Huawei AR530 series Configuration Manual page 197

Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
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Issue 01 (2014-11-30)
Configuration BPDUs are processed in a different way.
– Configuration BPDU transmission
In STP, the root bridge sends configuration BPDUs at Hello intervals after the topology
becomes stable. Non-root bridges send configuration BPDUs only after they receive
configuration BPDUs from upstream devices. This complicates the STP calculation and
slows down network convergence. RSTP allows non-root bridges to send configuration
BPDUs at Hello time intervals after the topology becomes stable, regardless of whether
they have received configuration BPDUs from the root bridge.
– BPDU timeout period
In STP, a device has to wait a Max Age period before determining a negotiation failure.
In RSTP, a device determines that the negotiation with the upstream device has failed
if the corresponding port does not receive any configuration BPDUs sent from the
upstream device for three consecutive Hello intervals.
– Processing of inferior BPDUs
When an RSTP port receives an RST BPDU from the upstream designated bridge, the
port compares the received RST BPDU with its own RST BPDU.
If its own RST BPDU is superior to the received one, the port discards the received RST
BPDU and immediately responds to the upstream device with its own RST BPDU. After
receiving the RST BPDU, the upstream device updates its own RST BPDU based on
the corresponding fields in the received RST BPDU.
In this manner, RSTP processes inferior BPDUs more rapidly, independent of any timer
that is used in STP.
Rapid convergence
– Proposal/agreement mechanism
In STP, a port that is selected as a designated port needs to wait at least one Forward
Delay interval (Learning state) before it enters the Forwarding state. In RSTP, the port
enters the Discarding state, and then the proposal/agreement mechanism allows the port
to immediately enter the Forwarding state. The proposal/agreement mechanism must
be applied on P2P links in full-duplex mode.
For details, see 6.2.6 RSTP Technology
– Fast switchover of the root port
If a root port fails, the best alternate port becomes the root port and enters Forwarding
state. This is because the network segment connected to this alternate port has a
designated port connected to the root bridge.
When the port role changes, the network topology changes accordingly. For details, see
6.2.6 RSTP Technology
– Edge ports
In RSTP, a designated port on the network edge is called an edge port. An edge port
directly connects to a terminal and does not connect to any other switching devices.
An edge port cannot receive or process configuration BPDUs and does not participate
in RSTP calculation. This port can transition from Disable to Forwarding state
immediately without a delay, just like an STP-incapable port. An edge port becomes a
common STP port once it receives a configuration BPDU. The spanning tree needs to
be recalculated, causing network flapping.
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Details.
Details.
6 STP/RSTP Configuration
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