ZyXEL Communications VMG1312-B Series User Manual Download

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Chapter 6 Broadband

VMG1312-B Series User’s Guide

104

IP Address Assignment

A st at ic I P is a fixed I P t hat  your I SP gives you. A dynam ic I P is not  fixed;  t he I SP assigns you a
different  one each t im e. The Single User Account  feat ure can be enabled or disabled if you have
eit her a dynam ic or st at ic I P. However t he encapsulat ion m et hod assigned influences your choices
for I P address and default  gat eway.

Introduction to VLANs

A Virt ual Local Area Net work ( VLAN)  allows a physical net work t o be part it ioned int o m ult iple logical
net works. Devices on a logical net work belong t o one group. A device can belong t o m ore t han one
group. Wit h VLAN, a device cannot  direct ly t alk t o or hear from  devices t hat  are not  in t he sam e
group( s) ;  t he t raffic m ust  first  go t hrough a rout er.

I n Mult i-Tenant  Unit  ( MTU)  applicat ions, VLAN is vit al in providing isolat ion and securit y am ong t he
subscribers. When properly configured, VLAN prevent s one subscriber from  accessing t he net work
resources of anot her on t he sam e LAN, t hus a user will not  see t he print ers and hard disks of
anot her user in t he sam e building.

VLAN also increases net work perform ance by lim it ing broadcast s t o a sm aller and m ore
m anageable logical broadcast dom ain. I n t radit ional swit ched environm ent s, all broadcast packet s
go t o each and every individual port . Wit h VLAN, all broadcast s are confined t o a specific broadcast
dom ain.

Introduction to IEEE 802.1Q Tagged VLAN

A t agged VLAN uses an explicit  t ag ( VLAN I D)  in t he MAC header t o ident ify t he VLAN m em bership
of a fram e across bridges -  t hey are not  confined t o t he swit ch on which t hey were creat ed. The
VLANs can be creat ed st at ically by hand or dynam ically t hrough GVRP. The VLAN I D associat es a
fram e wit h a specific VLAN and provides t he inform at ion t hat  swit ches need t o process t he fram e
across t he net work. A t agged fram e is four byt es longer t han an unt agged fram e and cont ains t wo
byt es of TPI D ( Tag Prot ocol I dent ifier) , residing w it hin t he t ype/ lengt h field of t he Et hernet  fram e)
and t wo byt es of TCI  ( Tag Cont rol I nform at ion) , st art s aft er t he source address field of t he Et hernet
fram e) .

The CFI  ( Canonical Form at  I ndicat or)  is a single- bit  flag, always set  t o zero for Et hernet  swit ches. I f
a fram e received at  an Et hernet  port  has a CFI  set  t o 1, t hen t hat  fram e should not  be forwarded as
it  is t o an unt agged port . The rem aining t welve bit s define t he VLAN I D, giving a possible m axim um
num ber of 4,096 VLANs. Not e t hat  user priorit y and VLAN I D are independent  of each ot her. A
fram e wit h VI D ( VLAN I dent ifier)  of null ( 0)  is called a priorit y fram e, m eaning t hat  only t he priorit y
level is significant  and t he default  VI D of t he ingress port  is given as t he VI D of t he fram e. Of t he
4096 possible VI Ds, a VI D of 0 is used t o ident ify priorit y fram es and value 4095 ( FFF)  is reserved,
so t he m axim um  possible VLAN configurat ions are 4,094.

ZyXEL Communications VMG1312-B Series, User Manual

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