Contemporary Control Systems Compact Managed Switches Software Manual for Console Access - Manual (Page 37)

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4.3.9 Configure Redundancy
Each managed switch from Contemporary Controls offers you a choice between the public
standard protocols known as Spanning Tree Protocol or Rapid Spanning Tree Protocol
and the proprietary redundancy protocol known as RapidRing™. By default, the screen of
Figure 31 ( Configure Redundancy) displays the public protocol RSTP and its basic
parameter values for this switch.
To choose RapidRing as your redundancy scheme, position your cursor in the line which
begins with the word Redundancy then press your keyboard space bar to toggle the
selection from STP/RSTP to RapidRing. Once this is done, a new screen appears with
options listed in Section 4.2.1.9.3.4 — but before adjusting these values, you should be
familiar with all of the material discussed in Section 4.2.1.9.3.
4.3.9.1 Rapid Spanning Tree Protocol
Rapid Spanning Tree Protocol (RSTP) provides network path redundancy but without
data loops that are prohibited. If more than one active path exists between two stations,
a confused forwarding algorithm could transmit duplicate frames — one along each path.
RSTP constructs a tree of all RSTP-compliant switches in the network. To avoid loops,
it forces each redundant path into an inactive state. If a segment is interrupted, an
algorithm reconfigures the tree by quickly activating a normally unused link to substitute
for the failed link.
By the exchange of messages, each switch in the tree collects information on all other
switches. This information includes switch and port priorities, Media Access Control
(MAC) addresses and path merit figures called “port costs”. This exchange results in
the election of one switch to perform as the root switch (the logical centre of the tree)
and also defines how ports are to be used on all other switches. The root port on a
switch will send traffic to the root switch along the most efficient path. If the root port is
disrupted, a backup port is activated as the substitute. A designated port provides
the best path for root-bound traffic from outlying switches. If the designated port is
disrupted, an alternate port is activated as the substitute.
If all RSTP-compliant switches in the network are enabled with default settings, the
switch with the lowest MAC address is elected the root switch. But due to network
traffic and architecture issues, the elected switch might not be the best to serve as the
root device. You can manually force the switch of your choice to serve as the root
device by increasing its priority so that the root-election algorithm chooses it as the root.
In general, you should configure your RSTP network so that the paths with the greatest
bandwidth are those which support traffic for the root switch. For conveying root traffic,
a fibre optic link would be preferred over a copper link and a 100 Mbps link would serve
better than one operating at 10 Mbps. Also, the tree should consist of only of devices
that are RSTP-compliant — non-compliant switches and hubs, if used at all, should only
occupy the periphery of the tree because they will not forward the special messages
needed for the construction and maintenance of the tree.