IPv6 NDP and Nexus (About the need to disable OMF)

[John Collins]

It was reported elsewhere that the OMF Feature (Optimised Multicast Flooding) when enabled on a Vlan, as is the default case, interferes with the fundamental IPv6 NDP mechanism.  The activation of this feature on a Vlan on the Nexus 7000 environment stops the flooding of ethernet frames with a destination MAC address 3333.ff00.0080 (for example) while letting destination MAC addresses like 3333.0000.0012 through.   This in effect disables the IPv6 neighbor solicitation mechanism.  So to allow NDP to work in order to allow the most fundamental IPv6 communication between LAN stations, as described in Cisco documentation on multicast, the OMF has to be turned off at least in the Vlan where IPv6 is being used.

My question is the following:  What does this OMF do exactly (couldn't find a decent decription of the feature anywhere) and what dangers arise when it is turned off ? And remember it must be turned off for IPv6 to work!  Is there some other less radical way of allowing the multicast frames through without turning OMF off.  OMF is also then turned off for IPv4 and it might be needed for IPv4 if not for IPv6.

Many thanks in advance for any comments.

[Michael Vincent]

See here:

http://www.cisco.com/en/US/docs/switches/datacenter/sw/nx-os/multicast/configuration/guide/b_multicast_chapter_0100.html

"When OMF is disabled, unknown IPv4 multicast traffic and all IPv6 multicast traffic is flooded to all ports in the VLAN.

(Unknown multicast traffic refers to multicast packets that have an active source, but have no receivers in the ingress VLAN. Having no receivers means that there is no group forwarding entry in the hardware.)"

Depending on your design and configuration, this may not be a problem if the source is routed to an RP or your sources sit in appropriately sized VLANs (based on IP(v6) mask and/or number of hosts).  With IPv4, our multicast sources are on their own /27.  With IPv6, they'd live on a /64, but you can limit the hosts on that network to multicast source only and keep the number low so traffic won't impact other hosts / servers.

cheers.