802.1D mode, 802.1D convergence times are used and rapid convergence is not realized.

For example, in the following figure, Switch 10 and Switch 30 receive legacy BPDUs from Switch 20. Ports on Switch 10 and Switch 30 begin sending BPDUs in STP format to allow them to operate transparently with Switch 20.

802.1W Bridges with an 802.1D Bridge

Once Switch 20 is removed from the LAN, Switch 10 and Switch 30 receive and transmit BPDUs in the STP format to and from each other. This state will continue until the administrator enables the force-migration-check command to force the bridge to send RSTP BPDU during a migrate time period. If ports on the bridges continue to hear only STP BPDUs after this migrate time period, those ports will return to sending STP BPDUs. However, when the ports receive RST BPDUs during the migrate time period, the ports begin sending RST BPDUs. The migrate time period is non-configurable. It has a value of three seconds.

Note: The IEEE standards state that 802.1W bridges need to interoperate with 802.1D bridges. IEEE standards set the path cost of 802.1W bridges to be between 1 and 200,000,000; whereas path cost of 802.1D bridges are set between 1 and 65,535. In order for the two bridge types to be able to interoperate in the same topology, the administrator needs to configure the bridge path cost appropriately. Path costs for either 802.1W bridges or 802.1D bridges need to be changed; in most cases, path costs for 802.1W bridges need to be changed.
Parent topic: 802.1W Rapid Spanning Tree (RSTP)