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

RSTP Overview
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.
Figure 9: 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.
Table 11: RSTP BPDU Flags
Bit
0
1
2–3:
00
01
10
11
4
5
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
56
Function
Topology change (TC)
Proposal
Port role:
Unknown
Alternate port
Root port
Designated port
Learning
Forwarding
Configuring Multiple Spanning-Tree Protocol
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