User`s guide

SET UP
Page 38 User’s guide ref 9018809-01 XSRING+ SHDSL switch
12 RSTP redundancy protocol
• Select “Set-up > Ethernet & switching > RSTP”.
• Select the RTSP mode.
The page is divided in two parts :
The general parameters.
The Ethernet & SHDSL ports parameters.
General parameters
“Bridge priority” parameter :
This value is prepended to the MAC address of the bridge to form the bridge ID.
This is used by the network to choose which bridge will become the root of the spanning tree. The root
is the bridge with the lowest bridge ID.
“Hello time” parameter :
The "Hello time" is the delay between 2 consecutive BPDU sent by a bridge.
“Forward delay” parameter :
When a port changes state following a topology change, it goes through 3 states :
• Blocked : All ingress traffic is discarded but BPDU frames (necessary for STP function)
• Listening/Learning : The port listens to the traffic but does not forward data. This is used to
detect transient loops that can be created during the convergence.
• Forwarding : The port forwards data.
The forward delay is the duration of the Listening/Learning state.
Remark : Ports configured as Edge ports or P2P ports ignore this setting and skip the listening step
altogether because it becomes unnecessary and increases the convergence time.
“Maximum age” parameter :
When the STP root transmits a BPDU, the information contained in it is forwarded from bridge to
bridge. At each retransmission, a bridge adds 1 to a counter in the data. When this counter exceeds
the "Max Age" value, the BPDU is not forwarded.
Note 1 : This acts like the TTL in IP packets.
Note 2 : This value must be large enough for the network. Each device must be able to receive the
BPDUs from the root, even if it is located a lot of devices away. For example, for a ring with 20
devices, you must have "Max Age" greater than 20. Otherwise strange problems like erratic
behaviour,
very long convergence time, or no convergence at all will occur
.
Ethernet & SHDSL ports parameters
“Port name” parameter :
Select one of ports of the product (Ethernet 1 to 4 or Shdsl 1 or 2).
“Disable RSTP on this port” parameter :
That check box must be selected.
“Port priority” parameter :
When "Port cost" is identical for two paths to the root bridge, the port priority can be used to break a
tie between two ports. The less the number, the higher the priority.
“Port cost” parameter :
The port cost reflects the data rate of the link. It is used in the calculation of the active topology to
prioritise a high data rate link versus a low data rate link. A high data rate link usually has a low cost.