Forescout NAC VLAN Miss: Switch SNMP/CLI When the Host Stays in Quarantine

Forescout · Techclick CLI troubleshooting lab

Forescout shows compliant. The switch port is still in the quarantine VLAN. User cannot reach AD.

This is a live-ticket style lesson from Techclick Infosec. You will isolate the fault with CLI first, then prove the fix with one clean packet or log.

What you will be able to do

• Name the first CLI command to run on Forescout for this ticket

• Separate the layer that is actually broken from the layer people blame

• Apply one scoped fix instead of a global disable

• Prove the ticket closed with a session, log, or counter

Quick answer

Forescout must be able to change the port (SNMP/CLI/RADIUS CoA). Compliance without a switch action leaves the old VLAN.

Why this ticket shows up

The policy can be true on the appliance and still fail to write the switch. That looks like 'NAC is broken' when it is a credentials or CoA path.

Symptoms operators actually see

• Compliant, wrong VLAN

• Switch action failed in Forescout log

• SNMP write broken after a switch change

• CoA not allowed

CLI first — copy this block

Run these on the Forescout device or console named in the ticket. Do not skip the first show command — it tells you which later command matters.

show authentication sessions interface Gi1/0/8 details
show vlan brief | include Gi1/0/8
show snmp user
show running-config | include radius|snmp
show logging | include VLAN|COA

How to read that output

If the session VLAN is still quarantine, Forescout's action never applied. Switch logs will not show a VLAN change.

Triage order

1. Read the Forescout switch-action log

2. Verify SNMP write or RADIUS CoA from the appliance

3. Manually set VLAN once to prove the port

4. Fix credentials/ACL

5. Re-run the policy

Classic traps

What you see	Real cause	Fix
Mark the host unmanaged	Action path is broken	Fix switch integration
Disable NAC globally	One switch profile is wrong	Fix that switch
Static the port forever	Next host will fail	Fix CoA/SNMP

Proof the ticket is closed

• Port VLAN updates without a human

• User reaches AD

• Action log success

Say this in an interview

Compliant plus quarantine VLAN means the switch action failed. I debug SNMP/CoA, not the posture plugin first.

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