RSTP Direct Failure

RSTP Direct Failure

RSTP has natively implemented the same type of mechanisms involved in UplinkFast and BackboneFast. In Figure 10-12, when Switch3 loses its RP, it immediately transitions the Alternate port into forwarding mode. In legacy STP mode, a direct failure of this type would have taken 30 seconds. Unlike the UplinkFast mechanism, the RSTP mechanism does not use dummy multicast generation to flush the CAM entries. TCs generated by RSTP to the upstream switch clear the appropriate CAM entries associated with the broken link.

When a TC bit is set, the switch starts a TC While timer equal to 4 seconds (2 * hello interval) for all its non-edge ports. It flushes the MAC addresses that were associated with that port. The upstream switch that received the TC BPDU will flush its MAC addresses from all ports except the port that received the BPDU. This process streamlines the convergence process. In legacy STP, the TCNs first needed to be propagated to the Root, which afterward generated configuration BPDUs that were propagated back to the spanning-tree domain. The amount of time it took to converge the network was contingent upon how big the spanning-tree domain was. In RSTP, the TCs are flooded quickly to non-edge ports and RPs, and the upstream switches flush their CAM entries, resulting in faster convergence time. The downside to this process is some flooding does take place in the network.