Multicast ASM & SSM

[Ab26]

Hi !

I have a multicast setup in my network using PIM-ASM.

For some reason a router in my network stopped responding to IGMP requests. Although the show commands for IGMP showed everything works. Has anyone experienced the same thing? I’m planning to upgrade and restart the router and see if this resolves the problem. 
Another question, one of my customers is complaining that the multicast traffic stops every 30 seconds and starts again. I have checked the multicast table (sh ip mroute) and I can see the streams have been there for several weeks. I can see the packet count is increasing (mfib). I have checked the unicast routing table, and the routes has been there for a long time (no route flapping). I have checked errors and physical interfaces and none is there. Does anyone have an advice? What kind of troubleshooting I could do to verify that there is no problem on the network side?

I want to ask about converting to SSM, would it be more beneficial if the customer has only few sites? So is there more benefit of using SSM over ASM other than the ease of configuration?

[AshSe]

Hello @Ab26 

Okay, let's break down these multicast issues and the potential migration to SSM.

IGMP Not Responding (But Show Commands Look OK)

This is a frustrating situation. Here's a breakdown of potential causes and troubleshooting steps, beyond a simple reboot (which, yes, is a reasonable first step):

1. Hidden Resource Exhaustion: Routers have limits on the number of multicast groups, interfaces participating in IGMP, and general memory allocated to multicast processes. show commands might not always reveal these limits being hit.

   1. Check Resource Usage: Look for specific commands related to multicast memory usage or group limits. The exact commands depend on your router vendor (Cisco, Juniper, etc.). For Cisco, consider show ip mroute summary, show processes memory sorted. High CPU utilization can also indirectly indicate resource issues.
   2. Temporary Workaround (If Possible): If you suspect a limit, try temporarily reducing the number of multicast groups on the affected router, if feasible, to see if it resolves the IGMP issue.

2. Software Bug: While less common, a bug in the router's IGMP implementation is possible.

   1. Check Release Notes: Before upgrading, carefully review the release notes for the target IOS/firmware version. Look for any bug fixes related to IGMP, PIM, or multicast in general. Upgrading without checking release notes can sometimes make things worse.
   2. Consider a Staged Upgrade: If possible, upgrade a test/non-production router first to evaluate the new version.

3. ACLs or Firewall Issues: Even if you think your ACLs are correct, double-check them. A misconfigured ACL could be blocking IGMP messages.

   1. Verify ACLs: Carefully examine any ACLs applied to interfaces facing the hosts sending IGMP reports and the upstream multicast router. Ensure they permit IGMP (protocol 2) and PIM (protocol 103).
   2. Temporary ACL Bypass (Use with Caution): As a temporary troubleshooting step (in a lab or maintenance window), you could remove the ACL to see if IGMP starts working. Do not leave the ACL removed in production.

4. PIM Issues: PIM and IGMP interact closely. Problems with PIM can sometimes manifest as IGMP issues.

   1. Check PIM Adjacencies: show ip pim neighbor (Cisco) or equivalent. Make sure the router has PIM neighbors on the interfaces where it should.
   2. PIM Configuration: Ensure PIM is correctly enabled on the relevant interfaces.

5. Hardware Issues: Although you mentioned no interface errors, a subtle hardware problem could be affecting IGMP processing.

   1. Run Diagnostics: If your router has built-in hardware diagnostics, run them.
   2. Swap Interface Cards (If Possible): If practical, try swapping the interface card experiencing the issue with a known good card.

Multicast Traffic Stopping Every 30 Seconds

This is more perplexing. The fact that show ip mroute shows the streams active and the MFIB packet count is increasing rules out some basic problems. Here's a more in-depth troubleshooting approach:

1. IGMP Membership Timeout: The most likely culprit, given the 30-second interval, is the IGMP membership timeout. Hosts send IGMP membership reports to indicate they want to receive a multicast stream. Routers maintain a membership state based on these reports. If a router doesn't receive a report for a group within the timeout period, it stops forwarding the multicast traffic. The default IGMP timeout is often around 260 seconds, but it can be configured.

   • Verify IGMP Configuration on the Router:
     1. show ip igmp interface <interface> (Cisco) or equivalent. Check the IGMP query interval and IGMP querier timeout. The timeout should be significantly longer than the query interval. The default query interval is often 60 seconds, and the timeout is 125 seconds. If the timeout is close to 30 seconds, that's a big clue.
   • Verify IGMP Configuration on the Hosts: Some operating systems allow you to configure the IGMP report interval. Make sure the hosts are sending reports frequently enough. You can often use packet captures on the host to verify the IGMP reports.
   • Check for IGMP Suppression: Some network devices have features that suppress IGMP reports to reduce network traffic. If this is enabled, it could be interfering with the router's ability to maintain membership state.
   • IGMP Version Mismatch: Ensure that the hosts and routers are using compatible IGMP versions (v2 or v3). Mismatched versions can lead to inconsistent membership reporting.

2. Multicast TTL (Time-to-Live): If the TTL of the multicast packets is too low, they might be expiring before reaching the receiver. However, this wouldn't typically cause a periodic drop every 30 seconds.

   • Check TTL: Use a packet capture to examine the TTL of the multicast packets. Ensure it's high enough to reach all receivers.

3. Network Congestion or Packet Loss: Although you checked for interface errors, transient congestion or packet loss could be causing IGMP reports to be lost.

   • Monitor Interface Statistics: Use more granular monitoring tools (SNMP, NetFlow, etc.) to track interface utilization, packet loss, and errors over time. Look for any spikes in traffic or errors that correlate with the multicast drops.
   • QoS Configuration: If you have QoS configured, ensure that multicast traffic is being prioritized appropriately. Poor QoS configuration could lead to multicast packets being dropped during congestion.

4. Unexpected PIM Assertions: In a shared network segment, multiple routers might be eligible to forward multicast traffic. PIM uses an assertion mechanism to elect a single forwarder. If there are issues with the assertion process, it could theoretically cause brief interruptions in traffic flow.

   • Check PIM Assertions: show ip pim assert (Cisco) or equivalent. Look for frequent changes in the assert winner.

5. Incorrect RP Configuration: If you are using Auto-RP or BSR, make sure you have redundancy implemented and that the RP is reachable. If the RP disappears for a short time, it could cause the multicast traffic to stop.

6. Hardware Offload Issues: Some network devices use hardware offloading to accelerate multicast forwarding. If there's a problem with the hardware offload engine, it could lead to intermittent issues.

   • Disable Hardware Offload (Use with Caution): As a temporary troubleshooting step (in a lab or maintenance window), you could try disabling hardware offload for multicast to see if it resolves the problem. This will likely impact performance, so don't leave it disabled in production.

Converting to SSM (Source-Specific Multicast)

You asked about the benefits of SSM, especially for a customer with only a few sites. Here's a comparison:

1. ASM (Any-Source Multicast):

   1. Complexity: More complex to configure, especially with Auto-RP or BSR. Requires an RP (Rendezvous Point) to be configured.
   2. Scalability: Scales well to large networks with many multicast groups and sources.
   3. Security: Vulnerable to inter-group forwarding. Any host can send to a multicast group.
   4. RP Configuration: Requires configuration and maintenance of the RP.

2. SSM (Source-Specific Multicast):

   1. Complexity: Simpler to configure. No RP is needed.
   2. Scalability: Scales well, but can be less efficient in very large networks with many sources for the same group.
   3. Security: More secure. Receivers explicitly request traffic from specific sources.
   4. Source Awareness: Requires receivers to know the source address of the multicast stream.

Benefits of SSM for a Few Sites:

1. Simplified Configuration: This is a significant advantage for smaller deployments. You eliminate the need to configure and maintain an RP, which simplifies the overall multicast setup.
2. Enhanced Security: SSM inherently provides better security because receivers explicitly join a multicast group for a specific source. This prevents unauthorized sources from injecting traffic into the group.
3. Reduced Overhead: No RP means less overhead in terms of control plane traffic and router processing.

Drawbacks of SSM:

1. Source Awareness: Receivers must know the source address of the multicast stream. This might require changes to applications or streaming devices.
2. Limited Compatibility: Older devices or applications might not support SSM.
3. Scalability Concerns: With a huge number of sources for one group, SSM can be less efficient than ASM.

Recommendation:

For a customer with only a few sites, SSM is generally the better choice due to its simplicity and enhanced security. The key is to ensure that the customer's applications and devices support SSM and that they can be configured to specify the source addresses of the multicast streams. If the customer is using older equipment that doesn't support SSM, then ASM might be necessary.

Steps to Migrate to SSM:

1. Verify SSM Support: Confirm that all routers and receivers support SSM.
2. Configure SSM Range: Configure an SSM range on your routers using the ip pim ssm range <acl> command. This defines the multicast group addresses that will use SSM.
3. Configure Receivers: Configure the receivers to join the multicast group for the specific source (using IGMPv3 or a similar mechanism).
4. Disable Auto-RP or BSR: If you're currently using Auto-RP or BSR for ASM, disable them.
5. Test Thoroughly: Test the multicast streams to ensure they are working correctly after the migration.

In Summary

• Troubleshoot the IGMP issue methodically, focusing on resource exhaustion, ACLs, and potential software bugs.
• The 30-second multicast drop strongly suggests an IGMP membership timeout problem. Investigate the IGMP configuration on your routers and hosts.
• Consider migrating to SSM for its simplicity and security benefits, especially for a smaller deployment. Ensure your customer's equipment supports SSM.

Good luck! Let me know if you have more questions.

HTH & Stay Curious!

AshSe

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