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Understanding Rapid Spanning Tree Protocol (802.1w) - Cisco

A port is designated if it can send the best BPDU on the segment to which it is connected.
An alternate port receives more useful BPDUs from another bridge and is a port blocked.
A backup port receives more useful BPDUs from the same bridge it is on and is a port blocked.
The rationale is that an alternate port provides an alternate path to the root bridge and therefore can replace the root port if it fails. Of course, a backup port provides redundant connectivity to the same segment and cannot guarantee an alternate connectivity to the root bridge.
A bridge now sends a BPDU with its current information every seconds (2 by default), even if it does not receive any from the root bridge.

Understanding Multiple Spanning Tree Protocol (802.1s) - Cisco

The MRecord contains enough information (mostly root bridge and sender bridge priority parameters) for the corresponding instance to calculate its final topology. The MRecord does not need any timer-related parameters such as hello time, forward delay, and max age that are typically found in a regular IEEE 802.1d or 802.1q CST BPDU. The only instance in the MST region to use these parameters is the IST; the hello time determines how frequently BPDUs are sent, and the forward delay parameter is mainly used when rapid transition is not possible (remember that rapid transitions do not occur on shared links). As MSTIs depend on the IST to transmit their information, MSTIs do not need those timers.