This option specifies the layer 2 link MTU CDRouter will use on the WAN. The layer 2 link MTU is the largest packet that can be transmitted on the layer 2 link without fragmentation. If jumboMtu is also set, the value of jumboMtu will override the value of ipv6WanMtu.
Please see this Knowledge Base article for more information on IPv6 MTU concepts within CDRouter.
This testvar was added in CDRouter 8.2