SIP ALG Problems: Complete Troubleshooting Guide for VoIP NAT Issues

SIP ALG Problems: Complete Troubleshooting Guide for VoIP NAT Issues

SIP ALG problems are among the most frustrating issues facing VoIP administrators and telecom operators today. When SIP Application Layer Gateway (ALG) functionality interferes with VoIP traffic, it causes registration failures, one-way audio, dropped calls, and complete communication breakdowns. This comprehensive troubleshooting guide covers everything you need to know about diagnosing and resolving SIP ALG problems across all major router brands and network configurations.

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Table of ContentsSIP ALG Problems: Complete Troubleshooting Guide for VoIP NAT Issues What is SIP ALG and Why Does It Break VoIP? How SIP ALG Works (In Theory) Why SIP ALG Causes Problems (SIP ALG Problems) Common SIP ALG Problem Symptoms (SIP ALG Problems) Symptom Checklist How to Disable SIP ALG on Popular Routers Cisco Routers Fortinet FortiGate MikroTik RouterOS TP-Link Routers Netgear Routers Asus Routers Ubiquiti UniFi / EdgeRouter NAT Traversal Solutions Beyond Disabling SIP ALG (SIP ALG Problems) NAT Traversal Methods Comparison VOS3000 NAT Configuration VOS3000 NAT-Related System Parameters VOS3000 Media Proxy Configuration Diagnosing SIP ALG Problems Testing for SIP ALG Presence Wireshark Filter Commands Related Resources (SIP ALG Problems) Frequently Asked QuestionsHow do I know if my router has SIP ALG enabled?Will disabling SIP ALG break other applications?Why do calls still drop after disabling SIP ALG?Can SIP ALG be disabled on ISP-provided routers?What is the difference between SIP ALG and SIP Helper? Get Expert Help with SIP ALG Problems Need Professional VOS3000 Setup Support?

What is SIP ALG and Why Does It Break VoIP?

SIP ALG (Application Layer Gateway) is a feature implemented in many routers and firewalls that is designed to help SIP traffic traverse NAT (Network Address Translation) boundaries. While the intention is good, SIP ALG implementations are notoriously problematic and often cause more harm than benefit for VoIP deployments.

How SIP ALG Works (In Theory)

The SIP ALG function monitors SIP signaling traffic and attempts to modify SIP headers and SDP (Session Description Protocol) payloads to help with NAT traversal. When a SIP message passes through a NAT device, the ALG examines the packet and rewrites IP addresses and port numbers to match the public-facing NAT address instead of the private internal address.

Why SIP ALG Causes Problems (SIP ALG Problems)

Common SIP ALG Problem Symptoms (SIP ALG Problems)

Identifying SIP ALG problems requires recognizing specific patterns in VoIP behavior. The following symptoms strongly indicate SIP ALG interference in your network:

Symptom Checklist

One-Way Audio: Call connects but only one party can hear audio, typically the internal party cannot hear external caller

No Audio on Answer: Phone rings and answers, but complete silence on both ends

Registration Expiry: Extensions register initially but lose registration within minutes

30-Second Call Drops: Calls disconnect precisely at 30-second intervals due to NAT binding timeout

Incoming Call Failures: Outbound calls work fine but inbound calls never reach the phone

Intermittent Issues: Problems appear and disappear without apparent pattern

VPN vs Direct: VoIP works through VPN but fails on direct internet connection

How to Disable SIP ALG on Popular Routers

Disabling SIP ALG is often the most direct solution to VoIP NAT problems. Below are instructions for major router brands commonly found in VoIP deployments:

Cisco Routers

On Cisco IOS routers, SIP ALG is implemented as SIP inspection in the firewall configuration:

! Check current SIP inspection status
show running-config | include sip

! Disable SIP inspection in class-map
configure terminal
class-map inspection_default
no match protocol sip

! Or remove from policy-map
policy-map global_policy
class inspection_default
no inspect sip

! Save configuration
write memory

Fortinet FortiGate

FortiGate firewalls have SIP ALG enabled by default. Disable through CLI or GUI:

! Via CLI – Check SIP helper status
diagnose sys sip-proxy status

! Disable SIP helper
config system settings
set sip-helper disable
set sip-nat-trace disable
end

! Also check VOIP profile
config voip profile
edit default
config sip
set status disable
end
next
end

MikroTik RouterOS

MikroTik routers use SIP helper for ALG functionality:

# Check SIP helper status
/ip firewall service-port print

# Disable SIP helper
/ip firewall service-port disable sip

# For older RouterOS versions
/ip firewall nat disable [find comment=”SIP”]

TP-Link Routers

TP-Link consumer and business routers have SIP ALG in different locations:

Netgear Routers

# Web Interface Navigation
# 1. Login to router admin panel
# 2. Go to Advanced → Setup → WAN Setup
# 3. Find “SIP ALG” or “SIP Connection Tracking”
# 4. Uncheck/disable the option
# 5. Apply changes and reboot router

Asus Routers

# Web Interface
# 1. Advanced Settings → WAN
# 2. NAT Passthrough tab
# 3. Set “SIP Passthrough” to “Disable”
# 4. Apply and reboot

# Via SSH/Telnet
nvram set sip_passthrough=0
nvram commit
reboot

Ubiquiti UniFi / EdgeRouter

# UniFi Security Gateway
# Via config.gateway.json:
{
“service”: {
“nat”: {
“rule”: {
“5000”: {
“description”: “Disable SIP ALG”,
“log”: “disable”,
“protocol”: “all”,
“source”: {
“group”: {
“network-group”: “net_LAN”
}
},
“type”: “masquerade”
}
}
}
}
}

# EdgeRouter CLI
configure
set service nat rule 5000 disable
commit
save

NAT Traversal Solutions Beyond Disabling SIP ALG (SIP ALG Problems)

In some network environments, simply disabling SIP ALG is not sufficient or may not be possible. Understanding and implementing proper NAT traversal techniques ensures reliable VoIP operation.

NAT Traversal Methods Comparison

VOS3000 NAT Configuration

For VOS3000 softswitch deployments, proper NAT configuration is essential. VOS3000 provides several parameters to handle NAT traversal scenarios:

VOS3000 NAT-Related System Parameters

VOS3000 Media Proxy Configuration

VOS3000 supports multiple media proxy modes to handle NAT scenarios. The SS_MEDIAPROXYMODE parameter controls this behavior:

Media Proxy Modes in VOS3000:

ON – Media proxy always enabled
All RTP flows through VOS3000 server
Highest server resource usage

OFF – Media proxy always disabled
RTP flows directly between endpoints
May fail with NAT issues

AUTO – VOS3000 decides based on conditions:
1. If caller/callee requires media proxy → Enable
2. If caller/callee disabled media proxy → Disable
3. If encryption enabled → Enable
4. If different networks (SS_MEDIAPROXYBETWEENNET) → Enable
5. If behind NAT (SS_MEDIAPROXYBEHINDNAT) → Enable
6. Otherwise → Disable

MUST ON – Forced media proxy regardless of settings
Used for specific troubleshooting scenarios

Diagnosing SIP ALG Problems

Testing for SIP ALG Presence

Before making configuration changes, confirm that SIP ALG is actually causing the problem:

Packet Capture Analysis: Use Wireshark to capture SIP traffic and compare original packets with received packets

Contact Header Check: Look for differences between internal IP and Contact header IP in SIP messages

SDP Analysis: Compare c= (connection) line in SDP with actual endpoint IP

Via Header Inspection: Check if received/rport parameters are being modified incorrectly

Online Tools: Use SIP ALG detection tools available from VoIP providers

Wireshark Filter Commands

# SIP traffic filter
sip

# SIP registration only
sip.Method == “REGISTER”

# SIP invite and responses
sip.Method == “INVITE” || sip.Status-Code

# RTP media streams
rtp

# Check for NAT-related issues
sip.Contact contains “192.168” || sip.Contact contains “10.”

Related Resources (SIP ALG Problems)

VOS3000 Extended Firewall Configuration Guide

VOS3000 One-Way Audio Troubleshooting

VOS3000 Server Configuration Guide

SIP Trunking with VOS3000

VOS3000 Downloads

Frequently Asked Questions

How do I know if my router has SIP ALG enabled?

The most reliable method is to capture SIP traffic using Wireshark and examine the Contact headers and SDP content. If the IP addresses in these fields show your public IP when they should show private IPs (or vice versa), SIP ALG is active. Many router admin interfaces also display SIP ALG status in the NAT or Firewall settings sections.

Will disabling SIP ALG break other applications?

In most cases, disabling SIP ALG does not negatively affect other applications. SIP ALG is specifically designed for SIP protocol and has no impact on web browsing, email, or other network services. However, some legacy SIP devices that rely on ALG for NAT traversal may require alternative NAT configuration after disabling.

Why do calls still drop after disabling SIP ALG?

If problems persist after disabling SIP ALG, other factors may be involved: firewall rules blocking RTP ports, incorrect NAT keep-alive settings, SIP session timer issues, or NAT binding timeouts. Check firewall rules for ports 5060 (SIP) and 10000-20000 (RTP), and verify SIP registration expiry settings.

Can SIP ALG be disabled on ISP-provided routers?

Many ISP-provided routers do not allow SIP ALG configuration through the web interface. Options include: contacting ISP to disable the feature, using bridge mode with a separate router, or replacing the ISP router entirely with a commercial router that offers full configuration access.

What is the difference between SIP ALG and SIP Helper?

SIP ALG and SIP Helper are essentially the same feature with different naming conventions across vendors. Cisco and MikroTik commonly use “SIP Helper,” while Fortinet and others use “SIP ALG.” Both refer to the router’s ability to inspect and modify SIP packets for NAT traversal purposes.

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