subinterface

[carl_townshend]

what is a sub interface, is this just used on routers ? why would I need one ?

thanks

Carlos

[simonstoll]

Hi

A sub-interface is a logical interface, instead of a physical interface like eth 0/0 or serial 0/0 (the sub-interfaces would be eth 0/0.1 for excempel). You can use them mostly on routers, to configure tagged (Trunks) links (ethernet) to switches or to configure framerelay enviroments etc. You can configure a lot on a subinterface, except physical things like speed (clockrate on serial) and duplex, that has to be done on the physical interface (but you can shutdown a single subinterface whithout affecting the other subinterfaces).

Hope that helps a bit

Simon

[carl_townshend]

So could I use a subinterface as an actual interface i.e set the gateway on my machine to the subinterface , or is it just used for trunks ?

[carl_townshend]

how to I create a sub interface ?

[Richard Burts]

Carl

If you create a subinterface, assign an address to that subinterface, then you could configure workstations to use that address as their default gateway. (This assumes that the workstations are in the same broadcast domain - same vlan/same subnet as the subinterface.)

Creating a subinterface is very much like creating a loopback interface. You name the subinterface in config mode and the IOS creates the virtual interface. You indicate the subinterface as the interface identifier then dot then an integer subinterface identifier. So if you wanted to create a subinterface on interface FastEthernet 1/0 you could do this:

interface fastethernet 1/0.1

and IOS would create the .1 subinterface under the physical fastethernet 1/0.

HTH

Rick

[carl_townshend]

is this not the same as a loopback more or less ?

[Richard Burts]

Carl

While I have said that they are similar, they are certainly not the same. A subinterface is associated with a physical interface and is subordinate to that physical interface. The subinterface is dependent on the operation of the physical interface.

A loopback interface is a separate and independent interface. Operation of the loopback interface is not subordinate to any other interface and operation of the loopback interface is not dependent on the operation of any other interface.

The fact that they are both virtual interfaces makes them similar but there are significant differences between a subinterface and a loopback interface.

HTH

Rick