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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 17-5
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 17-3
Bit
0
1
2–3:
4
5
6
7
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
17-12
Sequence of Events During Rapid Convergence
4. Agreement
5. Forward
2. Block
9. Forward
6. Proposal
Table 17-3
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
17-5.
1. Proposal
Edge port
3. Block
11. Forward
7. Proposal
Root port
Designated port
shows the RSTP flag fields.
Chapter 17
Configuring MSTP
10. Agreement
OL-8915-03
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