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Example: Configuring Source-Specific Multicast Groups with Any-Source Override

 

This example shows how to extend source-specific multicast (SSM) group operations beyond the default IP address range of 232.0.0.0 through 232.255.255.255. This example also shows how to accept any-source multicast (ASM) join messages (*,G) for group addresses that are within the default or configured range of SSM groups. This allows you to support a mix of any-source and source-specific multicast groups simultaneously.

Requirements

Before you begin, configure the router interfaces.

Overview

To deploy SSM, configure PIM sparse mode on all routing device interfaces and issue the necessary SSM commands, including specifying IGMPv3 or MLDv2 on the receiver's LAN. If PIM sparse mode is not explicitly configured on both the source and group members interfaces, multicast packets are not forwarded. Source lists, supported in IGMPv3 and MLDv2, are used in PIM SSM. Only sources that are specified send traffic to the SSM group.

In a PIM SSM-configured network, a host subscribes to an SSM channel (by means of IGMPv3 or MLDv2) to join group G and source S (see Figure 1). The directly connected PIM sparse-mode router, the receiver's designated router (DR), sends an (S,G) join message to its reverse-path forwarding (RPF) neighbor for the source. Notice in Figure 1 that the RP is not contacted in this process by the receiver, as would be the case in normal PIM sparse-mode operations.

Figure 1: Receiver Sends Messages to Join Group G and Source S
Receiver Sends Messages to Join Group
G and Source S

The (S,G) join message initiates the source tree and then builds it out hop by hop until it reaches the source. In Figure 2, the source tree is built across the network to Router 3, the last-hop router connected to the source.

Figure 2: Router 3 (Last-Hop Router) Joins the Source Tree
Router 3 (Last-Hop Router) Joins
the Source Tree

Using the source tree, multicast traffic is delivered to the subscribing host (see Figure 3).

Figure 3: (S,G) State Is Built Between the Source and the Receiver
(S,G) State Is Built Between the Source
and the Receiver

SSM can operate in include mode or in exclude mode. In exclude mode the receiver specifies a list of sources that it does not want to receive the multicast group traffic from. The routing device forwards traffic to the receiver from any source except the sources specified in the exclusion list. The receiver accepts traffic from any sources except the sources specified in the exclusion list.

This example works with the simple RPF topology shown in Figure 4.

Figure 4: Simple RPF Topology
Simple RPF Topology

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Step-by-Step Procedure

The following example requires that you navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure an RPF policy:

  1. Configure OSPF.

  2. Configure PIM sparse mode.

  3. Configure additional SSM groups.

  4. Configure the RP to accept ASM join messages for groups within the SSM address range.

  5. If you are done configuring the device, commit the configuration.

Results

Confirm your configuration by entering the show protocols and show routing-options commands.

Verification

To verify the configuration, run the following commands: