How to Find the Source of Multicast Traffic in an Office Network

To find the source and limit multicast traffic causing your internet provider to block service due to 27,000 packets per second, you need to use specialized multicast stream monitoring tools, check switch configurations for improper IGMP snooping settings, and apply traffic restrictions at the network edge.

Contents

• Main methods for finding multicast traffic source
• Monitoring and analysis tools for multicast traffic
• Practical steps for traffic limitation
• Network device configuration for multicast management
• Integration with provider monitoring system

Main methods for finding multicast traffic source

In this case, when Wireshark doesn’t show anomalies within the network but the provider detects high load, the problem may be related to several factors. First, check the Cisco CLI commands for analyzing multicast routing:

show ip mroute count
show ip mroute active

These commands will show traffic statistics for each (S,G) pair and help identify which sources and groups are generating excessive traffic. Also use:

• show ip igmp groups - to view active multicast groups
• show ip igmp interface - to check IGMP status on interfaces
• debug ip mpacket - for detailed multicast packet tracking

Remember that multicast sources are tracked by network nodes listening to streams, and the network adds receivers to the distribution tree when they request to join a group source.

Monitoring and analysis tools for multicast traffic

For accurately identifying the source of high-load multicast traffic, use specialized tools:

Enterprise solutions:

• SolarWinds Network Performance Monitor - automatically monitors multicast networks and alerts about performance issues
• NetFlow Analyzer - shows information about multicast traffic passing through interfaces
• Cisco Multicast Manager - provides a global network view and displays active sources and groups

Open-source tools:

• MNPA (Multicast Network Performance Analyzer) - for measuring network performance overloaded with multicast traffic
• Mtools - a set of tools for testing multicast connectivity
• Mausezahn - traffic generator for testing VoIP and multicast networks

Important: When analyzing, use Wireshark filters igmp || cgmp for analyzing IGMP and CGMP messages, as recommended in multicast video traffic analysis cases source.

Practical steps for traffic limitation

After identifying the multicast traffic source, take the following measures:

1. Rate limiting at the network edge:

   bash

   policy-map multicast-limit
     class class-default
       police cir 10000
   interface GigabitEthernet0/1
     service-policy output multicast-limit
   

2. Configure IGMP snooping on switches:

   bash

   ip igmp snooping
   ip igmp snooping querier
   ip igmp snooping immediate-leave
   

3. Block unwanted multicast groups:

   bash

   ip access-list extended BLOCK_MC
     deny ip any 224.0.0.0/4
     permit ip any any
   interface GigabitEthernet0/1
     ip access-group BLOCK_MC in
   

4. Use ip multicast ttl-threshold to prevent multicast traffic from leaking outside the network source.

Network device configuration for multicast management

To prevent similar situations in the future, configure switches and routers as follows:

On Layer 3 switches:

• Enable PIM (Protocol Independent Multicast)
• Configure Sparse-Mode instead of Dense-Mode for large networks
• Use ip pim spt-threshold infinity to prevent switching to SPT (Shortest Path Tree)
• Configure multicast source filtering using standard ACLs

On Layer 2 switches:

• Enable IGMP snooping
• Configure static membership for critical applications
• Use QinQ for isolating multicast domains

Tip: As noted in discussions on Reddit, sometimes it’s useful to set an explicit join on the VLAN interface to check traffic and disable switching to SPT using the command ip pim spt-threshold infinity source.

Integration with provider monitoring system

Since the provider is blocking the internet due to 27,000 packets per second, urgently coordinate with them:

1. Request detailed logs from the provider about the traffic type
2. Provide information about your multicast infrastructure
3. Discuss the possibility of configuring bandwidth for multicast traffic
4. Consider using a dedicated channel for critical multicast applications

Conclusion

To solve the problem with multicast traffic causing internet provider blocking:

1. Urgently use Cisco CLI commands (show ip mroute, show ip igmp groups) to identify the traffic source
2. Apply rate limiting at the network edge using policing policies
3. Configure IGMP snooping and multicast group filtering on switches
4. Integrate specialized monitoring tools to prevent future incidents
5. Coordinate with the provider to configure proper traffic throughput

The most effective solution will be combining technical restrictions with improved real-time multicast stream monitoring, which will not only solve the current problem but also prevent similar situations in the future.

Sources

1. Troubleshoot Multicast Networks with CLI Tools - Cisco
2. Multicast Traffic Monitoring with NetFlow | How does it Work?
3. Understanding IP Multicast Monitoring - Geek Speak - Resources - THWACK
4. Multicast Network Performance Analyzer (MNPA) - GitHub
5. How to generate and verify multicast traffic - Ostinato
6. Troubleshooting Common IP Multicast Issues | OrhanErgun.net Blog
7. Tools to test multicast routing - Server Fault
8. r/networking on Reddit: Multicast Routing Troubleshooting
9. Mastering the Art of Multicast Video Traffic Analysis: 3 Real-World Troubleshooting Case Studies
10. Monitoring and Maintaining IP Multicast [Networking Software (IOS & NX-OS)] - Cisco Systems