Bridge Protocol Data Unit Format And Processing - Cisco Catalyst 2950 Software Configuration Manual

Chapter 11 Configuring RSTP and MSTP
Figure 11-2 Sequence of Events During Rapid Convergence
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 11-2 RSTP BPDU Flags
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.
78-11380-04
4. Agreement
5. Forward
2. Block
9. Forward
6. Proposal
Table 11-2
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.
Catalyst 2950 Desktop Switch Software Configuration Guide
Understanding RSTP
10. Agreement
11-5