Administrator Guide

ICMPv6
ICMP for IPv6 combines the roles of ICMP, IGMP and ARP in IPv4. Like IPv4, it provides functions for reporting delivery and
forwarding errors, and provides a simple echo service for troubleshooting. The Dell EMC Networking OS implementation of
ICMPv6 is based on RFC 4443.
Generally, ICMPv6 uses two message types:
● Error reporting messages indicate when the forwarding or delivery of the packet failed at the destination or intermediate
node. These messages include Destination Unreachable, Packet Too Big, Time Exceeded and Parameter Problem messages.
● Informational messages provide diagnostic functions and additional host functions, such as Neighbor Discovery and Multicast
Listener Discovery. These messages also include Echo Request and Echo Reply messages.
The Dell EMC Networking OS ping and traceroute commands extend to support IPv6 addresses. These commands use
ICMPv6 Type-2 messages.
Path MTU discovery
The size of the packet that can be sent across each hop in the network path without being fragmented is called the path
maximum transmission unit (PMTU). The PMTU value might differ for the same route between two devices, mainly over a public
network, depending on the network load and speed, and it is not a consistent value. The MTU size can also be different for
various types of traffic sent from one host to the same endpoint.
Path MTU discovery (PMTD) identifies the path MTU value between the sender and the receiver, and uses the determined
value to transmit the packets across the network. Path MTU, in accordance with RFC 1981, defines the largest packet size that
can traverse a transmission path without suffering fragmentation. Path MTU for IPv6 uses ICMPv6 Type-2 messages to
discover the largest MTU along the path from source to destination and avoid the need to fragment the packet. The
recommended MTU for IPv6 is 1280. Greater MTU settings increase the processing efficiency because each packet carries more
data while protocol overheads (for example, headers) or underlying per-packet delays remain fixed.
Figure 60. Path MTU discovery process
IPv6 Neighbor Discovery
The IPv6 neighbor discovery protocol (NDP) is a top-level protocol for neighbor discovery on an IPv6 network.
In place of address resolution protocol (ARP), NDP uses “Neighbor Solicitation” and “Neighbor Advertisement” ICMPv6
messages for determining relationships between neighboring nodes. Using these messages, an IPv6 device learns the link-layer
addresses for neighbors known to reside on attached links, quickly purging cached values that become invalid.
IPv6 Routing
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