Multicast Load Balancing in SONiC:
How AsterNOS Uses ECMP to Break the RPF Bottleneck

Driven by digital transformation, modern campus networks are evolving from traditional office data transport networks into high-bandwidth, low-latency platforms for streaming services. In scenarios such as university research, remote medical education, and large-scale event broadcasting, the widespread adoption of 4K/8K ultra-high-definition video streams places unprecedented demands on the multicast distribution capabilities of the underlying network. Traditional Layer 2 multicast protocols can struggle to efficiently handle traffic at this scale, often resulting in poor bandwidth utilization or localized congestion.

Asterfusion has optimized its multicast protocol stack to support multicast load balancing in Equal-Cost Multi-Path (ECMP) environments, with a focus on traffic distribution and failure recovery. This article examines how AsterNOS, Asterfusion’s enterprise SONiC platform, implements multicast load balancing through ECMP, with a focus on RPF processing, traffic distribution, and failure recovery.

Traditional campus networks typically use a three-tier core-distribution-access architecture. While this model provides stable network connectivity, it can introduce significant latency and bandwidth bottlenecks when handling East-West traffic. With the rise of live streaming and event broadcasting, multicast traffic in campus networks is no longer limited to a north-south flow from the egress gateway to end devices. More traffic now involves real-time communication across departments and floors.

To address these requirements, Asterfusion has introduced a Spine-Leaf-based campus architecture that brings data center-style non-blocking forwarding and low latency to campus networks. In this architecture, each access switch (Leaf) connects to the aggregation or core layer (Spine) through multiple equal-cost links, creating a large number of ECMP paths. However, Protocol Independent Multicast (PIM) does not efficiently utilize these physical links by default.

Under standard Protocol Independent Multicast – Sparse Mode (PIM-SM), routers perform Reverse Path Forwarding (RPF) checks to prevent multicast loops. For a given multicast source, the RPF check requires the router to select a single upstream interface through which to receive traffic. When multiple equal-cost routes to the source exist in the routing table, PIM normally selects a single next hop as the RPF neighbor. This limits multicast load balancing across ECMP paths because multicast traffic is tied to a single RPF path.

This single-next-hop principle is simple and reliable, but it can become a serious bottleneck for bandwidth-intensive event broadcasting. For example, assume a campus core has four 100G ECMP links to the area where the video source is located. Due to the default behavior of PIM, all 4K/8K multicast streams are forwarded over a single 100G link, leaving the other 300G of available bandwidth unused. When the bandwidth required by a single multicast stream approaches or exceeds the capacity of one physical link, end users can still experience severe packet loss and video artifacts, even when sufficient aggregate bandwidth is available across the network.

To address the bottleneck described above, Asterfusion’s Multicast ECMP feature changes how RPF neighbors are selected to enable multicast load balancing across available ECMP paths.

The core of Multicast ECMP load balancing is its hashing algorithm. It uses the combination of the multicast source address (S) and group address (G) to distribute different (S,G) flows across different ECMP paths. This mechanism is particularly valuable in campus networks with high traffic volumes. For example, when multiple live channels with different camera views are being broadcast, each channel, represented by an (S,G) entry, can be forwarded through a different path based on the hash result. This enables multicast load balancing across physical ECMP links at the network level.

The path selection can be represented as:

NextHop=Hash(Source_IP,Group_IP)(modNumber_of_ECMP_Paths)

This means that even when the multicast source is the same, traffic from different live channels can be distributed across different physical links as long as they use different group addresses. This enables multicast load balancing across ECMP paths at the channel level.

Modern campus networks often require isolation between different services. For example, teaching, administrative, and security monitoring networks may need to remain logically isolated. Asterfusion’s Multicast ECMP is fully VRF-aware. Administrators can enable the feature in global configuration mode or within a specific VRF context. This ensures that multicast load-balancing policies in different service domains operate independently. This is particularly important for event broadcasting. Paid and public broadcast streams can be placed in different Virtual Routing and Forwarding (VRF) instances, with dedicated ECMP policies configured for each service domain.

Campus networks are dynamic, and physical link stability can be affected by various factors. When an interface in an ECMP group goes down, a traditional hashing algorithm can migrate the affected multicast flows to the remaining interfaces. However, this can result in highly uneven traffic distribution and reduce the effectiveness of multicast load balancing.

When Multicast ECMP Rebalance is disabled, PIM modifies only the (S,G) flows that were originally using the failed interface. For example, assume 100 flows are evenly distributed across four links, with 25 flows per link. If one link fails, its 25 flows may all move to the second link. The second link then carries 50 flows, while the third and fourth links continue to carry only 25 flows each.

With Multicast ECMP Rebalance enabled, the system detects changes in ECMP group membership and triggers the PIM process to recalculate the distribution of all multicast flows across the remaining active next hops. This multicast ECMP rebalancing mechanism ensures that network resources remain optimally distributed even after topology convergence.

In 8K ultra-high-definition video streaming, each multicast stream can require tens of Gbps of bandwidth. This makes multicast load balancing for 4K/8K video streaming particularly important in campus and event-broadcasting networks. When traffic becomes unevenly distributed, newly migrated flows can quickly exhaust the buffer capacity of an interface. This can cause both the affected stream and existing streams on the same interface to experience video artifacts or even service interruption.

This feature also applies to VRF environments. Administrators can configure it within a specific VRF context to provide enhanced resilience for individual service domains.

To provide a clearer view of Asterfusion’s multicast capabilities, we compare the support for key multicast features across different platforms.

The following table summarizes the key differences between community SONiC and Asterfusion AsterNOS in multicast forwarding and multicast load balancing for streaming media applications:

Community SONiC is primarily designed for unicast forwarding in large-scale data center networks. For the complex multicast applications found in campus environments, its feature coverage and operational stability may be more limited. Asterfusion strengthens the protocol stack by aligning complex hashing decisions with hardware state machines. This provides a more mature operational experience while retaining the flexibility of an open NOS.

Multicast load balancing is critical for maintaining bandwidth efficiency and reliability in modern campus networks. By introducing Multicast ECMP and Multicast ECMP Rebalance into its enterprise-grade SONiC-based system, Asterfusion addresses the performance limitations of traditional multicast forwarding in modern Spine-Leaf architectures.

By optimizing RPF processing, improving global traffic distribution, and providing deeper support for VRF and PTP, Asterfusion extends its multicast capabilities to demanding applications such as 4K/8K streaming and event broadcasting. Compared with community SONiC, Asterfusion AsterNOS for campus networks combines hardware and software capabilities to provide a commercial open NOS for high-performance multicast deployments. For campus networks that require high performance and availability while maintaining an open networking ecosystem, Asterfusion’s multicast load-balancing solution provides a practical approach to these requirements.