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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter18 Configuring MSTP
Understanding RSTP
If a designated port is in the forwarding state and is not configured as an edge port, it transitions to the
blocking state when the RSTP forces it to synchronize w ith new root information. In general, when the
RSTP forces a port to synchronize with root information and the port d oes no t satisf y any of t he a bove
conditions, its port state is set to blocking.
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 co nnecte d by a poin t-to -poi nt li nk
are in agreement about their port roles, the RSTP immedi ate l y tran sitions the port states to forwarding.
The sequence of events is shown in Figure 18-5.
Figure18-5 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. Table18-3 shows the RSTP flag fields.
2. Block
9. Forward
1. Proposal4. Agreement
6. Proposal
Root port
Designated port
8
. Agreement 10. Agreemen
t
Edge port
7. Proposal
5. Forward
3. Block
11. Forward
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Table18-3 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
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