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Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 21 Configuring MSTP
Information About Configuring MSTP
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 immedi ately transitions the port states to forwarding.
The sequence of events is shown in Figure 21-5.
Figure 21-5 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 BP DU 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 shows the RSTP flag fields.
2. Block
9. Forward
1. Proposal4. Agreement
6. Proposal
Root port
Designated port
8. Agreement 10. Agreement
Edge port
7. Proposal
5. Forward
3. Block
11. Forward
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Tab l e 21-3 RSTP BPDU Flags
Bit Function
0Topology change (TC)
1Proposal
2–3:
00
01
10
11
Port role:
Unknown
Alternate port
Root port
Designated port
4Learning
5Forwarding
6Agreement
7Topology change acknowledgement (TCA)