Ticket#7 – Multicast Video Freezes in Conference Room – Troubleshoot IGMP and PIM [CCNP ENTERPRISE]

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Problem Summary

In a conference room, video multicast stream intermittently freezes or buffers during a live broadcast.

The network uses multicast for IPTV using:

• IGMPv2 for group management
• PIM-Sparse Mode for multicast routing
• Cisco Layer 2 switches and Layer 3 routers

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Symptoms Observed

Symptom	Observed Impact
Video Freezing	Occurs every 2–3 mins
IGMP joins seen	But not always registered at the router
No new route in mroute	Upstream routers not forwarding traffic
Video works in some rooms	But not in the boardroom

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Root Cause Analysis

Multicast packets were not reaching the end hosts due to one or more of the following reasons:

• IGMP snooping not enabled or misconfigured on Layer 2 switches
• PIM Sparse Mode not enabled on all interfaces
• Missing or inconsistent RP configuration
• IGMP Join messages were not reaching the router due to filtering
• Multicast boundary set improperly

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The Fix

Here’s how the issue was resolved:

Fix Summary

Step	Description
1	Verified IGMP snooping on access switches
2	Enabled ip pim sparse-mode on all Layer 3 interfaces
3	Configured ip igmp snooping vlan X mrouter interface
4	Verified correct RP address with ip pim rp-address
5	Used show commands to verify group joins and traffic flow

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EVE-NG Lab Topology

Lab Diagram

Device Roles

• R1 – Multicast source router, PIM-SM enabled
• SW1/SW2 – L2 access switches with IGMP snooping
• RP (Rendezvous Point) – R2 configured as static RP

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CLI Configuration

Router (PIM-SM Config)

interface Gig0/0
 ip address 10.1.1.1 255.255.255.0
 ip pim sparse-mode

ip pim rp-address 10.2.2.2

Switch IGMP Snooping Config

ip igmp snooping
ip igmp snooping vlan 10 mrouter interface Gig1/0/1

RP Config

interface Loopback0
 ip address 10.2.2.2 255.255.255.255
!
ip pim rp-address 10.2.2.2

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Verification

Command	Output to Check
show ip mroute	Confirms multicast routes and incoming/outgoing interfaces
show ip igmp groups	Displays host group memberships
show ip pim neighbor	Confirms PIM neighbors are up
debug ip igmp	Verifies join/leave behavior
ping <multicast IP>	Tests multicast reachability (with helper tool)

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Key Takeaways

• IGMP snooping must be enabled and tuned per VLAN
• Ensure all Layer 3 interfaces running multicast have PIM enabled
• Always verify RP reachability and configuration
• Use debug and show tools to trace join/prune behavior
• Lab everything—multicast is often not plug-and-play

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Best Practices / Design Tips

• Use static RP or Auto-RP with redundancy
• Enable IGMP Querier where no L3 device exists
• Document VLANs and interfaces participating in multicast
• Use storm control to prevent multicast abuse
• Prefer Sparse-Mode PIM for scalable deployments

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FAQs

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1. Why does multicast video freeze intermittently?

Answer:
Freezing typically occurs when IGMP joins are not sustained, multicast routes are missing, or RP is unreachable intermittently.

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2. What’s the role of IGMP snooping in Layer 2 switches?

Answer:
IGMP snooping allows switches to listen to IGMP messages and forward multicast traffic only to ports with subscribed hosts, avoiding unnecessary flooding.

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3. What happens if IGMP snooping is disabled?

Answer:
Without IGMP snooping, multicast traffic is flooded across the VLAN, possibly overloading non-participating devices or causing loss.

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4. What is an mrouter interface in IGMP snooping?

Answer:
It tells the switch which interface connects to the router so it can forward IGMP join/prune messages to that router.

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5. How do I verify if multicast routing is working?

Answer:
Use show ip mroute, show ip pim neighbor, and show ip igmp groups to trace packet flow and neighbor relationships.

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6. What if the RP is misconfigured?

Answer:
The multicast tree can’t form properly. Traffic will not be routed, and clients won’t receive the stream.

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7. Can multicast work without PIM?

Answer:
No. PIM is required for multicast routing between routers. You may use PIM Sparse-Mode or Dense-Mode depending on design.

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8. Should the RP be reachable from all routers?

Answer:
Yes, all PIM routers must reach the RP and know its IP via routing (static or dynamic).

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9. What are common multicast IP ranges?

Answer:
Multicast uses 224.0.0.0 to 239.255.255.255. Reserved ranges like 224.0.1.x and 239.x.x.x are commonly used.

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10. How to simulate multicast traffic in EVE-NG?

Answer:
Use tools like VLC multicast stream or simulate IGMP joins using router-generated multicast packets.

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11. What is the purpose of ip pim sparse-mode?

Answer:
It enables the interface to participate in PIM-SM routing, necessary for tree building and RP communication.

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12. Can I use dense mode in enterprise networks?

Answer:
Sparse Mode is preferred. Dense mode floods traffic by default and is less efficient in large networks.

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13. How do I monitor multicast performance?

Answer:
Use SNMP traps, NetFlow, and show commands to monitor joins, prunes, and dropped packets.

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14. Can IGMP version mismatch cause issues?

Answer:
Yes. Ensure all devices use compatible IGMP versions (e.g., IGMPv2 or IGMPv3) across the VLAN.

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15. What tools help with multicast troubleshooting?

Answer:
Use Cisco tools: debug ip igmp, debug ip mpacket, Wireshark with multicast filters, and multicast-capable pings.

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YouTube Link

Watch the Complete CCNP Enterprise: Multicast Video Freezes in Conference Room – Troubleshoot IGMP and PIM Demo & Explanation on our channel:

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Final Note

Understanding how to differentiate and implement Multicast Video Freezes in Conference Room – Troubleshoot IGMP and PIM is critical for anyone pursuing CCNP Enterprise (ENCOR) certification or working in enterprise network roles. Use this guide in your practice labs, real-world projects, and interviews to show a solid grasp of architectural planning and CLI-level configuration skills.

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