Amer Networks SS2R48G4i V2 - Manual (Page 107)

SS2R24G4i/SS2R48G4i
96
Chapter 10 RSTP CONFIGURATION
10.1 INTRODUCTION TO RSTP
RSTP is the abbreviation of Rapid Spanning Tree Protocol, which may block the redundant paths in
exchanging network through rapid spanning tree algorithm and establish non-loop tree network. The
rapid spanning tree algorithm adopted by RSTP is a distributed algorithm. It operates on all bridges of a
Bridged-LAN, and is responsible for calculating a simple and interconnected active topology. It adopts a
bridge as root (root bridge) when conducting calculation. At the same time, it designates roles for all
ports of all bridges.
RSTP algorithm is basically consistent with the STP algorithm defined in the standard of IEEE 802.1D.
The only difference is that RSTP overcomes the shortcoming of STP algorithm – For changing the state
of any port from blocking state to forwarding state, it is necessary for STP algorithm to go through
2*forward-delay time. According to the different roles of ports in topology structure, RSTP may realize
instant or fast transferring from blocking state to forwarding state.
According to functions of ports in active topology, RSTP defines five port roles disabled port, root port,
designated port as well as alternate port and backup port which are specified for realizing instant
performance. Introduction to the functions of each port role in active topology is as follows
1. Disabled ports do not participate in the algorithm of RSTP;
2. The bridge where the root port is located is connected to Root Bridge. The path cost from the
bridge to Root Bridge through root port is the lowest.
3. The designated port connects a LAN to Root Bridge through the bridge connected to the port.
4. The alternate port provides alternate path from the bridge to Root Bridge other than the path
from root port to Root Bridge.
5. The backup ports provide the alternate path from LAN at bridge downstream (the direction
opposite to root) to Root Bridge.
The root port and designated port are part of active topology. They may conduct address learning
and normal data forwarding. The alternate port, backup port and disabled port are not part of active
topology. They do not conduct address-learning data forwarding.