ZyXEL VES1724-56B2 User Guide - Page 372
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CHAPTER 45 IPv6 Cache This chapter describe the IPv6 caches on the Switch. Refer to Section 8.6 on page 76 to see more information about IPv6. 45.1 Overview The Switch uses the Neighbor Discovery Protocol (NDP) to discover other IPv6 devices and track their reachability in a network. The Switch uses the following ICMPv6 messages types: • Neighbor solicitation: A request from a host to determine a neighbor's link-layer address (MAC address) and detect if the neighbor is still reachable. A neighbor being "reachable" means it responds to a neighbor solicitation message (from the host) with a neighbor advertisement message. • Neighbor advertisement: A response from a node to announce its link-layer address. • Router solicitation: A request from a host to locate a router that can act as the default router and forward packets. • Router advertisement: A response to a router solicitation or a periodical multicast advertisement from a router to advertise its presence and other parameters. An IPv6 host is required to have a neighbor cache, destination cache, prefix list and default router list. The Switch maintains and updates its IPv6 caches constantly using the information from response messages. In IPv6, the Switch configures a link-local address automatically, and then sends a neighbor solicitation message to check if the address is unique. If there is an address to be resolved or verified, the Switch also sends out a neighbor solicitation message. When the Switch receives a neighbor advertisement in response, it stores the neighbor's link-layer address in the neighbor cache. When the Switch uses a router solicitation message to query for a router and receives a router advertisement message, it adds the router's information to the neighbor cache, prefix list and destination cache. The Switch creates an entry in the default router list cache if the router can be used as a default router. When the Switch needs to send a packet, it first consults the destination cache to determine the next hop. If there is no matching entry in the destination cache, the Switch uses the prefix list to determine whether the destination address is on-link and can be reached directly without passing through a router. If the address is on-link, the address is considered as the next hop. Otherwise, the Switch determines the next-hop from the default router list or routing table. Once the next hop IP address is known, the Switch looks into the neighbor cache to get the link-layer address and sends the packet when the neighbor is reachable. If the Switch cannot find an entry in the neighbor cache or the state for the neighbor is not reachable, it starts the address resolution process. This helps reduce the number of IPv6 solicitation and advertisement messages. VES1724-56 User's Guide 372