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Diff  b/w Sub interfaces and Secondary

Posted on 2006-07-19
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Last Modified: 2010-03-19
hi
i have one more qsuesion and that is what is the difference between Subinterfces and Seconadry Command ... please give me some pracical answer rather than the defination... in what kind of envoirnment we can use ..... them

Regards
ASad
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Question by:annasad
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Expert Comment

by:Sorenson
ID: 17137620
Subinterfaces are used to define seperate routable layer three networks.  They are used in frame relay to define seperate point-to-point or point-to-multipoint circuits, each then has its own layer 3 address.  They are also used to define and route or access seperate vlans when the router is connected to a 802.1q trunked port of a switch.  Subinterfaces create logical seperate interfaces.  

The secondary command adds a second address to an existing layer 3 interface.  In general it is not good practice to use a secondary address except when absolutely necc.  It is used for migrations from one ip scheme to the next, or as a temporary solution.  A secondary address means that you are running two seperate address spaces in a single layer 2 network.
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rsivanandan earned 125 total points
ID: 17138283
Cisco routers had the capability of assigning more than one ip address to the interface which can be done both by using 'secondary' keyword or using subinterfaces.

However, 'secondary' was introduced first so that a single physical interface can have multiple networks and route among themselves.

Then came the limitation of parameters that can be defined on a single physical interface. So they introduced the subinterface component.

For example, frame-relay circuits can have different 'point-to-point' connections with different DLCI. In such a case if you use 'secondary' it wouldn't work. So when you use subinterface, you basically split out the interfaces from one another.

example would be;

int s0/0.1
frame-relay interface dlci 100

int s0/0.2
frame-relay interface dlci 200

See the flexibility ? So now subinterfaces are promoted and Cisco advises to use that instead of 'secondary'

Hope that helps understand.

Cheers,
Rajesh
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Expert Comment

by:rsivanandan
ID: 17145138
May I know why grade B ?

Cheers,
Rajesh
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Author Comment

by:annasad
ID: 17145184
YES YOU CAN .....
WHAT YOU HAVE EXPLAINED IS ..... CORRECT..... BUT WHAT I HAVE UNDERSTOOD WAS DIFFERENT ...... AND I NEED TO IMPLEMENT IT AND IT TAKES THE TIME WHEREEVER  QUESTION ARE FOR SHORT TIME ..

NEVER MIND ..... I WILL MAKE IT A ... IF I COULD IN CUMMING MAILS
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