Bridge Protocol Data Unit Format And Processing - Cisco Catalyst 3750-X Software Configuration Manual

Understanding RSTP
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 21-5
8. Agreement

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 21-3
Bit
0
1
2–3:
4
5
6
7
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
21-12
Sequence of Events During Rapid Convergence
4. Agreement
5. Forward
2. Block
9. Forward
6. Proposal
Table 21-3
RSTP BPDU Flags
Function
Topology change (TC)
Proposal
Port role:
00 Unknown
01 Alternate port
10 Root port
11 Designated port
Learning
Forwarding
Agreement
Topology change acknowledgement (TCA)
Figure 21-5.
1. Proposal
Edge port
3. Block
11. Forward
7. Proposal
Root port
Designated port
shows the RSTP flag fields.
Chapter 21 Configuring MSTP
10. Agreement
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