Redundant Ethernet Definition


Thread Starter

Ian Verhappen

Hello everyone,

I was in conversation with a coworker last week and we were wondering if you could help us determine through the consensus of this list what is meant by the term "redundant ethernet." Please "vote" for one of the following definitions or feel free to add more text. Thanks.

1. Redundant ports on a single I/O card

2. Redundant cards each with a single ethernet port

I realise that at some point in the system the redundancy will fail, either on the card or backplane, or... but as stated above this is simply to get a consensus on a definition.

- Ian Verhappen

Michael Griffin

In reply to Ian Verhappen: I think the phrase "redundant ethernet" is too vague to try to put such fine distinctions on it. As to which of the two methods would be "better" (one dual port card or two single port cards) would I believe depend upon the characteristics of the the particular hardware and software in question.
In my personal opinion you need both, so correct should be a third option:

3. Redundant cards with redundant ports

Redundant ports on a single I/O card does not do it because if the card fails you are down on a single point of failure so option 1 is out. Redundant cards each with a single ethernet port does not work because if the switch or wire breaks, e.g. for the primary card the secondary takes over but the secondary is not able to talk to active primary cards on the other controllers. That is, option 3 is required for peer-to-peer communication and some other functions that may be done across Ethernet. This is why you find that DCS controllers have redundant processor modules with redundant ports.

With any system using redundancy, one has to evaluate what failure modes you are trying to protect against. Jonas makes a good point in that just having two cards will provide no protection if both cards are wired to a common hub/switch/router that provides a single point of failure. Likewise, what will happen when the backplane that the two cards are plugged into fail?

I once worked at a company that paid handsomely to have electrical power supplied to the plant from two separate power company substations separated by several miles. One day the entire plant was shutdown by a squirrel who entered the transfer switch that selected which power feed the plant used.

Most redundant configurations I have seen have only been partial implementations and still contained significant single points of failure.

Also be sure that the additional complexity of a redundant system does not become the cause of additional system failures.

Ron Davis