1. Broker takes client order.

2. broker trades his own account first (with knowledge of client order)

3. Broker executes client order.

If you don't have clients you can't be front running. It is black and white and as simple as that.

It's also Front running and illegal to pay a Broker for prior knowledge in front of their clients trades.

Also, the term may apply to using insider knowledge. Khan & Lu (2008: 1) define front running as "trading by some parties in advance of large trades by other parties, in anticipation of profiting from the price movement that follows the large trade". They find evidence consistent with front-running through short sales ahead of large stock sales by CEOs on the New York Stock Exchange. http://en.wikipedia.org/wiki/Front_running

" "Front running" is sometimes used informally for a broker's tactics related to trading on proprietary information before its clients have been given the information.

For example, analysts and brokers who buy shares in a company just before the brokerage firm is about to recommend the stock as a strong buy, are practising this type of "front running". Brokers have been convicted of securities laws violations in the United States for such behavior. "

This is not a problem when the principal has an exchange seat and may execute directly. Where firms are performing multiple-exchange arbitrage, and the principal splits a large order into several for execution across multiple exchanges, there is now an opportunity for something like front-running: the arbitrage firm can detect the principal's intent from the first order to hit the first exchange, and then try to move the market profitably (against the principal) on the other exchanges before the correlated orders can execute. While somewhat analagous, this activity should not be referred to as front-running.