Web browsing and video become slow after connecting to the corporate VPN: how can you distinguish full tunnelling, DNS, and MTU problems?

Compare default routes, interface metrics, DNS, egress location, path MTU, and corporate bandwidth before deciding whether split tunnelling is appropriate.

1. Conclusion and scope

Prepare the client and server versions, domain membership, DNS and gateway settings, network location, full error text, event timestamps, and recent changes. The reserved example domain corp.example is used throughout; no customer domain, IP address, account, or device identifier is included.

This issue falls under Network, VPN and firewall. Logs and configuration can often be collected remotely first. Bulk permission changes, switch-path work, production cutovers, and recovery drills should use a controlled implementation window.

2. Symptoms and environment

• Capture the complete error text, event-log timestamp, and failed action rather than relying on a verbal description.
• Record the affected scope, first occurrence, reproducibility, and whether the result changes on another subnet.
• If a firewall rule is open but the application still fails, inspect the server gateway, policy routing, session table, NAT, and path symmetry.

3. Troubleshooting sequence

1. Compare default routes, interface metrics, and DNS before and after VPN connection to determine whether full tunnelling sends all internet traffic through the corporate gateway.
2. For split tunnelling, push only corporate routes and the required internal DNS scope instead of sending all internet traffic through the company gateway.
3. If some pages load while uploads stall or large packets fail, check effective VPN MTU, MSS adjustment, and path fragmentation.
4. A connected VPN only proves that the tunnel is established; client routes, internal DNS, access control, server firewalls, and the return path must also be correct.
5. Inspect WinHTTP, user proxy settings, PAC files, and security-agent proxy remnants; browser access does not prove that Office or system services use the same path.
6. Change one variable at a time and export the current configuration before making changes.

route print
ipconfig /all
Get-NetIPInterface | Sort-Object InterfaceMetric
ping public.example -f -l 1400

4. Safe remediation and rollout

Start with read-only queries, configuration exports, and one-system validation. Once the root cause is confirmed, define the target scope, change window, and rollback method. Validate policy changes with a test source and a limited time window, record rule hits and sessions, keep a rollback export, and expand scope gradually.

• A connected VPN only proves that the tunnel is established; client routes, internal DNS, access control, server firewalls, and the return path must also be correct.
• Inspect WinHTTP, user proxy settings, PAC files, and security-agent proxy remnants; browser access does not prove that Office or system services use the same path.
• Change one variable at a time and export the current configuration before making changes.

5. Validation, rollback and common mistakes

Do not stop when the service works once. Revalidate with the user workflow, logs, a restart or fresh sign-in, another network location where relevant, and the next policy or backup cycle.

Validation and rollback checks

• Change one variable at a time and export the current configuration before making changes.
• A connected VPN only proves that the tunnel is established; client routes, internal DNS, access control, server firewalls, and the return path must also be correct.
• Inspect WinHTTP, user proxy settings, PAC files, and security-agent proxy remnants; browser access does not prove that Office or system services use the same path.

Common mistakes to avoid

• Assuming the network is healthy because the VPN status says connected.
• Using any-destination or any-service rules instead of least privilege.
• Changing client routes without checking the server return path and firewall sessions.