ipv6 packet-too-big drop
Function
The ipv6 packet-too-big drop configures the switch to discard packets with the length larger than the IPv6 MTU of the outbound interface, and enables IPv6 PMTU discovery.
The command undo ipv6 packet-too-big drop restores the default policy for processing packets with the length larger than the IPv6 MTU of the outbound interface.
By default, the switch properly forwards packets with the length larger than the IPv6 MTU of the outbound interface and IPv6 PMTU discovery is disabled.
Only the S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-HI, S6730-H, S6730S-H, S6730-S, and S6730S-S support this command.
Usage Guidelines
Usage Scenario
When the switch functions as an intermediate node on an IPv6 network, you can configure the ipv6 packet-too-big drop command to discard packets with the length larger than the IPv6 MTU of the outbound interface and enable IPv6 PMTU discovery.
In an IPv4 network, oversized packets need to be fragmented. When a transit device receives a packet exceeding the maximum transmission unit (MTU) size of its outbound interface from a source node, the transit device fragments the packet before forwarding it to the destination node. In an IPv6 network, however, only the source node can fragment packets, which reducing pressure on transit devices. When an interface on a transit device receives a packet whose size exceeds the MTU, the transit device discards the packet and sends an ICMPv6 Packet Too Big message to the source node. The ICMPv6 Packet Too Big message contains the MTU value of the outbound interface. The source node fragments the packet based on the MTU and resends the packet, increasing traffic overhead. The Path MTU Discovery protocol dynamically discovers the MTU value of each link on the transmission path, reducing excessive traffic overhead.
The PMTU Discovery protocol is implemented through ICMPv6 Packet Too Big messages. A source node first uses the MTU of its outbound interface as the PMTU and sends a probe packet. If a smaller PMTU exists on the transmission path, the transit device sends a Packet Too Big message to the source node. The Packet Too Big message contains the MTU value of the outbound interface on the transit device. After receiving this message, the source node changes the PMTU value to the received MTU value and sends packets based on the new MTU. This process repeats until packets are sent to the destination address. The source node obtains the PMTU of the destination address.
Precautions
The switch supports IPv6 MTU discovery, but does not support IPv6 packet fragmentation when it functions as an intermediate node. IPv6 packets can be fragmented only on source nodes.