After roles of all RSTP ports are determined, the topology convergence is completed.
l RSTP redefines port states.
RSTP deletes two port states defined in STP and reduces the number of port states to 3.
Depending on whether a port can forward user traffic and learn MAC addresses, the port
may be in any of the following states:
– If the port does not forward user traffic or learn MAC addresses, it is in the Discarding
state.
– If the port does not forward user traffic but learns MAC addresses, it is in the Learning
state.
– If the port forwards user traffic and learns MAC addresses, it is in the Forwarding state.
Table 6-12 compares the port states defined in STP and RSTP.
NOTE
Port states are not necessarily related to port roles. Table 6-12 lists possible states for different port
roles.
Table 6-12 Comparison between port states defined in STP and RSTP
STP Port State RSTP Port State Port Role
Forwarding Forwarding Root port or designated port
Learning Learning Root port or designated port
Listening Discarding Root port or designated port
Blocking Discarding Alternate port or backup port
Disabled Discarding Disabled port
l RSTP changes the configuration BPDU format and uses the Flags field to describe port
roles.
RSTP retains the basic configuration BPDU format defined in STP and makes minor
changes:
– The value of the Type field is changed from 0 to 2, so devices running STP will drop
the configuration BPDUs sent from devices running RSTP.
– The Flags field uses the six bits reserved in STP. This configuration BPDU is called an
RST BPDU. Figure 6-11 shows the Flags field in an RSTP BPDU.
Figure 6-11 Format of the Flags field in an RST BPDU
TCA
Agreement Forwarding Learning Port role Proposal TC
Topology Change
Acknowledgment flag
Topology
Change flag
Port role = 00 Unknown
01 Root port
10 Alternate/Backup port
11 Designated port
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
6 STP/RSTP Configuration
Issue 01 (2014-11-30) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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