Overview of video fallback

Video fallback, also known as waterfall or client-side mediation, maximizes the likelihood of filling an impression opportunity when you’re using VAST redirects. With fallback, Ad Manager determines the eligibility of ads, ranks them accordingly, and returns a slate of ads, rather than just one.

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How video fallback works

Ad Manager sends multiple redirect ads to your SDK when a request is made. The SDK then requests the first ad. If the server fails to respond with a valid VAST response, the SDK requests the second, and continues to "fall back" until it either successfully gets and plays a creative (which could be an Ad Manager hosted creative, as it assumes that the creative would definitely render) or reaches the fallback limit you’ve set for the network. Fallback is a combination of VAST redirect creatives and an Ad Manager hosted creative.

For single video ads, fallback can occur until the first frame of an ad is played. If the ad is unable to play, fallback is ready with another one.


Possible fallback triggers

  • VAST response times out
  • Blank VAST XML response
  • HTTP error
  • Media file fails to load (not a trigger for pod fallback)
  • VPAID creative loads, but fails to load a video (not a trigger for pod fallback)

Some errors (for example, 202) can occur even if the video ad plays. These errors do not trigger fallback.

Errors cannot be detected in media file timeouts or errors associated with VPAID creatives for ads within pods.

Enable video fallback

  1. Sign in to Google Ad Manager.

  2. Click Admin and then Video and then Video settings.
  3. Select Enable video fallback.
  4. Click Save.

All IMA3 GPT video tags will use fallback when you enable at the network level.

You can add nofb=1 to disable fallback for individual tags.

Requirements and limitations

  • Only line items containing VAST redirect creative sets trigger fallback. The fallback chain could have line items with different creative types.
  • Fallback requires the IMA 3 SDK and Google Publisher Tags.
  • Fallback considers dynamic allocation ads for competition only in the first spot of the fallback chain.
  • For most line item types, only one line item is returned for each priority level in the fallback chain. For multiple line items of the same type to be returned in the fallback response, they should generally have different priorities.
    Exceptions to this are price priority line items, and when Ad Exchange is eligible. In either of these situations, multiple line items of the same priority can appear in the chain, as the eCPM is used. When Ad Exchange is eligible (either through dynamic allocation or First Look), only one line item per type is returned.

Report on fallback position

  1. Sign in to Google Ad Manager.

  2. Follow the instructions to generate a report.
  3. Select Fallback position from the "Video" section of report dimensions. This allows you to view impressions, clicks, conversions, and other metrics that result from fallback and non-fallback ads that were served.

VAST 3 fallback report metrics by error code

If you use VAST 3, your report includes VAST errors, categorized by response status codes. You can use these metrics to determine the number of times fallback has occurred for a particular creative or line item.

  • VAST errors 300, 301, 302, and 303 indicate errors at the ad request level, which trigger fallback.
  • VAST errors 400-403, 405, 900, and 901 indicate errors during asset playback, which trigger asset-level fallback.
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