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Appendix C IPv6
VMG5313-B10A/-B30A Series User’s Guide
379
• 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 VMG maintains and updates its IPv6 caches constantly using the information from
response messages. In IPv6, the VMG 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 VMG also sends out a neighbor solicitation message. When the VMG
receives a neighbor advertisement in response, it stores the neighbor’s link-layer address in the
neighbor cache. When the VMG 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 VMG creates an entry in the default router list cache if the
router can be used as a default router.
When the VMG 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 VMG 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 unlink, the address is considered as the next hop. Otherwise, the
VMG determines the next-hop from the default router list or routing table. Once the next hop IP
address is known, the VMG looks into the neighbor cache to get the link-layer address and sends
the packet when the neighbor is reachable. If the VMG 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.
Multicast Listener Discovery
The Multicast Listener Discovery (MLD) protocol (defined in RFC 2710) is derived from IPv4's
Internet Group Management Protocol version 2 (IGMPv2). MLD uses ICMPv6 message types, rather
than IGMP message types. MLDv1 is equivalent to IGMPv2 and MLDv2 is equivalent to IGMPv3.
MLD allows an IPv6 switch or router to discover the presence of MLD listeners who wish to receive
multicast packets and the IP addresses of multicast groups the hosts want to join on its network.
MLD snooping and MLD proxy are analogous to IGMP snooping and IGMP proxy in IPv4.
MLD filtering controls which multicast groups a port can join.
MLD Messages
A multicast router or switch periodically sends general queries to MLD hosts to update the multicast
forwarding table. When an MLD host wants to join a multicast group, it sends an MLD Report
message for that address.
An MLD Done message is equivalent to an IGMP Leave message. When an MLD host wants to leave
a multicast group, it can send a Done message to the router or switch. The router or switch then
sends a group-specific query to the port on which the Done message is received to determine if
other devices connected to this port should remain in the group.