Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 298)

FIGURE 61 Completing the TCN propagation
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.
Compatibility of 802.1W with 802.1D
298 FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide
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