To enable Layer 3 multicast functionality on an AOS-CX switch, it is essential to understand the roles of different protocols and where they are applied in the hierarchy of the switch configuration.
L3 vs. L2 Multicast:
IGMP (Internet Group Management Protocol): This is a Layer 3 protocol used by hosts to report their multicast group memberships to adjacent routers. It is configured on a Layer 3 interface, such as an SVI (Switch Virtual Interface) or a routed port.
IGMP Snooping: This is a Layer 2 optimization feature. It allows a switch to " listen in " on IGMP conversations between hosts and routers to constrain multicast traffic to only those ports that have requested it, preventing the traffic from flooding the entire VLAN.
Enabling L3 Multicast Locally: When the " core switch " acts as the default gateway for multicast receivers, it must be able to process membership reports from those receivers. By enabling IGMP on the SVI (e.g., interface vlan 100), the switch begins listening for IGMP Join/Leave messages from clients on that subnet. This is a prerequisite for the switch to populate its multicast routing table and, when combined with a routing protocol like PIM (Protocol Independent Multicast), forward traffic from a source to those local receivers.
Analysis of Options:
Option A (Correct): IGMP must be enabled on the SVI so the switch can function as a multicast-aware gateway for that specific subnet.
Option B: In AOS-CX syntax, you do not configure " IGMP " under the L2 vlan [ID] context; it is an L3 function.
Option C: Enabling IGMP-snooping on a VLAN only manages the L2 delivery of traffic; it does not enable L3 routing or the processing of multicast packets across different subnets.
Option D: While IGMP-snooping is often used in conjunction with L3 multicast, it is technically a VLAN-level configuration, not a property of the SVI itself.
Key CLI Commands:
To fully enable this, a professional would typically use:
Bash
router multicast
routing
!
interface vlan 100
ip igmp
ip pim-sm
