Users Guide

Extended Address Space
The address format is extended from 32 bits to 128 bits. This not only provides room for all anticipated needs, it allows for the
use of a hierarchical address space structure to optimize global addressing.
Stateless Autoconfiguration
When a booting device comes up in IPv6 and asks for its network prefix, the device can get the prefix (or prefixes) from an IPv6
router on its link. It can then autoconfigure one or more global IPv6 addresses by using either the MAC address or a private
random number to build its unique IPv6 address.
Stateless autoconfiguration uses three mechanisms for IPv6 address configuration:
• Prefix Advertisement — Routers use “Router Advertisement” messages to announce the network prefix. Hosts then use their
interface-identifier MAC address to generate their own valid IPv6 address.
• Duplicate Address Detection (DAD) — Before configuring its IPv6 address, an IPv6 host node device checks whether that
address is used anywhere on the network using this mechanism.
• Prefix Renumbering — Useful in transparent renumbering of hosts in the network when an organization changes its service
provider.
NOTE: As an alternative to stateless autoconfiguration, network hosts can obtain their IPv6 addresses using the dynamic
host control protocol (DHCP) servers via stateful auto-configuration.
NOTE: Dell Networking OS provides the flexibility to add prefixes on Router Advertisements (RA) to advertise responses to
Router Solicitations (RS). By default, RA response messages are sent when an RS message is received.
Dell Networking OS manipulation of IPv6 stateless autoconfiguration supports the router side only. Neighbor discovery (ND)
messages are advertised so the neighbor can use this information to auto-configure 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 flags
• Reachable time
• Retrans timer
• MTU options
• Preferred and valid lifetime values for the same prefix
Only management ports support stateless auto-configuration 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 fixed length of 40 bytes. This fixed length provides 16 bytes each for source and destination information
and 8 bytes for general header information.
The IPv6 header includes the following fields:
• Version (4 bits)
• Traffic Class (8 bits)
• Flow Label (20 bits)
IPv6 Routing 363