Bridge Protocol Data Unit Format And Processing - Cisco Catalyst 2960 series Configuration Manual

If a designated port is in the forwarding state and is not configured as an edge port, it transitions to the blocking
state when the RSTP forces it to synchronize with new root information. In general, when the RSTP forces a
port to synchronize with root information and the port does not satisfy any of the above conditions, its port
state is set to blocking.
After ensuring that all of the ports are synchronized, the switch sends an agreement message to the designated
switch corresponding to its root port. When the switches connected by a point-to-point link are in agreement
about their port roles, the RSTP immediately transitions the port states to forwarding.
Figure 16: Sequence of Events During Rapid Convergence

Bridge Protocol Data Unit Format and Processing

The RSTP BPDU format is the same as the IEEE 802.1D BPDU format except that the protocol version is
set to 2. A new 1-byte Version 1 Length field is set to zero, which means that no version 1 protocol information
is present.
Table 32: RSTP BPDU Flags
Bit
0
1
2–3:
00
01
10
11
Function
Topology change (TC)
Proposal
Port role:
Unknown
Alternate port
Root port
Designated port
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
Information About MSTP
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