IPv6 routers do not fragment packets.
The source node can send packets less than or equal to the maximum transmission unit (MTU) of the link layer.
As the packet travels through the network to the source it can encounter a link with a smaller MTU. If so, the router sends the source node an ICMP error message that contains the MTU size of the next link. The source IPv6 node then resends a packet equal to the size of the MTU included in the ICMP message.
The default Layer 3 IPv6 MTU value is 1500 where the system MTU default value is 1950.
The default IPv6 MTU value is always less than the default System MTU value.
You can configure the MTU for each IPv6 interface.
Note
To configure separate Layer 3 MTU values for IPv4 and IPv6 packets on the same VLAN interface, you must disable Unicast Reverse Path Forwarding (uRPF) mode. If you enable the uRPF mode using the command boot config flags urpf-mode, the MTU values for both IPv4 and IPv6 packets on the same VLAN are matched. Different Layer 3 MTU sizes on the same VLAN are not allowed in uRPF mode.