138 Understanding IPv6,Second Edition TypeL =134 Code e w =o Checksum Current Hop Limit Managed Address Configuration Flag Other Stateful Configuration Flag Home Agent Flag Default Router Preference Reserved Router Lifetime Reachable TimeLwlLI Retrans Timer Options lwwL... Figure 6-13 The structure of the Router Advertisement message The fields in the Router Advertisement message are as follows: Type The value of this field is 134. Code The value of this field is 0. Checksum The value of this field is the ICMPv6 checksum. Current Hop Limit The Current Hop Limit field indicates the default value of the Hop Limit field in the IPv6 header for packets sent by hosts that receive this Router Adver- tisement message.The size of this field is 8 bits.A current hop limit of 0 indicates that the default value of the Hop Limit field is not specified by the router. Managed Address Configuration flag The Managed Address Configuration flag indi- cates,when set to 1,that hosts receiving this Router Advertisement message must use an address configuration protocol such as DHCPv6 to obtain addresses in addition to the addresses that might be derived from stateless address autoconfiguration.The size of this field is 1 bit. Other Stateful Configuration flag The Other Stateful Configuration flag indicates. when set to 1,that hosts receiving this Router Advertisement message must use an address configuration protocol (for example,DHCPv6)to obtain non-address configu- ration information.The size of this field is I bit. Home Agent flag The Home Agent flag is defined in RFC 3775 for Mobile IPv6.For more information,see Appendix F. Default Router Preference The Default Router Preference field indicates the level of preference for this router as the default router.If multiple routers advertise themselves as default routers,you can configure the routers so that they advertise with different pref- erence levels.Valid values in binary are 01 (High),00(Medium),and 11 (Low).If the
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Chapter 6 Neighbor Discovery 139 preference is set to 10,the receiving host should assume a value of 0 for the Router Lifetime field,effectively disabling the advertising router as a default router.The size of this field is 2 bits.This field is described in RFC 4191. A typical configuration that can use the default router preference is a subnet that has two routers connected to the Internet or an organization intranet-one router is the primary router,and another router is a slower,secondary router intended to provide fault toler- ance for the primary router.Both routers advertise themselves as default routers:how- ever,the primary router advertises a default router preference of 0l (High)and the secondary router advertises a default router preference of 00(Medium).If the primary router becomes unavailable,the hosts on the subnet will use the secondary router until the primary router becomes available. Reserved This is a 3-bit field reserved for future use and set to 0. Router Lifetime The Router Lifetime field indicates the lifetime (in seconds)of the router as the default router.The size of this field is 16 bits.The maximum Router Life- time value is 65,535 seconds (about 18.2 hours).A value of 0 indicates that the router cannot be considered a default router:however,all other information contained in the Router Advertisement is still valid.A computer running Windows Server 2008. Windows Vista,Windows XP,or Windows Server 2003 acting as an IPv6 router that does not have a default route that is configured to be published sets the Router Lifetime field to 0. Reachable Time The Reachable Time field indicates the amount of time (in millisec- onds)that a node can consider a neighboring node reachable after receiving a reachabil- ity confirmation.The size of this field is 32 bits.A value of 0 indicates that the router does not specify the reachable time.For more information,see the "Neighbor Unreach- ability Detection"section in this chapter. Retransmission Timer The Retransmission Timer field indicates the amount of time(in milliseconds)between retransmissions of Neighbor Solicitation messages.The size of this field is 32 bits.The retransmission timer is used during neighbor unreachability detection.A value of 0 indicates that the router does not specify the retransmission timer value. The options that can be present in a Router Advertisement message are the following: Source Link-Layer Address option When present,the Source Link-Layer Address option contains the link-layer address of the interface on which the Router Advertise- ment message was sent.This option can be omitted when the router is load-balancing across multiple link-layer addresses. MTU option When present,the MTU option contains the MTU of the link.It is typi- cally sent on links that have a variable MTU or in switched environments that have mul- tiple link-layer technologies on the same link
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