Fix “The Peer Is Not Responding to Phase 1 ISAKMP Requests” in SonicWall Global VPN Client

A remote user once had a SonicWall Global VPN Client (GVC) connection that worked from one network but failed from another. The GVC log kept reporting:

The peer is not responding to Phase 1 ISAKMP requests.

That message makes it sound as if the SonicWall is refusing the connection. It doesn’t necessarily mean that. It’s a generic timeout: the client sent its Phase 1 request and never got the response it expected. SonicWall’s own troubleshooting guidance lists several possible causes, from the client having no internet access, to the peer gateway or GroupVPN policy not being available, to an upstream NAT device dropping the traffic (SonicWall KB: The Peer is Not Responding to Phase 1 ISAKMP Requests error in GVC).

So the useful question isn’t “why is the VPN broken?” It’s this:

Did the Phase 1 packet actually reach the SonicWall?

Once you know that, the troubleshooting path becomes much clearer.

What Phase 1 ISAKMP is doing

Phase 1 is the first stage of an IPsec VPN negotiation. The GVC contacts the SonicWall and starts the IKE negotiation, and the two sides agree the parameters for a security association before moving on to Phase 2. If Phase 1 doesn’t complete, the VPN never gets as far as Phase 2.

The initial negotiation uses UDP 500. If a NAT device is detected between the two ends, the traffic moves to UDP 4500 for NAT Traversal.

The important point is that the GVC error doesn’t tell you where the failure happened. The client may be sending the packet correctly while it disappears somewhere between the client and the firewall. Or the packet may reach the SonicWall and be dropped there. Everything that follows is about working out which.

If you only have access to the client

If you’re on the helpdesk or desktop support and can’t get into the SonicWall, there are still three quick tests worth doing before you escalate.

Try the same laptop on another network

The quickest test is a phone hotspot. Connect the same laptop to it and try the GVC connection again. If it works through the hotspot but fails on the user’s normal connection, the laptop and VPN profile are probably fine. The network path has changed, which points the investigation at the original router, modem, NAT device, ISP or carrier network.

This is especially useful when a user says the VPN works at home but not from a hotel, an office, public Wi-Fi or mobile broadband.

Check the GVC log

The GVC has a Log Viewer that records connection activity and errors. Look for repeated Phase 1 attempts and retransmissions around the time the connection fails, and check that the peer gateway address is the one you expect. The log tells you what the client is experiencing, but it can’t prove whether the SonicWall received anything. For that, you need a capture on the firewall.

Enable “Restrict the size of the first ISAKMP packet sent”

SonicWall added a connection property specifically for this error in GVC 4.9.14: Restrict the size of the first ISAKMP packet sent. SonicWall explains that the first ISAKMP packet can be fragmented because of its size, and a router in the path may not allow fragmented packets while the VPN is being established (SonicWall KB).

To enable it:

1. Open the VPN connection’s Properties.
2. Select the General tab.
3. Tick Restrict the size of the first ISAKMP packet sent.
4. Click OK, then try the connection again.

In my experience, this is the first SonicWall-specific setting to test when the basic configuration is correct and the problem follows a particular network connection.

Here’s why it works. The first Phase 1 packet can be larger than the path allows without fragmentation. Once it’s fragmented, every fragment has to reach the SonicWall, and some devices, particularly restrictive routers and mobile connections, drop them instead. The GVC waits for a response that never comes and reports the Phase 1 error. That’s also why the same VPN configuration can work perfectly from one connection and fail from another. The VPN policy hasn’t changed. The network path has.

If you have access to the SonicWall

If the client-side tests don’t solve it, the next step is to prove whether the SonicWall actually receives the Phase 1 traffic. Use Packet Monitor, which on SonicOS 7 is under Monitor > Tools & Monitors > Packet Monitor. Start a capture, reproduce the GVC failure, then look at what arrived from the client.

The fragmentation trap

Don’t just filter the capture on UDP port 500. If the ISAKMP packet has been fragmented, only the first fragment carries the UDP header. The later fragments don’t, so a port-based filter hides the very fragments you’re trying to find.

Filter on the client’s public source IP instead. Get it from any “what is my IP” site on the client before you start. Packet Monitor supports source IP filters and can also monitor fragmented traffic, which gives you a far more useful capture for this problem.

Nothing arrives at the SonicWall

If the client is retrying but Packet Monitor shows nothing from its public IP, the problem is before the SonicWall. Look at the client’s router, hotel or public Wi-Fi equipment, LTE or 5G routers, upstream NAT, ISP filtering, or fragmented UDP being dropped along the way.

At this point, changing SonicWall VPN settings won’t help. The firewall can’t respond to a packet it never received. This is where the Restrict setting above is most likely to fix it.

Packets arrive at the SonicWall

If the Phase 1 packets do reach the firewall, stop blaming the ISP. The investigation moves to the SonicWall’s configuration and security services.

Firewall-side checks when the packets arrive

WAN GroupVPN

On SonicOS 7, go to Network > IPSec VPN > Rules and Settings. Make sure VPN is enabled and the WAN GroupVPN policy is enabled. SonicWall’s own guidance recommends confirming the GroupVPN policy is enabled before anything more complicated, and it’s a quick check.

Access rules restricting IKE

Some administrators restrict which source addresses can use GVC by editing the WAN-to-WAN access rules for IKE (SonicWall KB: How to restrict who can connect through GVC). If someone has done this, a user connecting from a new location will be silently dropped with exactly this error. Check those rules if the user’s public IP has recently changed.

NAT Traversal

If there’s a NAT device between the client and the SonicWall, check Network > IPSec VPN > Advanced > Enable NAT Traversal. It’s enabled by default, and it’s a global setting, so disabling it affects every GVC and site-to-site VPN where a gateway is behind NAT (SonicWall KB: How to enable or disable NAT Traversal). If another router sits in front of the SonicWall, make sure it’s forwarding UDP 500 and UDP 4500 correctly.

Geo-IP and Botnet filtering

This one is easy to overlook, especially when the user is travelling or connecting from an unusual IP range. Geo-IP filtering can block connections based on the country an IP address is associated with, and Botnet filtering blocks addresses linked to known botnets. On SonicOS 7, both are under Policy > Security Services.

Check their configuration and logs for the client’s public IP. You don’t need to disable either service to test this. Work out whether it’s blocking the traffic, then add an exclusion or adjust the policy if you need to.

SonicWall VPN logs

Correlate the firewall’s log timestamps with the GVC connection attempt and your Packet Monitor capture. Look for Phase 1 attempts, rejected negotiations, NAT Traversal activity and security service drops. The exact logging menus vary between SonicOS versions, so check the path for the release you’re running.

A less common cause: negotiation throttling

SonicOS limits how many VPN negotiations it will accept per second. When many GVC clients reconnect at the same time, they can exceed that limit, and Packet Monitor shows packets reaching the firewall with no response. SonicWall documented this for SonicOS 7.0.1. Later builds log “Negotiation under throttling” and let you raise the threshold (SonicWall KB: Firewall not responding to VPN requests intermittently in GVC).

This isn’t where I’d start for one user’s problem. It becomes relevant when many users hit the same error at around the same time, for example after an internet outage when every client reconnects at once.

What not to change first

Two troubleshooting moves are tempting but make poor first-line fixes.

Lowering the SonicWall WAN MTU. It’s common advice to drop the WAN interface from 1500 to something like 1400. It doesn’t address this problem. If the client’s first ISAKMP packet is being fragmented or dropped before it reaches the SonicWall, changing the firewall’s own MTU doesn’t make the client’s packet any smaller, and it changes the behaviour of every other packet on that interface. If the evidence points to a path MTU problem, investigate the path rather than guessing at the firewall.

Disabling security features at random. Switching off broad firewall functionality one piece at a time makes troubleshooting messy fast. If the packet never reaches the firewall, no firewall setting can fix it. If Geo-IP or Botnet filtering is the suspect, investigate that service specifically. Let the packet capture tell you where to look before you touch any security control.

A practical troubleshooting sequence

If you only have access to the user’s PC

1. Confirm the user has normal internet access.
2. Check the GVC peer gateway address.
3. Try the same laptop through a phone hotspot.
4. In the connection’s Properties > General tab, enable Restrict the size of the first ISAKMP packet sent.
5. Check the GVC Log Viewer for repeated Phase 1 attempts.
6. If it still fails, escalate with the user’s public IP, network type and GVC log.

If you have access to the SonicWall

1. Confirm VPN and the WAN GroupVPN policy are enabled.
2. Run Packet Monitor while reproducing the problem, filtered on the client’s public source IP, not just UDP 500.
3. If nothing arrives, investigate the network path and the Restrict setting.
4. If packets arrive, check access rules, NAT Traversal, Geo-IP, Botnet filtering and the VPN logs.
5. If many users are affected at once, check for negotiation throttling.

Find the packet first

The message “The peer is not responding to Phase 1 ISAKMP requests” doesn’t prove the SonicWall is refusing the connection. It helps to think of the traffic as a chain:

GVC client -> network path -> SonicWall -> network path -> GVC client

Something in that chain has failed. If nothing reaches the SonicWall, investigate the path. If the packet arrives but there’s no response, investigate the firewall. If the SonicWall responds but the client never receives it, investigate the return path.

This error can look like a complicated IPsec configuration problem when the real issue is a packet being dropped somewhere between the user and the firewall. The fastest client-side test is a different network, such as a phone hotspot, and the fastest fix is often the GVC Restrict the size of the first ISAKMP packet sent option. If you have firewall access, Packet Monitor gives you the definitive answer.

Stop guessing. Find the packet first, then troubleshoot the device that actually lost it.

Sources

• SonicWall KB: The Peer is Not Responding to Phase 1 ISAKMP Requests error in GVC
• SonicWall KB: How to enable or disable NAT Traversal in VPN settings
• SonicWall KB: How to restrict who can connect through GVC
• SonicWall KB: Firewall not responding to VPN requests intermittently in GVC
• SonicWall KB: Troubleshooting common log errors for IPsec VPN policies and GVC

Daniel Lutton

From my early days on the helpdesk through roles as a service desk manager, systems administrator, and network engineer, I’ve spent more than 25 years in the IT world. As I transition into cyber security, my goal is to make tech a little less confusing by sharing what I’ve learned and helping others wherever I can.