Compatibility with 802.1D from Understanding Rapid Spanning Tree Protocol (802.1w)

Compatibility with 802.1D

RSTP is able to interoperate with legacy STP protocols. However, it is important to note that the inherent fast convergence benefits of 802.1w are lost when it interacts with legacy bridges.

Each port maintains a variable that defines the protocol to run on the corresponding segment. A migration delay timer of three seconds also starts when the port comes up. When this timer runs, the current STP or RSTP mode associated to the port is locked. As soon as the migration delay expires, the port adapts to the mode that corresponds to the next BPDU it receives. If the port changes its mode of operation as a result of a BPDU received, the migration delay restarts. This limits the possible mode change frequency.

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For instance, suppose Bridges A and B in the preceding figure both run RSTP, with Switch A designated for the segment. A legacy STP Bridge C is introduced on this link. As 802.1D bridges ignore RSTP BPDUs and drop them, C believes there are no other bridges on the segment and starts to send its inferior 802.1D-format BPDUs. Switch A receives these BPDUs and, after twice hello-time seconds maximum, changes its mode to 802.1D on that port only. As a result, C now understands the BPDUs of Switch A and accepts A as the designated bridge for that segment.

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Notice in this particular case, if Bridge C is removed, Bridge A runs in STP mode on that port even though it is able to work more efficiently in RSTP mode with its unique neighbor B. This is because A does not know Bridge C is removed from the segment. For this particular (rare) case, user intervention is required in order to restart the protocol detection of the port manually.

When a port is in 802.1D compatibility mode, it is also able to handle topology change notification (TCN) BPDUs, and BPDUs with TC or TCA bit set.

 

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

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