Bridge Protocol Data Unit Format And Processing - Cisco Catalyst 2960 Software Configuration Manual

Understanding RSTP
After ensuring that all of the ports are synchronized, the switch sends an agreement message to the
designated switch corresponding to its root port. When the switches connected by a point-to-point link
are in agreement about their port roles, the RSTP immediately transitions the port states to forwarding.
The sequence of events is shown in
Figure 17-5
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 1-byte Version 1 Length field is set to zero, which means that no version 1 protocol
information is present.
Table 17-3
Bit
0
1
2–3:
4
5
6
7
Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE
17-12
Sequence of Events During Rapid Convergence
4. Agreement
5. Forward
2. Block
9. Forward
6. Proposal
Table 17-3
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)
Figure 17-5.
1. Proposal
Edge port
3. Block
11. Forward
7. Proposal
Root port
Designated port
shows the RSTP flag fields.
Chapter 17 Configuring MSTP
10. Agreement
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