Cisco Systems 3750E manual Bridge Protocol Data Unit Format and Processing, Bit Function, 19-12

Chapter 19 Configuring MSTP

Understanding RSTP

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. The sequence of events is shown in Figure 19-5.

Figure 19-5 Sequence of Events During Rapid Convergence

4. Agreement

1. Proposal

	5. Forward
	Edge port
2. Block	3. Block
9. Forward	11. Forward

8. Agreement 6. Proposal 7. Proposal

Root port

Designated port

10. Agreement

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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 19-3shows the RSTP flag fields.

Table 19-3 RSTP BPDU Flags

Bit

0

1

2–3:

00

01

10

11

4

Function

Topology change (TC)

Proposal

Port role:

Unknown

Alternate port

Root port

Designated port

Learning

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
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