What is a VAST Tag / Router

Created by Andrew Baritz, Modified on Wed, 25 Jun at 12:52 PM by Andrew Baritz

What Is a VAST Tag and VAST Router?

If you're working in OTT streaming or AVOD (Advertising Video on Demand), you've likely heard terms like VAST tag and VAST router thrown around. But what exactly are they, and why are they so important to video advertising?

Let’s break it down.

✅ What Is a VAST Tag?

VAST stands for Video Ad Serving Template. A VAST tag is a URL that a video player calls to fetch video ads. This tag points to a VAST XML file that tells the player:

What ad to play
Where to find the media file
How long the ad is
Where to track impressions and clicks

? Think of It Like This:

A VAST tag is like giving your video player a map. The map tells it where to find the ad, how to display it, and where to report back once the ad is shown.

? What Does a VAST Tag Look Like?

Here’s an example of a basic VAST tag URL:

arduino
CopyEdit
https://adserver.com/vast?placement_id=12345

When the player calls that URL, it gets back an XML response that looks something like this:

xml
CopyEdit
<VAST version="4.0"> <Ad> <InLine> <Creatives> <Creative> <Linear> <MediaFiles> <MediaFile delivery="progressive" type="video/mp4"> https://cdn.adserver.com/media/ad1.mp4 </MediaFile> </MediaFiles> </Linear> </Creative> </Creatives> </InLine> </Ad> </VAST>

? What’s Inside a VAST File?

Element	What It Does
`<Ad>`	Defines the ad or wrapper
`<MediaFile>`	Points to the video ad file
`<Tracking>`	Tracks impressions, clicks, completes
`<ClickThrough>`	URL to visit when someone clicks the ad
`<Duration>`	How long the ad should play

? What Is a VAST Router?

A VAST Router (sometimes called a VAST Wrapper Router) is a server that sits between the video player and multiple demand sources (advertisers, SSPs, DSPs).

Instead of pointing your player directly to one VAST tag, you point it to the VAST router, which does the following:

? How It Works

The player requests an ad from the router.
The router queries multiple VAST tags from different ad sources.
It picks the best ad (based on price, quality, fill, etc.).
It either:
- Responds with that ad’s VAST directly, or
- Wraps the winning ad in another VAST file (a wrapper).

?️ Diagram: Player > Router > Demand Sources

sql
CopyEdit
Video Player     |     v [VAST Router]     |     +--> SSP 1 (VAST)     |     +--> SSP 2 (VAST)     |     +--> DSP (VAST)     |     v
Returns best VAST ad

? Why Use a VAST Router?

Benefit	Why It Matters
Unified Traffic	Route all demand through one point of contact.
Optimization	Choose best ad based on CPM, latency, or creative quality.
Fallback Handling	If one demand source fails, try another instantly.
Tracking	Centralized tracking of performance, requests, and fills.
A/B Testing	Easily split traffic to test different demand paths.

? Potential Issues with VAST Tags & Routers

Issue	Fix or Prevention Strategy
Too many wrappers	Limit wrappers to 2 or fewer to avoid timeouts.
Long load times	Optimize router response speed.
No-fill from sources	Include fallback tags or default creative.
Broken media files	Always QA test final VAST responses.

?‍? Real-World Example

If you're a publisher like Revry, you might route all of your AVOD traffic through a VAST router to control who gets access to your inventory and to select the highest-paying ads programmatically.

text
CopyEdit
Player → Revry VAST Router → Index Exchange, Magnite, OpenX, etc.

? Glossary

SSP: Supply-Side Platform – sells ad inventory
DSP: Demand-Side Platform – buys ad inventory
Wrapper: A VAST file that points to another VAST tag
Inline Ad: A VAST response that includes the final media file

? Summary

Term	What It Means
VAST Tag	URL that returns a VAST ad file
VAST Router	Middleware that selects the best ad and serves the VAST response

?️ Visual Recap

? VAST Tag Path:

nginx
CopyEdit
Player ➜ VAST Tag ➜ Ad Creative

? VAST Router Path:

nginx
CopyEdit
Player ➜ VAST Router ➜ Multiple Demand ➜ Best Ad ➜ Ad Creative