Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 437

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) IPv6 Routing 437