IBM 12.1(22)EA6 User Manual
10-8
Cisco Systems Intelligent Gigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
24R9746
Chapter 10 Configuring MSTP
Understanding RSTP
Figure 10-2
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 1-byte version 1 Length field is set to zero, which means that no version 1 protocol
information is present.
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.
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.
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.
role in the agreement message is always set to the root port.
2. Block
9. Forward
1. Proposal
4. Agreement
6. Proposal
Root port
Designated port
Designated port
8. Agreement
10. Agreement
Edge port
7. Proposal
5. Forward
3. Block
11. Forward
11. Forward
74008
Table 10-2
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)