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.

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)

Requirements and eligibility

  • Fallback requires Google Publisher Tags; and for VAST 2.0 ad requests, the IMA 3 SDK.
  • Only line items containing VAST redirect creative sets trigger fallback. The fallback chain could have line items with different creative types.
  • Fallback generally considers dynamic allocation ads and Programmatic Guaranteed for competition only in the first spot of the fallback chain. There may be some scenarios with optimized video pods where additional ads may be returned.
  • For most line item types, only one line item is returned for each priority level in the chain. For multiple line items of the same type to be returned in the response, they should generally have different priorities.

    Multiple standard line item types may serve, and multiple price priority line items may serve, as the eCPM is used. For sponsorships, only one sponsorship may serve, even if multiple sponsorships have different priorities.
  • Certain line item and/or creative settings can prevent an ad from being selected as part of the fallback chain. For example, line items with sequential creative rotation are restricted from being selected for fallback due to the fact that they're not guaranteed to show, and can break the creative rotation.*
  • Some errors (for example, 202) can occur even if the 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.

* There may be additional line item settings the prevent an ad from being selected.

Ad buffet fallback functionality for optimized pods

Ad buffet can be used to provide fallback functionality to optimized pods. It refers to standalone ads available to serve in the pod, that were not part of the original ads to be served in that pod. Optimized pods serve ads in a specific sequence. Ad buffet ads are served using their own sequence. Ad Exchange ads may be eligible to be served in fallback positions in optimized pod responses.


For example, for an optimized pod that contains 3 sequential ads and 3 buffet ads, if the second ad is a redirect that does not return an ad, the first buffet ad is selected.

Additional eligibility requirements for optimized pods

  • Ad buffet defaults to 3 ads, but you can increase the value to allow up to 9 ads.
  • Ads with pod or stream frequency caps are eligible to serve to the ad buffet.
  • Line items with competitive exclusion labels are not eligible to serve to the ad buffet.**
  • Ad buffet ads cannot exceed the max duration of the original ads in the pod (for example, if all ads are 30 seconds, then a buffet ad cannot be greater than 30 seconds).

** If a line item with a competitive exclusion label serves in the main pod, then other line items with the same label may not serve to the ad buffet. Having a label itself does not preclude the line item from being selected as part of the buffet.

Activate video fallback and video ad buffet

  1. Sign in to Google Ad Manager.

  2. Click Admin and then Video and audio and then Video and audio settings.
  3. Activate Video fallback and Video ad buffet.

    When you activate video ad buffet, you can set the maximum number of ad buffet ads, up to 9.
  4. Click Save.

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

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

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, 900-901 indicate errors during playback, which trigger asset-level fallback.

Asset-level fallback

Conventional fallback allows the IMA SDK to attempt to serve ads in sequence and "fall back" to the next ad only when the VAST wrapper itself doesn't resolve to an inline ad. With asset-level fallback, the SDK goes one level further—even if the individual VAST wrapper resolves to an inline ad, but encounters other errors (for example, a VPAID that doesn't track an impression, a media file that times out, or other VAST errors), the SDK continues to fall back to the next ad in the list.

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