Multicast PIM-ASM & Cisco SD-Access

Multicast PIM-ASM & Cisco SD-Access

In today's lesson, we will discuss the basics of multicast used in Cisco SD-Access. Through the Cisco SDA infrastructure, Cisco offers native multicast support with the Overlay Multicast (PIM-SM overlay) to support any future multicast application. 

How multicast design will be in the Cisco SDA environment?
Below are the highlights points of the SDA Fabric Multicast Design

  • Multicast is delivered in overlay
  • Multicast replication is done at head-end by default. 
  • Multicast can use underlay multicast capability (aka, native multicast). 
  • PIM SSM is used in the underlay for multicast transport.  
  • Multicast is using: PIM SSM for default SSM range ( and PIM ASM for rest
  • In each SDA site, border node will be used as RP.  For large branch, MSDP will be running between two border nodes. 
  • It requires an extra IP pool for PIM-SM neighboring.   
Fig 1.1- Multicast PIM-ASM in Cisco SDA

How Multicast Works with PIM ASM 
A shortest path failover occurs once the first multicast packet is delivered to the Fabric Edge (FE) and traffic is forwarded between the Fabric Border (FB) and Fabric Edge (FE) directly. The Fabric Edge (FE) knows from the first multicast packet received that the Fabric Border is now the owner of the multicast group, so it sends a PIM join directly to the Fabric Border (FB).

The Fabric Border (FB) is now aware of which Fabric Edge (FE) have clients who are interested in the specific multicast group. It performs headend replication and VXLAN encapsulation of the multicast traffic and unicasts it to the interested Fabric Edge (FE).

During multicast traffic, the overlay transmits the packets to the Fabric Edge (FE), which decapsulates them, applies policy, and forwards the packets to the Access Point (AP).After removing the VXLAN header, the AP sends an IP multicast packet in its place.