ZyXEL Communications OLT2406, User Manual

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Chapter 55 IPv6
OLT2406 User’s Guide
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IPv6 Cache
An IPv6 host is required to have a neighbor cache, destination cache, prefix list and default router list.
The OLT maintains and updates its IPv6 caches constantly using the information from response
messages. In IPv6, the OLT 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
OLT also sends out a neighbor solicitation message. When the OLT receives a neighbor advertisement in
response, it stores the neighbor’s link-layer address in the neighbor cache. When the OLT 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 OLT creates an entry in
the default router list cache if the router can be used as a default router.
When the OLT 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 OLT 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 OLT determines the next-hop
from the default router list or routing table. Once the next hop IP address is known, the OLT looks into the
neighbor cache to get the link-layer address and sends the packet when the neighbor is reachable. If
the OLT 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. If the leave mode is not set to
immediate
, the router or switch 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.
MLD Port Role
A port on the OLT can be either a downstream port or upstream port in MLD. A downstream port ( DSP in
the figure) connects to MLD hosts and acts as a multicast router to send MLD queries and listen to the
MLD host’s Report and Done messages. An upstream port (
USP in the figure) connects to a multicast
router and works as a host to send Report or Done messages when receiving queries from a multicast
router.