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 feature can be enabled or disabled if you have
either a dynam ic or st at ic I P. How ever the encapsulat ion m ethod assigned influences your choices
for I P address and default gateway.
Introduction to VLANs
A Virt ual Local Area Net work ( VLAN) allows a physical network to be partit ioned int o m ultiple logical
net works. Devices on a logical network belong to one gr oup. A device can belong t o m or e t han one
group. Wit h VLAN, a device cannot directly talk to or hear from devices t hat are not in the sam e
group( s); t he t raffic m ust first go t hrough a router.
I n Mult i-Tenant Unit (MTU) applicat ions, VLAN is vital in providing isolation and security am ong the
subscribers. When properly configured, VLAN prevents one subscriber from accessing the network
resources of another 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 the sam e building.
VLAN also increases net work perfor m 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 ents, all broadcast packet s
go t o each and every individual por t . With VLAN, all broadcast s are confined to a specific broadcast
dom ain.
Introduction to IEEE 802.1Q Tagged VLAN
A tagged VLAN uses an explicit tag ( VLAN I D) in t he MAC header t o identify the VLAN m em bership
of a fram e across bridges - t hey are not confined to the switch on which they were created. The
VLANs can be created st at ically by hand or dynamically t hrough GVRP. The VLAN I D associates a
fram e with a specific VLAN and provides the inform at ion t hat switches need t o process the fram e
across the net work. A t agged fram e is four byt es longer t han an unt agged fram e and contains tw o
byt es of TPI D ( Tag Prot ocol I dentifier), residing wit hin the t ype/ length field of the Et hernet fram e)
and t wo byt es of TCI (Tag Cont rol I nfor m ation) , starts after the 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 switches. I f
a fram e received at an Et hernet port has a CFI set t o 1, t hen that fram e should not be forwarded as
it is t o an untagged port . The rem aining t welve bits define t he VLAN I D, giving a possible m axim um
num ber of 4,096 VLANs. Not e that user pr iorit y and VLAN I D are independent of each ot her. A
fram e with VI D (VLAN I dent ifier) of null ( 0) is called a priority fram e, m eaning that only the priority
level is significant and the default VI D of t he ingress port is given as the VI D of the fram e. Of the
4096 possible VI Ds, a VI D of 0 is used to ident ify priority fram es and value 4095 ( FFF) is reserved,
so the m axim um possible VLAN configurations are 4,094.
TPI D
2 Bytes
User Priority
3 Bit s
CFI
1 Bit
VLAN I D
12 Bit s
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