Cisco ACI BUM forwarding (Broadcast, Unknown Unicast, and Multicast) is handled differently from classic switching, and that difference matters for design, troubleshooting, and scale.
In this guide, you’ll see exactly how BUM forwarding works in Cisco ACI fabric through clear packet flow diagrams (Slides Included).
Summary
- BUM traffic stands for Broadcast (like ARP request or DHCP discovery messages), Unknown unicast (for Layer-2 traffic only; it happens when the switch doesn’t know how to forward a frame to a destination MAC address), and Multicast (which can be Layer-2 or Layer-3 multicast traffic).
- BUM forwarding is different from unicast forwarding because it may have multiple destinations. In BUM forwarding, the packet must be replicated several times to reach multiple destinations.
- Just as in traditional networking, flooding is the default forwarding mechanism for BUM traffic in ACI. The only exception is the L2 unknown unicast forwarding, which defaults to a hardware proxy.
- In ACI, each Bridge Domain is associated with a Group IP outer Multicast address (GIPo).
- The GIPo is a multicast group address used to flood the BUM traffic within the bridge domain. It is used as the dst outer IP address in the encapsulated iVXLAN packet.
- There are 16 FTAG trees, but only 12 are used for user traffic. These multiple trees achieve load balancing across the ACI fabric during BUM traffic flooding, where inner IP payloads are hashed across the FTAG topologies.
Additional Resources: Cisco Application Centric Infrastructure (ACI) Design Guide
