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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP Understanding RSTP
Figure 12-2 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 one-byte version 1 Le ngth field is set to zero, which means that no version 1 protocol
information is present. Table 12-2 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 send ing switch s ets the agreem ent flag in th e RSTP BPDU to ac cept the pre vi ous propos al. The p ort
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
Table 12-2 RSTP BPDU Flags
Bit Function
0 To pology c hange (TC )
1 Proposal
23:
00
01
10
11
Port role:
Unknown
Alterna te po rt
Root port
Designate d port
4Learning
5Forwarding
6 Agreement
7 To pology cha nge acknowledg ement (TCA )