Cisco APIC Layer 3 Networking Configuration Guide, Release 6.2(x)

Guidelines, limitations, and expected behaviors for layer 3 IPv4 and IPv6 multicast

Updated: April 17, 2026

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Provides guidelines, limitations, and expected packet behaviors for Layer 3 IPv4 and IPv6 multicast in ACI fabric environments.

Note

Cisco ACI does not support IP fragmentation. Therefore, when you configure Layer 3 Outside (L3Out) connections to external routers, or Multi-Pod connections through an Inter-Pod Network (IPN), it is recommended that the interface MTU is set appropriately on both ends of a link. On some platforms, such as Cisco ACI, Cisco NX-OS, and Cisco IOS, the configurable MTU value does not take into account the Ethernet headers (matching IP MTU, and excluding the 14-18 Ethernet header size), while other platforms, such as IOS-XR, include the Ethernet header in the configured MTU value. A configured value of 9000 results in a max IP packet size of 9000 bytes in Cisco ACI, Cisco NX-OS, and Cisco IOS, but results in a max IP packet size of 8986 bytes for an IOS-XR untagged interface.

For the appropriate MTU values for each platform, see the relevant configuration guides.

We highly recommend that you test the MTU using CLI-based commands. For example, on the Cisco NX-OS CLI, use a command such as ping 1.1.1.1 df-bit packet-size 9000 source-interface ethernet 1/1.

Guidelines and limitations for IPv4 and IPv6 multicast

Layer 3 IPv4 and IPv6 multicast support depends on the platform generation, interface type, and deployment configuration.

Layer 3 IPv4 and IPv6 multicast is supported on second-generation leaf switches. Product IDs for these switches have an -EX, -FX, -FX2, -FX3, -GX, or later suffix.
Custom QoS policies are not supported for Layer 3 multicast traffic that enters the Cisco Application Centric Infrastructure fabric through an L3Out.
You can enable PIMv4 or PIMv6 and Advertise Host Routes on a bridge domain.
Layer 3 multicast is enabled at the VRF level. Multicast protocols operate within the VRF instance, and you can enable or disable multicast independently for each VRF instance.
After you enable multicast for a VRF instance, you can enable multicast for individual bridge domains and L3Outs in that VRF instance. Multicast is disabled by default on all bridge domains and L3Outs.
Bidirectional PIMv4 and PIMv6 are not supported.
Multicast routers are not supported in pervasive bridge domains.
The combined IPv4 and IPv6 multicast route scale is 2,000. Each IPv4 route counts as one route, and each IPv6 route counts as four routes. The IPv6 route-scale limit remains 2,000, including on node profiles that support a higher multicast scale.
PIMv4 and PIMv6 are supported on L3Out routed interfaces and routed subinterfaces, including Layer 3 port channels and Layer 3 port-channel subinterfaces.

Beginning with Cisco ACI Release 5.2(3), PIMv4 is supported on L3Out SVI interfaces for physical interfaces and directly connected port channels. PIMv4 and PIMv6 are not supported on L3Out SVIs with vPC interfaces.
Enabling PIMv4 or PIMv6 on an L3Out creates an implicit external network. As a result, the L3Out is deployed and its protocols might become operational even if you have not defined an external network.
If a multicast source connects to Leaf-A through an orphan port, an L3Out is configured on Leaf-B, and the two leaf switches form a vPC pair, deploy the EPG encapsulation VLAN associated with the multicast source on Leaf-B.
Filters are not supported for inter-VRF multicast communication.
Do not use the clear ip mroute command in a production network. The command is intended only for internal debugging.

IPv4 and IPv6 packet behaviors

Ingress and egress leaf switches handle Layer 3 IPv4 and IPv6 multicast packets differently.

For Layer 3 IPv4 multicast, when an ingress leaf switch receives a packet from a source attached to a bridge domain that is enabled for IPv4 multicast routing, the switch sends only a routed VRF copy to the fabric. Routing decrements the TTL by 1 and replaces the source MAC address with the pervasive subnet MAC address.

The egress leaf switch routes the packet to receivers in all applicable bridge domains. A receiver on the same bridge domain as the source but on a different leaf switch receives a routed copy. This behavior also applies when the source and receiver are on the same bridge domain and leaf switch if PIM is enabled on the bridge domain.
For Layer 3 IPv6 multicast, when an ingress leaf switch receives a packet from a source attached to a bridge domain that is enabled for IPv6 multicast routing, the switch sends only a routed VRF copy to the fabric. Routing decrements the TTL by 1 and replaces the source MAC address with the pervasive subnet MAC address.

The egress leaf switch routes the packet to the receivers and decrements the TTL by 1 again. As a result, the TTL is decremented twice. For ASM, the multicast group must have a valid RP configuration.

Guidelines and limitations for IPv4 multicast

Additional guidelines and limitations apply specifically to IPv4 multicast configurations.

If multicast is running on the border leaf switches in the Cisco ACI fabric, disabling multicast on the L3Out while unicast reachability remains available causes traffic loss when the external peer is a Cisco Nexus 9000 Series switch.

This limitation applies when traffic enters the fabric from an external source and has receivers inside the fabric. It also applies when both the source and receivers are outside the fabric and the fabric provides transit connectivity.
Any Source Multicast (ASM) and Source-Specific Multicast (SSM) are supported for IPv4.
You can configure a maximum of four SSM multicast ranges in the route map for each VRF instance.
IGMP snooping cannot be disabled on a pervasive bridge domain when multicast routing is enabled.
Multicast sources and receivers that connect through FEX ports are not supported.

Guidelines and limitations for IPv6 multicast

Additional guidelines and limitations apply specifically to IPv6 multicast configurations.

Source-Specific Multicast (SSM) is supported for IPv6. The SSM range is fixed and cannot be changed, as specified in RFC 3306: Unicast-Prefix-based IPv6 Multicast Addresses.
You can configure a maximum of four SSM multicast ranges in the route map for each VRF instance.
Any Source Multicast (ASM) is supported for IPv6.
The OIF and VRF scale limits for IPv6 are the same as the limits for IPv4.
Only static RP configuration is supported for PIM6. Auto-RP and BSR are not supported for PIM6.
Receivers inside the fabric are not supported. Disable the MLD Snoop Policy before enabling IPv6 multicast. MLD snooping and PIM6 cannot be enabled in the same VRF instance.
Layer 3 Multicast Listener Discovery (MLD) is not supported with Cisco ACI.
Fabric Rendezvous Point (RP) is not supported for IPv6 multicast.
Cisco Multi-Site Orchestrator is not supported.

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