19-13
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 19 Configuring Standard-Compliant IEEE MST
Understanding RSTP
Figure 19-5 Sequence of Events During Rapid Convergence
Bridge Protocol Data Unit Format and Processing
These sections describe bridge protocol data unit (BPDU) format and processing:
• BPDU Format and Processing Overview, page 19-13
• Processing Superior BPDU Information, page 19-14
• Processing Inferior BPDU Information, page 19-14
BPDU Format and Processing Overview
The RSTP BPDU format is the same as the 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 19-3 describes the RSTP flag fields.
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
88761
Table 19-3 RSTP BPDU Flags
Bit Function
0 Topology change (TC)
1 Proposal
2–3:
00
01
10
11
Port role:
Unknown
Alternate port or backup port
Root port
Designated port
4Learning
5Forwarding
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