ZyXEL Communications MSC1000G Series, Руководство пользователя

Страница: 476 / 1168

Chapter 12 Statistics Screens
Management Switch Card User’s Guide
476
12.16 IPv6 Neighbor Cache
The MSC uses the Neighbor Discovery Protocol (NDP) to discover other IPv6 devices and track their
reachability in a network. The MSC 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 MSC maintains and updates its IPv6 caches constantly using the information from response
messages. In IPv6, the MSC 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 MSC also sends out a neighbor solicitation message. When the MSC receives a
neighbor advertisement in response, it stores the neighbor’s link-layer address in the neighbor
cache. When the MSC 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 MSC creates an entry in the default router list cache if the router can be
used as a default router.
When the MSC 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 MSC 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 MSC determines the next-hop from the default router list or routing table. Once the next hop IP
address is known, the MSC looks into the neighbor cache to get the link-layer address and sends
the packet when the neighbor is reachable. If the MSC 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.
12.16.1 Neighbor Cache
Click Statistics > IPv6 in the navigation panel to open this screen. The neighbor cache is similar
to a MAC address table in IPv4. Use this screen to view the link-layer addresses of the MSC’s
interfaces and neighboring devices. You can also see if a neighbor is still reachable from the MSC.