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Configuring PIM Auto-RP

 

Understanding PIM Auto-RP

You can configure a more dynamic way of assigning rendezvous points (RPs) in a multicast network by means of auto-RP. When you configure auto-RP for a router, the router learns the address of the RP in the network automatically and has the added advantage of operating in PIM version 1 and version 2.

Although auto-RP is a nonstandard (non-RFC-based) function that typically uses dense mode PIM to advertise control traffic, it provides an important failover advantage that simple static RP assignment does not. You can configure multiple routers as RP candidates. If the elected RP fails, one of the other preconfigured routers takes over the RP functions. This capability is controlled by the auto-RP mapping agent.

Configuring PIM Auto-RP

For correct operation, every multicast router within a PIM domain must be able to map a particular multicast group address to the same rendezvous point (RP). The auto-RP mechanism is one way that a multicast router can learn the set of group-to-RP mappings. Auto-RP automatically distributes mapping information to routing devices. It simplifies use of multiple RPs for different multicast group ranges, thus allowing multiple RPs to act as backups for each other. Auto-RP relies on a router to act as the RP mapping agent. Potential RPs announce themselves to the mapping agent, and the mapping agent resolves any conflicts.

The mapping agent sends the multicast group-RP mapping information to the other routers using PIM dense mode. The specific groups used are 224.0.1.39 and .40. The first (.39) is used to advertise, the second (.40) is used for discovery. Because PIM dense mode is necessary to enable auto-RP to work, which in turns enables PIM sparse mode to work, you must configure PIM sparse-dense mode in the PIM domains that use auto-RP.

Although auto-RP is a nonstandard (non-RFC-based) function requiring dense mode PIM to advertise control traffic, it provides an important failover advantage that static RP assignment does not. That is, you can configure multiple routing devices as RP candidates. If the elected RP fails, one of the other preconfigured routing devices takes over the RP functions. This capability is controlled by the auto-RP mapping agent.

In most cases, how the routing device handles auto-RP discovery, announce, or mapping messages depends on whether the routing device is an RP (configured as local RP) or not. Table 1 shows how the routing device behaves depending on the local RP configuration.

Table 1: Local RP and Auto-RP Message Types

Auto-RP Message Type

Local RP?

Routing Device Behavior

discovery

No

Listen for auto-RP mapping messages.

discovery

Yes

Listen for auto-RP mapping messages.

announce

No

Listen for auto-RP mapping messages.

announce

Yes

Listen for auto-RP mapping messages. Send auto-RP announce messages.

mapping

No

Listen for auto-RP mapping messages. Listen for auto-RP announce messages. If elected mapping agent, send auto-RP mapping messages.

mapping

Yes

Listen for auto-RP mapping messages. Send auto-RP announce messages. Listen for auto-RP announce messages. If elected mapping agent, send auto-RP mapping messages.

Note

If the routing device receives auto-RP announcements split across multiple messages, the routing device loses the information in the previous part of the message as soon as the next part of the message is received.

You can configure auto-RP properties globally or for a routing instance. This example shows the global configuration.

To configure auto-RP properties:

  1. Configure PIM in sparse-dense mode on all routing devices in the PIM domain.

    This configuration allows the routing device to operate in sparse mode for most groups and dense mode for others. The default is to operate in sparse mode unless the routing device is specifically informed of a dense mode group.

  2. Configure a routable loopback interface address on all routing devices in the PIM domain.

    The routing device joins the auto-RP groups on the configured interfaces and on the loopback interface lo0.0. For auto-RP to work correctly, configure a routable IP address on the loopback interface. You cannot use the loopback address 127.0.0.1. Also, you must enable PIM sparse-dense mode on the lo0.0 interface if you do not specify interface all.

  3. Configure the two multicast dense groups on all the routing devices.

    Auto-RP requires multicast flooding to announce potential RP candidates and to discover the elected RPs in the network. Multicast flooding occurs through a PIM dense mode model, where group 224.0.1.39 is used for announce messages and group 224.0.1.40 is used for discovery messages.

    Tip

    Step 3 is required. When auto-RP is enabled, the auto-RP announce group (224.0.1.39) and auto-RP-discovery group (224.0.1.40) must be configured explicitly as dense groups. When the auto-RP discovery group is not configured as a dense group, auto-RP is not enabled. When the auto-RP announce group is not configured as a dense group, auto-RP is enabled in the discovery mode only, and mapping and announce modes are disabled.

  4. Configure the auto-RP announce option.

    At least one routing device in the PIM domain must announce auto-RP messages and at least one must map them, or you can configure a routing device to perform both functions.

    When a routing device sends announce messages in the network, it is advertising itself as a candidate RP. A routing device configured with this option must also be configured as an RP, or announce messages are not sent.

    Note

    You cannot include the auto-rp announce option at the [edit logical-systems logical-system-name routing-instances routing-instance-name protocols pim] hierarchy level.

  5. Configure the auto-RP mapping agent.

    The mapping agent sends discovery messages to the network, informing all routing devices in a multicast group of which RP to use. If the mapping agent is also an RP, the mapping option also allows the routing device to send auto-RP announcements (mapping on an RP allows the routing device to perform both the announcement and mapping functions).

    If the mapping agent is also an RP, configure the mapping agent as a local RP.

  6. Configure mapping agent election.

    If you configure the mapping option on more than one routing device in the PIM domain, configure mapping agent election on each potential mapping agent.

    Auto-RP specifications state that mapping agents do not send mapping messages if they receive messages from a mapping agent with a higher IP address. However, some vendors' mapping agents continue to announce mappings, even in the presence of higher-addressed mapping agents. In other words, some mapping agents will always send mapping messages.

    The default auto-RP operation is to perform mapping agent election. To explicitly configure mapping agent election, you can include the mapping-agent-election statement. When this option is configured, the mapping agent will stop sending mapping messages if it receives messages from a mapping agent with a higher IP address.

    Mapping message suppression is disabled with the no-mapping-agent-election statement. When this option is configured, the mapping agent will always send mapping messages even in the presence of higher-addressed mapping agents.

    To disable mapping agent election for compatibility with other vendors' equipment, include the no-mapping-agent-election statement.

  7. Configure the remaining routing devices in the PIM domain to discover the RP.

    Discovery enables the routing devices to receive and process discovery messages from the mapping agent. This is the most basic auto-RP option.

  8. Monitor the operation of PIM auto-RP routers by running the following commands:
  9. Issue the show pim rps extensive command to see information about how an RP is learned, what groups it handles, and the number of groups actively using the RP.
    user@host> show pim rps extensive

    In the example, the RP at 192.168.5.1 was learned through auto-RP. The RP is able to support all groups in the 224.0.0.0/4 range (all possible groups). The local router has sent PIM control traffic for the 224.2.2.100 group to the RP.

    Additionally, the presence of a Tunnel Physical Interface Card (PIC) in an RP router creates a de-encapsulation interface, which allows the RP to receive multicast traffic from the source. This interface is indicated by pd-0/0/0.32769.