ZyXEL Communications MES3500-24S User Manual page 409

ICMPv6
Internet Control Message Protocol for IPv6 (ICMPv6 or ICMP for IPv6) is defined in RFC 4443. ICMPv6 has
a preceding Next Header value of 58, which is different from the value used to identify ICMP for IPv4.
ICMPv6 is an integral part of IPv6. IPv6 nodes use ICMPv6 to report errors encountered in packet
processing and perform other diagnostic functions, such as "ping".
Neighbor Discovery Protocol (NDP)
The Neighbor Discovery Protocol (NDP) is a protocol used to discover other IPv6 devices and track
neighbor's reachability in a network. An IPv6 device 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.
IPv6 Cache
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 onlink, 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.
Appendix C IPv6
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