Concept Guide

• Prex Advertisement — Routers use “Router Advertisement” messages to announce the network prex. Hosts then use their
interface-identier MAC address to generate their own valid IPv6 address.
• Duplicate Address Detection (DAD) — Before conguring its IPv6 address, an IPv6 host node device checks whether that address is
used anywhere on the network using this mechanism.
• Prex Renumbering — Useful in transparent renumbering of hosts in the network when an organization changes its service provider.
NOTE: As an alternative to stateless autoconguration, network hosts can obtain their IPv6 addresses using the dynamic host
control protocol (DHCP) servers via stateful auto-conguration.
NOTE: The system provides the exibility to add prexes on Router Advertisements (RA) to advertise responses to Router
Solicitations (RS). By default, RA response messages are sent when an RS message is received.
The manipulation of IPv6 stateless autoconguration supports the router side only. Neighbor discovery (ND) messages are advertised so
the neighbor can use this information to auto-congure its address. However, received ND messages are not used to create an IPv6
address.
NOTE: Inconsistencies in router advertisement values between routers are logged per RFC 4861. The values checked for
consistency include:
• Cur Hop limit
• M and O ags
• Reachable time
• Retrans timer
• MTU options
• Preferred and valid lifetime values for the same prex
Only management ports support stateless auto-conguration as a host.
The router redirect functionality in the neighbor discovery protocol (NDP) is similar to IPv4 router redirect messages. NDP uses ICMPv6
redirect messages (Type 137) to inform nodes that a better router exists on the link.
IPv6 Headers
The IPv6 header has a xed length of 40 bytes. This xed length provides 16 bytes each for source and destination information and 8 bytes
for general header information.
The IPv6 header includes the following elds:
• Version (4 bits)
• Trac Class (8 bits)
• Flow Label (20 bits)
• Payload Length (16 bits)
• Next Header (8 bits)
• Hop Limit (8 bits)
• Source Address (128 bits)
• Destination Address (128 bits)
IPv6 provides for extension headers. Extension headers are used only if necessary. There can be no extension headers, one extension
header or more than one extension header in an IPv6 packet. Extension headers are dened in the Next Header eld of the preceding IPv6
header.
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IPv6 Routing