Appendix E IPv6
VMG1312-B Series User’s Guide
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• Neighbor advertisement: A response from a node t o announce its link-layer address.
• Router solicitat ion: A request from a host t o locat e a router that can act as t he default rout er and
forward packet s.
• Router advertisem ent : A response t o a router solicitat ion or a periodical m ulticast advertisem ent
from a rout er t o advertise it s presence and ot her param eters.
IPv6 Cache
An I Pv6 host is required t o have a neighbor cache, dest ination cache, prefix list and default router
list . The Device m aintains and updates it s I Pv6 caches const ant ly using t he inform at ion from
response m essages. I n I Pv6, t he Device configures a link- local address aut om at ically, and then
sends a neighbor solicit ation m essage t o check if the address is unique. I f t her e is an address t o be
resolved or verified, t he Device also sends out a neighbor solicitation m essage. When the Device
receives a neighbor advertisem ent in response, it stores t he neighbor’s link- layer addr ess in the
neighbor cache. When t he Device uses a rout er solicitat ion m essage t o query for a rout er and
receives a router advert isem ent m essage, it adds the rout er ’s infor m ation to the neighbor cache,
prefix list and destination cache. The Device creates an entry in the default rout er list cache if the
router can be used as a default rout er.
When t he Device needs t o send a packet, it first consults t he destination cache t o det erm ine t he
next hop. I f t her e is no m at ching ent ry in the destination cache, t he Device uses t he prefix list to
det erm ine whether t he dest ination address is on-link and can be reached directly without passing
through a rout er. I f t he address is unlink, t he address is considered as t he next hop. Otherwise, t he
Device determ ines the next- hop from the default rout er list or rout ing t able. Once t he next hop I P
address is known, the Device looks into the neighbor cache t o get the link- layer address and sends
the packet when t he neighbor is reachable. I f t he Device cannot find an entry in the neighbor cache
or t he state for the neighbor is not reachable, it start s the address resolut ion process. This helps
reduce t he num ber of I Pv6 solicitation and advert isem ent m essages.
Multicast Listener Discovery
The Multicast List ener Discovery ( MLD) prot ocol ( defined in RFC 2710) is derived from I Pv4's
I nternet Group Managem ent Prot ocol version 2 (I GMPv2). MLD uses I CMPv6 m essage t ypes, rat her
than I GMP m essage types. MLDv1 is equivalent t o I GMPv2 and MLDv2 is equivalent to I GMPv3.
MLD allows an I Pv6 switch or rout er t o discover the presence of MLD list eners who w ish t o receive
m ulticast packets and the I P addresses of m ult icast groups t he host s want t o j oin on its network.
MLD snooping and MLD proxy are analogous t o I GMP snooping and I GMP proxy in I Pv4.
MLD filtering cont rols which m ulticast groups a port can j oin.
MLD Messages
A m ulticast r out er or swit ch periodically sends general queries t o MLD host s t o updat e the m ult icast
forwarding table. When an MLD host wants t o j oin a m ulticast group, it sends an MLD Report
m essage for that address.
An MLD Done m essage is equivalent to an I GMP Leave m essage. When an MLD host wants t o leave
a m ulticast group, it can send a Done m essage to the router or switch. The rout er or swit ch then
sends a group- specific quer y to the port on which t he Done m essage is received t o determ ine if
other devices connected to t his port should rem ain in the group.
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