Core Issue
IP multicasting provides a method to send information to a group of hosts at the same time, which is different from the normal IP unicast routing that sends information only to one specific host at a time. IP multicast relies on a data distribution tree built by a multicast routing protocol to deliver packets from the source to the receivers when they are connected to different networks. The most common issue in a multicast network is packets transmitted by the source not reaching receivers. Other issues could be related to the formation of the distribution tree itself, and unwanted flooding in LAN environments that use switches.
Resolution
One of the common causes of packets not reaching receivers is Reverse Path Forwarding (RPF) check failure. Once a multicast data distribution tree is formed using a multicast routing protocol, routers use RPF check to forward multicast packets from one interface to another. The RPF check verifies whether the packet arrived on the correct interface pointing toward the source to avoid loops. RPF failures may occur when there are multiple paths between devices that forward multicast traffic from a source to receivers, and the unicast routing topology is not congruent or the same as the data distribution tree of multicast topology.
Another reason packets may not reach receivers is that the Time To Live (TTL) value of packets is not set enough to reach the receivers available in the various parts of the network. The TTL value of a packet is decremented at every hop as in unicast routing, and a packet is not forwarded if its TTL value is less than the threshold value that can be configured under an interface on the router to define multicast boundaries.
Another multicast issue could be related to the building of the data distribution tree itself. Each multicast routing protocol has its own mechanism to build and maintain the tree. Dense mode protocols rely on flood and prune behavior, which should not cause problems. Sparse mode protocols rely on a device functioning as Rendezvous Point (RP) to build the tree for a multicast group, and every router in the network needs to know the RP-to-group mapping. This is done through manual configuration or automatic information distribution, which also involves some basic configuration. There are also mechanisms like filters available to control the distribution of RP information. Improper configuration of these features could lead to failure in building the distribution tree and affect the forwarding of multicast traffic.
In LAN environments, comprising LAN switches (which is just a Layer 2 (L2) device forwarding packets based on MAC addresses) will lead to unwanted flooding of packets to parts of the network where there are no receivers. There are methods like Internet Group Management Protocol (IGMP) snooping and proprietary Cisco Group Management Protocol (CGMP) which avoid this unwanted flooding. Improper configuration of these features, or placement of source and receivers connected across the switch can still lead to unwanted flooding, which needs to be addressed.
Using Multicast with HSRP. When using multicast in an HSRP environment, care must be used in configuring multicast with the physical IP address as opposed to the HSRP logical address as the IP Address. If the HSRP address is used, multicast traffic will not be sent.
For more information on these issues and how to solve them using various commands, refer to the IP Multicast Troubleshooting Guide.
Source and Destination Physical Connectivity
Source and destination exist on different subnetwork
