ZyXEL Communications VMG1312-B Series User Manual page 355

• Neighbor advert isem ent : A response from a node t o announce it s link- layer address.
• Rout er solicit at ion: A request from a host t o locat e a rout er t hat can act as t he default rout er and
forward packet s.
• Rout er advert isem ent : A response t o a rout er solicit at ion or a periodical m ult icast advert isem ent
from a rout er t o advert ise it s presence and ot her param et ers.
IPv6 Cache
An I Pv6 host is required t o have a neighbor cache, dest inat ion cache, prefix list and default rout er
list . The Device m aint ains and updat es 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 t hen
sends a neighbor solicit at ion m essage t o check if t he address is unique. I f t here is an address t o be
resolved or verified, t he Device also sends out a neighbor solicit at ion m essage. When t he Device
receives a neighbor advert isem ent in response, it st ores t he neighbor 's link- layer address in t he
neighbor cache. When t he Device uses a rout er solicit at ion m essage t o query for a rout er and
receives a rout er advert isem ent m essage, it adds t he rout er 's inform at ion t o t he neighbor cache,
prefix list and dest inat ion cache. The Device creat es an ent ry in t he default rout er list cache if t he
rout er can be used as a default rout er.
When t he Device needs t o send a packet , it first consult s t he dest inat ion cache t o det erm ine t he
next hop. I f t here is no m at ching ent ry in t he dest inat ion cache, t he Device uses t he prefix list t o
det erm ine whet her t he dest inat ion address is on- link and can be reached direct ly wit hout passing
t hrough a rout er. I f t he address is unlink, t he address is considered as t he next hop. Ot herwise, t he
Device det erm ines t he next- hop from t he default rout er list or rout ing t able. Once t he next hop I P
address is known, t he Device looks int o t he neighbor cache t o get t he link- layer address and sends
t he packet when t he neighbor is reachable. I f t he Device cannot find an ent ry in t he neighbor cache
or t he st at e for t he neighbor is not reachable, it st art s t he address resolut ion process. This helps
reduce t he num ber of I Pv6 solicit at ion and advert isem ent m essages.
Multicast Listener Discovery
The Mult icast List ener Discovery ( MLD) prot ocol ( defined in RFC 2710) is derived from I Pv4's
I nt ernet Group Managem ent Prot ocol version 2 ( I GMPv2) . MLD uses I CMPv6 m essage t ypes, rat her
t han I GMP m essage t ypes. MLDv1 is equivalent t o I GMPv2 and MLDv2 is equivalent t o I GMPv3.
MLD allows an I Pv6 swit ch or rout er t o discover t he presence of MLD list eners who wish t o receive
m ult icast packet s and t he I P addresses of m ult icast groups t he host s want t o j oin on it s net work.
MLD snooping and MLD proxy are analogous t o I GMP snooping and I GMP proxy in I Pv4.
MLD filt ering cont rols which m ult icast groups a port can j oin.
MLD Messages
A m ult icast rout er or swit ch periodically sends general queries t o MLD host s t o updat e t he m ult icast
forwarding t able. When an MLD host want s t o j oin a m ult icast group, it sends an MLD Report
m essage for t hat address.
An MLD Done m essage is equivalent t o an I GMP Leave m essage. When an MLD host want s t o leave
a m ult icast group, it can send a Done m essage t o t he rout er or swit ch. The rout er or swit ch t hen
sends a group- specific query t o t he port on which t he Done m essage is received t o det erm ine if
ot her devices connect ed t o t his port should rem ain in t he group.
VMG1312-B Series User's Guide
Appendix E IPv6
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