Posts Tagged ‘multicast troubleshooting’



This publication illustrates some common techniques for troubleshooting multicast issues in IP networks. Common problems and their causes are discussed, troubleshooting techniques demonstrated. PIM Sparse mode is used for most of the examples, due to the fact that this is the most complicated mode of multicast signaling. The suggested troubleshooting approach separates control plane from data-plane troubleshooting and heavily relies on the mroute command for the control-plane verification. This publication requires solid understanding of intra-domain multicast routing technologies.

Common Reasons for Multicast Problems

In short, one common reason for all issues with multicast routing is the PIM and logical/physical topology incongruence. Ideally, multicast should be deployed in a single IGP domain with PIM enabled on all links running the IGP with all links preferably being point-to-point or broadcast multiple-access. If you have multicast running across the domain that has multiple IGPs, or you don not have PIM enabled on all links or finally you have NBMA links in the topology – you have open possibilities for a problem. Unfortunately, the “problem” conditions just described are very common in the CCIE lab exam environment.

The most common type of multicast issue is the RPF Failure. RPF checks are used both at the control and data plane of multicast routing. Control plane involves PIM signaling – some PIM messages are subject to RPF checks. For example, PIM (*,G) Joins are sent toward the shortest path to RP. Next, the BSR/RP address in the BSR messages is subject to RPF check as well. Notice that this logic does not apply to PIM Register messages – the unicast register packet may arrive on any interface. However, RPF check is performed on the encapsulated multicast source to construct the SPT toward the multicast source.

Data plane RPF checks are performed every time a multicast data packet is received for forwarding. The source IP address in the packet should be reachable via the receiving interface, or the packet is going to be dropped. Theoretically, with PIM Sparse-Mode RPF checks at the control plane level should preclude and eliminate the data-plane RPF failures, but data-plane RPF failures are common during the moments of IGP re-convergence and on multipoint non-broadcast interfaces.

PIM Dense Mode is different from SM in the sense that data-plane operations preclude control-plane signaling. One typical “irresolvable” RPF problem with PIM Dense mode is known as “split-horizon” forwarding, where packet received on one interface, should be forwarded back out of the same interface in the hub-and-spoke topology. The same problem may occur with PIM Sparse mode, but this type of signaling allows for treating the NBMA interface as a collection of point-to-point links by the virtue of PIM NBMA mode.
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