Azure front door session affinity question

Question

Azure front door session affinity question

Eddy Lynch 60 Microsoft Employee

I have session affinity enabled on an origin group. I also have latency sensitivity set to 50ms. However, I noticed that users get redirected to different nodes within that latency threshold, after they already went to a different node. This goes against the expected behaviour of session affinity. Why were they not pinned to the same node the whole session? Or does latency sensitivity affect that?.

Ravi Varma Mudduluru 12,370 Reputation points Microsoft External Staff Moderator

2026-04-29T10:45:22.75+00:00
Hello @ Eddy Lynch,

Thank you for reaching out to Microsoft Q&A.

It looks like what you’re seeing comes down to how Front Door combines session affinity with its latency-based load-balancing. Session affinity in Standard/Premium works by setting a cookie (ASLBSA/ASLBSACORS) on the first response and then trying to send subsequent requests back to that same origin but Front Door still applies your “Latency sensitivity” threshold first.

Here’s the sequence:

Front Door measures latencies to all healthy origins and finds the one with the best RTT.

It filters out any origins whose latency is more than (best RTT + your sensitivity threshold).

From that filtered set, if you have a valid affinity cookie pointing to an origin that’s still in the set, it picks it.

If your pinned origin falls outside the latency window (or becomes unhealthy), the affinity cookie can’t “stick” you to it anymore, and Front Door picks the next-best origin within your threshold.

So yes, latency sensitivity does affect stickiness. if your original node’s latency drifts beyond best RTT + 50 ms, users will be sent elsewhere even if they already had a session cookie pointing at the first node.

What you can try:

Increase your latency sensitivity (say to 100 ms or more) so small swings don’t kick your users off their pinned origin.

If you really need guaranteed stickiness regardless of latency, consider using Priority-based load balancing instead of latency.

Double-check that your backend responses aren’t cacheable (because if Front Door can’t set the session cookie on the response, affinity never gets established).

Use your browser dev tools or Fiddler to confirm the ASLBSA/ASLBSACORS cookies are being set on the client.

Verify your health probes so the origin doesn’t get marked unhealthy spuriously.

Reference: https://learn.microsoft.com/en-us/azure/frontdoor/routing-methods

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Alex Burlachenko 22,120 Reputation points MVP Volunteer Moderator

2026-04-30T07:38:46.1333333+00:00
Eddy Lynch hi & thx for join me here at Q&A portal,

latency sensitivity can override affinity, affinity is soft, not guaranteed... this is expected behavior once u mix session affinity + latency sensitivity, AFD does not treat affinity as hard pinning. session affinity in Front Door is cookie-based (ARRAffinity/afd cookie) but it is best-effort, not strict stickiness. the routing engine still evaluates health + latency and can override affinity if another origin is considered better within policy. latency sensitivity = how aggressively AFD re-evaluates “better” origins, so with 50ms threshold u basically tell it “feel free to switch if another node is within 50ms and looks better” so users can bounce between origins even with affinity cookie present..... also affinity can break if cookie is not returned (cross-domain, HTTPS redirect, client behavior) or if origin becomes temporarily degraded.

if u want stronger stickiness, increase latency sensitivity (higher ms) or reduce routing aggressiveness, and ensure cookies are preserved end-to-end.

but there is no true hard session pinning in AFD like classic load balancers

rgds,

Alex

&

if my answer helps pls accept it.
Eddy Lynch 60 Reputation points Microsoft Employee

2026-04-30T08:03:40.95+00:00

Hi, so if I set sensitivity to 0, can users still "bounce" between origins?
Ravi Varma Mudduluru 12,370 Reputation points Microsoft External Staff Moderator

2026-04-30T09:35:25.0333333+00:00

Hello @ Eddy Lynch

Thank you for the follow up question.

With latency sensitivity set to 0 ms, Azure Front Door selects the fastest healthy origin for the initial request. After that, the session affinity cookie keeps the user pinned to that same origin for the rest of the browser session.

The above statement is mentioned in the below document. https://learn.microsoft.com/en-us/azure/frontdoor/routing-methods

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Alex Burlachenko 22,120 Reputation points MVP Volunteer Moderator

2026-04-30T07:38:46.1333333+00:00

Eddy Lynch hi & thx for join me here at Q&A portal,

latency sensitivity can override affinity, affinity is soft, not guaranteed... this is expected behavior once u mix session affinity + latency sensitivity, AFD does not treat affinity as hard pinning. session affinity in Front Door is cookie-based (ARRAffinity/afd cookie) but it is best-effort, not strict stickiness. the routing engine still evaluates health + latency and can override affinity if another origin is considered better within policy. latency sensitivity = how aggressively AFD re-evaluates “better” origins, so with 50ms threshold u basically tell it “feel free to switch if another node is within 50ms and looks better” so users can bounce between origins even with affinity cookie present..... also affinity can break if cookie is not returned (cross-domain, HTTPS redirect, client behavior) or if origin becomes temporarily degraded.

if u want stronger stickiness, increase latency sensitivity (higher ms) or reduce routing aggressiveness, and ensure cookies are preserved end-to-end.

but there is no true hard session pinning in AFD like classic load balancers

rgds,

Alex

&

if my answer helps pls accept it.
Eddy Lynch 60 Reputation points Microsoft Employee

2026-04-30T08:03:40.95+00:00

Hi, so if I set sensitivity to 0, can users still "bounce" between origins?
Ravi Varma Mudduluru 12,370 Reputation points Microsoft External Staff Moderator

2026-04-30T09:35:25.0333333+00:00

Hello @ Eddy Lynch

Thank you for the follow up question.

With latency sensitivity set to 0 ms, Azure Front Door selects the fastest healthy origin for the initial request. After that, the session affinity cookie keeps the user pinned to that same origin for the rest of the browser session.

The above statement is mentioned in the below document. https://learn.microsoft.com/en-us/azure/frontdoor/routing-methods

Answer 1

Hello @ Eddy Lynch,

Thank you for reaching out to Microsoft Q&A.

I checked with the PG team, and they confirmed that if the sensitivity is set to 0, it reduces the eligible origins to only the fastest one. If the fastest origin changes, requests will be routed to a different origin.

In Azure Front Door, session affinity applies only within the latency-selected origin pool. If an origin falls outside the latency sensitivity window, Front Door will select another origin, so session stickiness is not guaranteed under latency-based routing.

Setting the sensitivity to 0 may not help, as it always selects the fastest origin, which can change.
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The above statement is mentioned in the below document. https://learn.microsoft.com/en-us/azure/frontdoor/routing-methods

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