An agency ad server is a specific version of an ad server which is dedicated to the management of advertisers’ campaigns at the agency level.
Agency ad servers allow to centralize ad data and campaign results at the agency level when the campaign is delivered on a range of publishers’ websites.
When the publisher ad tag is loaded, the visitor’s browser calls the ad server for an ad. Then the ad server delivers another ad tag that points toward the agency ad server. Finally, the browser calls the agency ad server which sends directly the creative or points to the location of the creative on a CDN.
Some ad serving discrepancies can be observed between agency side servers and publisher’s servers.
Agency ad server in the ad delivery chain:
Source image wiki Appnexus