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How Intermarket Sweep Orders Bypass Cross-Exchange Routing

Article Quant Q&A · Author: Prashanth
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Summary

The document describes the purpose of an intermarket sweep order in the context of Regulation NMS and displayed quotes across U.S. exchanges. Ordinarily, when an order reaches the best displayed price, routing rules may require the order to access other exchanges showing that price before executing at a worse price. An ISO lets the submitting trader direct an exchange to execute the order there, subject to available volume and price, without routing it to other venues.

The response frames this as useful when a trader believes the national best bid or offer is stale and has already checked the relevant liquidity elsewhere. It says these orders are often submitted with immediate-or-cancel instructions and place responsibility for that diligence on the trader. The explanation is brief and gives no execution data, legal detail, or examples of how to verify quotes. It also cautions that most traders may have little need to use ISOs, so the order type is a specialized execution tool rather than a general-purpose way to fill large orders.

Key ideas

  • An ISO directs an exchange to execute eligible quantity locally without routing to other exchanges.
  • The order type is explained as a response to Regulation NMS routing requirements.
  • A trader may use an ISO when they believe the displayed national best quote is stale.
  • The trader assumes responsibility for checking that bypassing other venues is appropriate.
  • The answer presents ISOs as a specialized tool with limited need for most traders.

Tags

ExecutionMarket microstructureUS markets
Full text
# Purpose of ISOs


# Purpose of ISOs












Why was ISO (Intermarket Sweep Order) introduced? I read that it was introduced to help fill large orders. Some explanation is here: http://tabbforum.com/opinions/why-hfts-have-an-advantage-part-3-intermarket-sweep-orders

Do you have any more information?

Thanks.

## Answer by chollida (score 2)

https://quant.stackexchange.com/a/22005

For most traders they wouldn't have a need to use ISO orders.

It tells the exchange to fill the order completely at the exchange, assuming the required volume and price are met without routing the order to another exchange.

It was introduced when RegNMS came in as otherwise once an exchange filled the order at the top price level it would have to send the order to the other exchanges showing that particular price level before it could fill the order at a lower price level.

So if you happened to be a shop who knows the current NBBO is stale, you can issue an ISO order(typically Immediate or Cancel) that tells the exchange to fill your order entirely at the exchange without trying to source liquidity at other exchanges.

Exchanges like it as they get the entire fill at their exchange and the user takes the liability of telling them that they have done the diligence to know that they are getting the NBBO.

Shown in full with attribution under the source's licence. Licence: CC BY-SA 4.0 (Stack Exchange)

This summary was written by Stratmill's research agent from the original; it is not a copy of the source.