Multicast Load Splitting

Implementation

By default, if multiple equal-cost optimal routes are available, a router selects the route with the largest next-hop address from the IGP routing table according to the RPF check policy.

Multicast load splitting enables a router to distribute multicast traffic to multiple equal-cost routes, instead of selecting only one route based on the RPF check policy.

In Figure 1, the multicast source (Source) sends multicast streams to group G. RouterA and RouterD run an Interior Gateway Protocol (IGP), such as OSPF, to implement IP interworking. Two equal-cost paths are available: RouterA → RouterB → RouterD and RouterA → RouterC → RouterD. Based on the default RPF check policy, multicast streams are forwarded through interface Int1 of RouterA because Int1 has a larger IP address than Int0. After multicast load splitting is configured on RouterA, RouterA does not select forwarding paths by comparing the next-hop IP addresses. Instead, multicast streams are forwarded along both of the two equal-cost paths.

Figure 1 Multicast forwarding without and with multicast load splitting

Multicast Load Splitting Modes

Various methods are available to split (*, G) and (S, G) data streams in different scenarios.

• Load splitting by group address

  In Figure 2, the source sends data streams to different multicast groups (G1 to G10). Router7, Router6, and Router5 each have two equal-cost paths to the source. These routers use route selection algorithms to select an optimal path for data sent to each group. In this load splitting mode, streams transmitted on different paths are sent to different groups.

  Figure 2 Load splitting by group address
  

• Load splitting by source address

  In Figure 3, different sources (S1 to S10) send data streams to the same group. Router7, Router6, and Router5 each have two equal-cost paths to the sources. These routers use route selection algorithms to select an optimal path for data from each source. In this load splitting mode, streams transmitted on different paths are sent from different sources.

  Figure 3 Load splitting by source address
  

• Load splitting based on source and group addresses

  In Figure 4, different sources (S1 to S10) send data streams to different groups (G1 to G10). Router7, Router6, and Router5 each have two equal-cost paths to the sources. These routers use route selection algorithms to select an optimal path for each (S, G) stream. In this load splitting mode, streams transmitted on different paths have different source and group addresses.

  Figure 4 Load splitting based on source and group addresses
  

• Other load splitting methods

  Figure 5 Load splitting network
  

  The following load balancing methods can also be used on the network shown in Figure 5.

  • Stable-preferred load splitting

    When route flapping occurs on a multicast network, frequent changes of multicast traffic distribution on equal-cost paths will cause the route flapping to become worse. Stable-preferred load splitting can be configured to solve this problem. When route flapping occurs, a router with stable-preferred load splitting adjusts traffic distribution on equal-cost paths until route flapping ends. When the network topology becomes stable, the router evenly distributes (S, G) streams from the same source to equal-cost paths.

  • Balance-preferred load splitting

    Balance-preferred load splitting enables routers to immediately adjust traffic distribution among equal-cost paths when route flapping occurs on a multicast network. When the network topology becomes stable, the router evenly distributes (S, G) streams from the same source to equal-cost paths.

  • Unbalanced load splitting

    Unbalanced load splitting is a supplement to stable-preferred and balance-preferred load splitting and does not change their behavior. In unbalanced load splitting mode, (S, G) streams are distributed to equal-cost paths in proportion to the weights of the paths. As transmission paths on a network have different capabilities, load on some paths may need to be manually adjusted. In this case, you can configure load splitting weights on upstream interfaces of a router to implement unbalanced load splitting. A larger weight on an upstream interface allows the corresponding path to transmit more (*, G) and (S, G) streams.