Etic Telecom XLAN+ Series, Setup Manual

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SETUP
Page 26
DOC_DEV_XS_Setup guide_C
6 RSTP
6.1 Overview
RSTP, standing for "Rapid Spanning Tree Protocol" is specified by the IEEE in the 802.1D-2004 document.
Loops in Ethernet networks are highly unwelcome, as they can cause broadcast storms, eating up all the
available bandwidth and causing network outage. But sometimes they are necessary, in order to have backup
paths in case a device or a link fails.
The goal of RSTP is to eliminate loops dynamically by calculating a spanning tree of the network. This spanning
tree becomes the topology of the network and is created by disabling some ports.
The algorithm work by exchanging periodically small Ethernet frames : BPDUs (Bridge Protocol Data Unit).
These frames contain information that allow the network to choose a root bridge, which is the root of the
spanning tree. BPDUs originating from the root bridge are forwarded from bridge to bridge, to ensure
propagation of the information in the network.
When a bridge receives BPDUs from root from 2 of its ports, meaning there is a loop in the network, it will only
forward the best one, and block the port where the others were received.
When a link fails, BPDU are not received anymore by the bridge, and it will enable previously blocked ports in
response.
RSTP supersedes the old STP for loop and redundancy management.
RSTP is retro-compatible with STP, but has extensions for faster recovery times :
•
Ports can be put in "Edge" mode, meaning they will never be blocked.
•
Ports can be put in "Peer to Peer" mode, meaning they transition quickly from "Blocked" to "Forwarding"
when necessary.
This protocol can be used with devices from other manufacturers because it is interoperable.
On the SHDSL side of the product, convergence is achieved in around 10 seconds.
Ports classification :
An Edge port is a port on the network edge; it connects an RSTP switch to equipments not acting as a bridge;
for example, an Ethernet port that connects the XS to a PC or a PLC is an Edge Port