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Complexities of Monitoring Multicast Networks

In a previous blog, we covered the basics of multicast routing. Multicast is preferred over unicast and broadcast by content and service provider networks for IPTV, video content distribution, stock market live feeds, and in data centers to synchronize data between servers. Large enterprises also use multicast for video surveillance and teleconferencing. Because multicast is used for these critical and high-volume applications, any downtime can lead to major loss of revenue and customer churn.

In this blog post, we will look at the challenges of running multicast applications and how you can monitor and manage multicast networks.

Multicast versus Unicast

Though multicast requires considerably less resources and reduces delay when compared to unicast, it does not provide the same level of visibility. With unicast, a single source sends to a single destination and the IP address of each participating device is known. But with multicast, a single source sends to a group of devices – a host group – that can be dynamic. This makes managing and monitoring network performance more complex and many network operators are challenged to know what multicast addresses are used in their network and what they are used for. For example, a financial services organization we talked to estimated there are sixteen thousand multicast groups in their network. While they would like to reduce that number, they can’t because they do not know what each group is used for.

Challenges with Multicast Routing

There are other challenges associated with running a multicast network. Here are four common issues:

  • If multicast is not enabled on an upstream interface that connects to the Rendezvous Point (RP), it can result in what is known as an RPF (Reverse Path Forwarding) failure, which will break multicast routing.
  • To enable multicast on an interface, PIM must be enabled on all the routers connected to that interface. But if it is a low capacity interface, the high volume of multicast traffic reaching it may result in congestion. In some cases, this can cause downstream receivers to request missing packets. This in turn increases the volume of multicast traffic and can possibly result in a total network meltdown.
  • Problems occur when multicast group to RP mappings are misconfigured. Because these mappings are statically configured, an incorrect or different configuration on routers can result in some receivers never receiving the data.
  • Changes made to the underlying unicast routing that affect the choice of the upstream multicast router can result in the branch from the multicast tree to be grafted to another router, leading to multicast routing failure. Diagnosing such changes requires visibility not only into multicast, but also into the underlying unicast routing.

Monitoring Multicast Networks

Traditional network and traffic monitoring tools are insufficient for monitoring multicast routing. Information from SNMP MIBs provides no visibility into multicast logical constructs and these tools often lack information about the underlying IGP and BGP control plane.

Enterprises, content and network service providers who operate multicast networks require specialized tools that collect multicast routing topology (groups, trees, sources and receivers), establish baselines, detect changes and correlate them with underlying IGP and BGP routing changes. With this information, when a link goes down, they can be alerted that all multicast trees using the link will have to re-converge.

Monitoring tools must also track multicast-specific issues, such as RP mismatches, interfaces without PIM enabled, PIM interfaces with no router connection, groups with no receivers, etc., and issue alerts so that preventive actions can be taken quickly.

We hope to have highlighted some of the complexities of managing and monitoring multicast networks. Have you faced any of these challenges in your multicast network?

If you have or would like to learn more about monitoring and managing multicast better, download our whitepaper that covers all this and more in detail:

Understanding & Managing Multicast Routing