IPv6 Addressing | 493Path MTU (Maximum Transmission Unit) defines the largest packet size that can traverse atransmission path without suffering fragmentation. Path MTU for IPv6 uses ICMPv6 Type-2messages to discover the largest MTU along the path from source to destination and avoid the need tofragment the packet.The recommended MTU for IPv6 is 1280. Greater MTU settings increase processing efficiency becauseeach packet carries more data while protocol overheads (headers, for example) or underlying per-packetdelays remain fixed.Figure 22-2. MTU Discovery PathIPv6 Neighbor DiscoveryIPv6 NDP is supported on platforms c e sNeighbor Discovery Protocol (NDP) is a top-level protocol for neighbor discovery on an IPv6 network. Inlieu of ARP, NDP uses "Neighbor Solicitation" and "Neighbor Advertisement" ICMPv6 messages fordetermining relationships between neighboring nodes. Using these messages, an IPv6 device learns thelink-layer addresses for neighbors known to reside on attached links, quickly purging cached values thatbecome invalid.With ARP, each node broadcasts ARP requests on the entire link. This approach causes unnecessaryprocessing by uninterested nodes. With NDP, each node sends a request only to the intended destinationvia a multicast address with the unicast address used as the last 24 bits. Other hosts on the link do notparticipate in the process, greatly increasing network bandwidth efficiency.Router BRouter AMTU = 1600 MTU = 1400 MTU = 1200ICMPv6 (Type 2)Use MTU = 1400Packet (MTU = 1600)Packet (MTU = 1400)ICMPv6 (Type 2)Use MTU = 1200Packet (MTU = 1200)Packet ReceivedSource Destination
