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 56. 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.
NOTE:
If a neighboring node does not have an IPv6 address assigned, it must be manually pinged to allow the IPv6
device to determine the relationship of the neighboring node.
IPv6 Routing 401