21.1.2 Distributing information

There are two types of address distribution by using RAs: periodic distribution by routers and distribution in response to requests from terminals. Both types of address distribution are performed by using ICMPv6 RAs (the type field in the messages indicates 134). When a terminal wants to find a router, it sends an ICMPv6 Router Solicitation message (the value of type field in the packet is 133).

When a terminal receives an RA, it generates a global address by combining the given prefix and the 64-bit interface ID it owns (the interface ID is usually generated based on the 48-bit MAC address).Then the terminal sets the global address for the interface that received the RA. At the same time, the terminal sets the source address of the RA (link-local address of the router interface that sent the RA) as its default gateway address. The following figure shows how an interface ID is generated based on a MAC address.

Figure 21-2: ID generation of interfaces from MAC addresses

The prefix sent from the router to the terminal is usually the prefix of the address set for the interface that sends RAs. However, routers can advertise other prefixes as well. You can configure the maximum and minimum intervals for sending RAs from routers for each interface. The following table describes the information distributed in an RA.

Table 21-1 Informations distributed over RA
Distributed information	Description	Specifiable value	Default
Managed address configuration flag (ManagedFlag)	This flag tells terminals to perform IPv6 address auto-configuration upon reception of the RA using a method other than RAs (such as DHCPv6). Regardless of the value set for this flag, address configuration triggered by RAs is always performed. This flag is usually set to off.	ON/OFF	OFF
Other stateful configuration flag (OtherConfigFlag)	This flag tells terminals to perform auto-configuration of information (such as the address of a DNS server) other than IPv6 addresses upon reception of the RA using a method other than RAs (such as DHCPv6). This flag is usually set to off.	ON/OFF	OFF
Link MTU (LinkMTU)	The MTU size to be used by terminals in actual communication. The MTU size usually used is that of the interface that received an RA.If you do not want the terminals to send or receive the packets of the MTU size set for an interface, specify a value smaller than the MTU size in this field. Routers cannot provide an MTU size that is larger than the MTU set for the interface.	0(not distributed) or MTU of 1280 to interfaces	MTU for the interface
Reachable time (ReachableTime)	In IPv6, terminals use ICMPv6 messages to check the reachability of neighboring nodes.This field indicates the length of time terminals assume a neighbor is reachable after they have received a reachability confirmation. If the reachable time is not specified or 0 is specified, the default value determined for each terminal is used. When a value other than 0 is set in this field, the value is also used as the base for the reachable state transition time for neighbor entries that is learned on the applicable interface on the Switch.	0 to 4294967295 (ms)	0
Retransmission timer (RetransTimer)	In IPv6, terminals use ICMPv6 messages to check the reachability of neighboring nodes. This field specifies the interval of sending ICMPv6 packets to terminals. If the retransmission timer is not specified or 0 is specified, the default value determined for each terminal is used. When a value other than 0 is specified, the value is also used as the interval for resending Neighbor Solicitation messages for address resolution that use neighbor entries and neighbor unreachability detection leaned on the applicable interface of the Switch.	0, Or 1000 to 4294967295 (ms)	0
Current hop limit (CurHopLimit)	This field indicates the value to be specified in the Hop Limit field in the IPv6 header of packets sent by terminals. The value in the Hop Limit field indicates how many hops packets are forwarded.	0~255	64
Router lifetime (DefaultLifetime)	Expiration time of the default router confirmed by terminals by receiving the RA. When this field is set to 0, terminals do not regard the source of the received RA as the default gateway.	0, Or RA send interval max. to 9000 (seconds)	1800(seconds)
Link layer option (SourceLink-layerAddressOption)	The link-layer address associated with the IPv6 address of the source of the RA. For the Switch, this field contains the MAC address of the port on the interface that sent the RA only when the interface is Ethernet or Gigabit Ethernet. When load balancing is performed by using link-layer addresses, this field is set to 0, allowing each terminal to resolve the link-layer address of the default gateway.	ON/OFF	ON
Router preference (DefaultRouterPreference)	This field indicates the priority level of an RA.When a terminal receives RAs from multiple routers, it uses this information to determine which RA it accepts.	high, medium, low	medium
Prefix (PrefixList)	A prefix advertised in an RA. When no prefix is specified in this field, routers advertise the prefix set for the advertising interface (except for the prefix in a link-local address). This field is also used to advertise prefixes other than the prefix assigned to the advertising interface or to set a time limit for the prefix assigned to an interface.	Prefix in a global or site-local address	Prefix in an address assigned to an interface other than link-local addresses
Autonomous address-configuration flag (AutonomousFlag)	Prefixes are not provided to terminals when this flag is set to off. This flag is always set to on except for testing RAs.	ON/OFF	ON
On-link flag (OnLinkFlag)	Routers do not send ICMPv6 Redirect messages to terminals when this flag is set to off. This flag is always set to on except for testing RAs.	ON/OFF	ON
Preferred lifetime (PreferredLifetime)	The length of time during that terminals are allowed to use the prefix provided in the RA in the source address of packets sent by terminals. If a terminal does not receive a new RA after the preferred lifetime is exceeded, the terminal tries to use an address that does not contain the previously provided prefix as the source address for communication. However, if the terminal does not have another appropriate prefix, it uses the prefix for communication even if the prefix has exceeded the preferred lifetime.	0, Or RA send interval max. to 4294967295 (seconds)	604800(seconds)
Valid lifetime (ValidLifetime)	The expiration time of the prefix provided by an RA. If a terminal does not receive a new RA after the valid lifetime is exceeded, the terminal deletes the address containing the expired prefix.	0, Or RA send interval max. to 4294967295 (seconds)	2592000 (seconds)
DNS Servers (RDNSS)	Specifies IPv6 of DNS servers that the terminal uses for DNS resolution.	Global address or link-local address	No distribution
DNS server-information expiration date (Lifetime)	The maximum amount of time that DNS servers advertised by RA can be used.	0 to 4294967295 (seconds)	Three times the maximum RA transmission interval
DNS search list (DNSSL)	Specifies the domainname that the terminal uses to perform DNS resolution.	Domain name of up to 253 characters	No distribution
DNS search-list expiration date (Lifetime)	The limit for how long the domain-name advertised by RA can be used.	0 to 4294967295 (seconds)	Three times the maximum RA transmission interval