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Chapter 15
Configuring MSTP
Figure 15-3
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 one-byte Version 1 Length field is set to zero, which means that no version 1 protocol
information is present.
Table 15-2
Bit
0
1
2–3:
4
5
6
7
The sending switch sets the proposal flag in the RSTP BPDU to propose itself as the designated switch
on that LAN. The port role in the proposal message is always set to the designated port.
The sending switch sets the agreement flag in the RSTP BPDU to accept the previous proposal. The port
role in the agreement message is always set to the root port.
78-17058-01
Sequence of Events During Rapid Convergence
4. Agreement
5. Forward
2. Block
9. Forward
6. Proposal
Table 15-2
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)
1. Proposal
Edge port
3. Block
11. Forward
7. Proposal
Root port
Designated port
shows the RSTP flag fields.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
Understanding RSTP
10. Agreement
15-9
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