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• 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.
Appendix C IPv6
VMG3925-B10C/B30C User's Guide
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