IPv6 on NX-OS

So you thought you’d enable IPv6 on your new Nexus chassis and get ready for the future of the internet? Create some IPv6 SVIs and away you’d go? Wrong! Out of the box the Nexus is configured such that Neighbor Discrovery will not work.
A bit of googling will eventually lead you to this command:

no ip igmp snooping optimised-multicast-flood

Either apply it globally or per-VLAN. Sounds easy, away we go, but what does it really mean?

Lets look at the Cisco configuration document, it mentions the F2 line card specifically, but it is worth noting the defaults used by the other line cards:

F2 Disable OMF is using any v6 (unicast or multicast)
F2E OMF is disabled by default
F3 For v6 unicast, no configuration necessary is off by default. For v6 multicast disable OMF
M series Disable OMF is using v6 multicast

The above information was gleaned from TAC and the BU. I still find it strange that IPv6 unicast and multicast are treated in isolation, when the forwarding of IPv6 multicast traffic is at the core of the protocol functioning!
It is also worth noting that this is a hardware limitation affecting all Nexus platforms (7k and 9k), it cannot be fixed by firmware.

Once you have OMF disabled, as the configuration document specifies all IPv6 multicast traffic will be flooded in the Bridge Domain. Multicast efficiency lost to broadcast traffic.

Under these scenarios on the Catalyst platform MLD snooping would be used to optimize the flooding of multicast packets, however NX-OS has no implementation of this function but it is touted as being included in the 7.x software train.

Until then, if you want IPv6 on your Nexus you will have to pay the penalty of IPv6 multicast traffic being amplified as it is broadcast within the Layer2 domain.

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