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14-9
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 14 Configuring MSTP
Understanding RSTP
Figure 14-3 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 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 14-2 shows the RSTP flag fields.
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.
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
74008
Ta b l e 14-2 RSTP BPDU Flags
Bit Function
0 Topology change (TC)
1 Proposal
2–3:
00
01
10
11
Port role:
Unknown
Alternate port
Root port
Designated port
4 Learning
5 Forwarding
6 Agreement
7 Topology change acknowledgement (TCA)