Concept Guide

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 50. 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.
NOTE: To avoid problems with network discovery, Dell EMC Networking recommends configuring the static route last or
assigning an IPv6 address to the interface and assigning an address to the peer (the forwarding router’s address) less than
10 seconds apart.
With ARP, each node broadcasts ARP requests on the entire link. This approach causes unnecessary processing by uninterested
nodes. With NDP, each node sends a request only to the intended destination via a multicast address with the unicast address
used as the last 24 bits. Other hosts on the link do not participate in the process, greatly increasing network bandwidth
efficiency.
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IPv6 Routing