September 2011
55
V1.44
Copyright © 2011 Adept Systems, Inc. and Control Network Solutions Ltd All Rights Reserved.
3.3. Redundant Twin Mode (Cost Option)
The Twin Redundant Mode enables two eNodeIV devices to operate as a redundant pair for
high availability applications without generating duplicate traffic. This enhanced capability
increases reliability and eliminates some single mode failure sources. A simple diagram showing
a redundant connection between two channels is shown below.
Fig.3.6: Two redundant routers between the same channels
Although it is possible for a pair of conventional 709.1 routers to be identically configured and
connected between the same two channels, this configuration induces a doubling of the traffic
between those two channels. The built-in duplicate detection mechanism in 709.1 discards the
duplicate packets at each receiving node. However, the doubled traffic load could tax network
bandwidth and create other problems.
In
Redundant Twin Mode
(or for the sake of brevity,
Twin Mode
), both routers are identically
configured and connected between the same two channels as per the case described above but
unlike the case above only one of the two routers is forwarding packets. This feature achieves
the increased system reliability of having a redundant backup router without the drawbacks of
doubled traffic. The Twin Mode routers monitor each other’s health and operational status and
dynamically activate forwarding as needed should one of the other fail. Failures are detected,
diagnosed, and reported so that repairs can be made to maintain continuous availability.
Should there be a fault in either interface then both routers will go active and forward traffic
until the fault has been healed. In addition, the router configuration is periodically
automatically synchronized between the two routers to reduce fail-over time and increase the
fidelity between the backup and primary router operation. Also supported is manual
synchronization which makes it more convenient to replace one of the redundant pair and
replicate its configuration. A high availability building network can be constructed using pairs of
redundant twin mode routers and a redundant switched Ethernet network. An example
network showing the application of Twin Mode is shown below.
