A Protocol Independent Multicast (PIM) router uses Reverse Path Forwarding (RPF) procedure to select an upstream interface and router in order to build forwarding state. When there are equal-cost multipaths (ECMPs), existing implementations often use hash algorithms to select a path. Such algorithms do not allow the spread of traffic among the ECMPs according to administrative metrics. This usually leads to inefficient or ineffective use of network resources. PIM ECMP Redirect (RFC 6754) provides a mechanism to improve the RPF procedure over ECMPs. It allows ECMP selection to be based on administratively selected metrics, such as data transmission delays, path preferences, and routing metric. An interface identifier option is used in PIM hello messages as a tiebreaker during ECMP path selection.

Following is a brief description of terms and concepts used to describe the PIM-ECMP Redirect protocol:

ECMP refers to parallel, single-hop, equal-cost links between adjacent nodes.

An ECMP bundle is a set of PIM-enabled interfaces on a router, where all interfaces belonging to the same bundle share the same routing metric. The next hops for the ECMP are all one hop away. There can be one or more ECMP bundles on any router, while one individual interface can only belong to a single bundle. ECMP bundles are created on a router via configuration.

Reverse Path Forwarding (RPF) is an optimized form of flooding, in which the router accepts a packet from SourceA through Interface IF1, only if IF1 is the interface the router uses to reach SourceA. To determine if the interface is correct, it consults its unicast routing tables. The packet that arrives through interface IF1 is forwarded because the routing table lists this interface as the shortest path. The router's unicast routing table determines the shortest path for the multicast packets. Because a router accepts a packet from only one neighbor, it floods the packet only once, meaning that (assuming point-to-point links) each packet is transmitted over each link, once in each direction.

Towards the root of the multicast forwarding tree. An upstream router refers to a router that is forwarding, or potentially capable of forwarding, data packets onto interfaces in an ECMP bundle. When there are multiple routers forwarding packets onto interfaces in the ECMP bundle, all these routers are called upstream routers.

Away from the root of the multicast forwarding tree. A downstream router is a router that uses an interface in the ECMP bundle as an RPF interface for a multicast forwarding entry

When a PIM router downstream of the ECMP interfaces creates a new (*,G) or (S,G) entry, it will populate the RPF interface and RPF neighbor information according to the rules specified by [RFC4601]. This router will send its initial PIM Joins to that RPF neighbor. When the RPF neighbor router receives the Join message and finds that the receiving interface is one of the ECMP interfaces, it will check if the same flow is already being forwarded out of another ECMP interface. If so, this RPF neighbor router will send a PIM ECMPRedirect message onto the interface the Join was received on. The PIM ECMP Redirect message contains the address of the desired RPF neighbor, an Interface ID [RFC6395], and the other parameters used as tiebreakers. In essence, a PIM ECMP Redirect message is sent by an upstream router to notify downstream routers to redirect PIM Joins to the new RPF neighbor via a different interface. When the downstream routers receive this message, they SHOULD trigger PIM Joins toward the new RPF neighbor specified in the packet.

This PIM ECMP Redirect message has similar functions as the existing PIM Assert message:

However, the existing Assert message is used to select an upstream router within the same multi-access network (such as a LAN), while the Redirect message is used to select both a network and an upstream router.

ECMP Redirects are sent by an upstream router under either of the following conditions:

In both cases, an ECMP Redirect is sent to the non-desired interface. An outgoing interface is considered non-desired when:

Receiving ECMP Redirect

When a downstream router receives an ECMP Redirect, and detects that the desired RPF path from its upstream router's point of view is different from its current one, it should choose to join the newly suggested path and prune from the current path.

If a downstream router receives multiple ECMP Redirects sent by different upstream routers, it SHOULD use the Preference, Metric, or other fields as specified below as the tiebreakers to choose the most preferred RPF interface and neighbor. The tie-break procedure is the same as that used in PIM Assert processing described by [RFC4601].

If an upstream router receives an ECMP Redirect, it SHOULD NOT change its forwarding behavior even if the ECMP Redirect makes it a less preferred RPF neighbor on the receiving interface.

This section provides the configuration steps for configuring PIM ECMP Redirect and examples for a relevant scenario.

In this network topology, the source address is 172.31.1.52 and the group address is set to 224.0.1.3.

Configure PIM router-id on all of the PIM routers inside the PIM domain:

Configure PIM ECMP Bundle on all of the PIM routers inside the PIM domain:

Bind an ECMP Bundle to an interface on the PIM routers inside the PIM domain:

Enter the commands listed in this section to confirm the previous configurations.

The following output displays the bundle information:

The following output displays the interface details:

Initially router_1 sends the (*, G) to Router_2 IF-2, as Router_2 IF-2 is RIB indicated RPF neighbor.The RIB indicated RPF neighbor can be checked using command show ip rpf

The show ip pim mroute command displays the IP multicast routing table. In this table, the following fields are defined:

Router-2 upon receiving (*, G) on IF-2, which is rib indicated RPF, sends an ECMP redirect message to Router-1 IF-2 to intimate that, subsequent joins should be sent to IF-1 being the desired path with a (*,G). Since, Router-2 IF-1 already has a (*, G), the show ip pim mroute command output suggests 192.168.1.152 as the RPF neighbor, which is ECMP redirected RPF neighbor.

The below output displays (*,G) at router_2 IF-1 using the command show ip pim mroute detail: