Downstream fragmentation with small first IPv6 fragment

In the downstream direction, the existing tunnel-mtu command is used to set the size of the frames in the downstream direction.

Classic CLI:

config
   router / service vprn 
      nat
         inside
               dual-stack-lite
                         address  <AFTR IPv6 address> 
                              tunnel-mtu <464..1280>

MD-CLI:

configure {
   router {
      nat {
         inside {
            large-scale {
               dual-stack-lite {
                  endpoint <AFTR IPv6 address> {
                     tunnel-mtu <464..1280>
                  }
               }
            }
         }
      }
   }
}

The tunnel-mtu value represents the size of the IPv4 payload which is encapsulated in the IPv6 packet with an additional 48-byte header. These IPv6 packets can then be fragmented when fragmentation is enabled.