Bedienungsanleitung ZyXEL VES1724-56 (Seite 372 von 414) (Englisch)

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VES1724-56 User’s Guide 372

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.

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