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2 rstp operation, Rstp operation, Fig. 8.14 – Siemens S223 User Manual

Page 205: Alternate port and backup port

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User Manual UMN:CLI
SURPASS hiD 6615 S223/S323 R1.5

A50010-Y3-C150-2-7619 205

8.3.2 RSTP

Operation

STP or RSTP is configured on network where Loop can be created. However, RSTP is
more rapidly progressed than STP at the stage of reaching to the last topology. This sec-
tion describes how the RSTP more improved than STP works. It contains the below sec-
tions.

Port

States

BPDU

Policy

Rapid Network Convergence

Compatibility with 802.1d.

Port States

RSTP defines port states as discarding, learning, and forwarding. Blocking of 802.1d and
listening is combined into discarding. Same as STP, root port and designated port are de-
cided by port state. But a port in blocking state is divided into alternate port and backup
port. Alternate port means a port blocking BPDUs of priority of high numerical value from
other switches, and backup port means a port blocking BPDUs of priority of high numeri-
cal value from another port of same equipment.

Switch B

Switch A

Alternate

Port

Switch C

Switch D

Backup

Port

Path 2

Path 1

ROOT

Designated

Port

Fig. 8.14

Alternate Port and Backup port

The difference of between alternate port and backup port is that alternate port can alter-
nate path of packet when there is a problem between Root switch and SWITCH C but
Backup port cannot provide stable connection in that case.

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