In 802.1D, BPDUs basically originate from the Root Bridge and are relayed by all switches down through the tree. Because of this propagation of BPDUs, 802.1D convergence must wait for steady-state conditions before proceeding.
RSTP uses the 802.1D BPDU format for backward compatibility. However, some previously unused bits in the Message Type field are used. The sending switch port identifies itself by its RSTP role and state. The BPDU version also is set to 2 to distinguish RSTP BPDUs from 802.1D BPDUs. In addition, RSTP uses an interactive process so that two neighboring switches can negotiate state changes. Some BPDU bits are used to flag messages during this negotiation.
BPDUs are sent out every switch port at Hello Time intervals, regardless of whether BPDUs are received from the root. In this way, any switch anywhere in the network can play an active role in maintaining the topology. Switches also can expect to receive regular BPDUs from their neighbors. When three BPDUs are missed in a row, that neighbor is presumed to be down, and all information related to the port leading to the neighbor immediately is aged out. This means that a switch can detect a neighbor failure in three Hello intervals (default 6 seconds), versus the Max Age timer interval (default 20 seconds) for 802.1D.
Because RSTP distinguishes its BPDUs from 802.1D BPDUs, it can coexist with switches still using 802.1D. Each port attempts to operate according to the STP BPDU that is received. For example, when an 802.1D BPDU (version 0) is received on a port, that port begins to operate according to the 802.1D rules.
However, each port has a measure that locks the protocol in use, in case BPDUs from both 802.1D and RSTP are received within a short time frame. This can occur if the switches in a network are being migrated from one STP type to another. Instead of flapping or toggling the STP type during a migration, the switch holds the protocol type for the duration of a migration delay timer. After this timer expires, the port is free to change protocols if needed.
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