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How to distinguish physical interface and logical interface(subinterface) in Cisco Router/Switch?

Answered Question
Sep 9th, 2014
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Hi Expert,

    How to distinguish physical interface and logical interface(subinterface) in Cisco Router/Switch? Can you please specify a formal way for this if have?

Correct Answer by Marvin Rhoads about 2 years 11 months ago

A physical interface will be numbered with the same interface name as is printed on the physical port. For example "GigabitEthernet 0/1" corresponds to port 1 on module 0 (or the base unit).

A logical interface can be a subinterface on a routed port and will have a dot (".") preceding the subinterface number (ex. GigabitEthernet 0/1.1). It can also be a loopback or virtual interface (on a router this could also include interfaces like tunnel and virtual tunnel or VTI types). A switch can also have VLAN type logical interfaces (e.g. interface vlan 1) which are used as layer 3 virtual interfaces.

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Correct Answer
Marvin Rhoads Tue, 09/09/2014 - 19:02
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A physical interface will be numbered with the same interface name as is printed on the physical port. For example "GigabitEthernet 0/1" corresponds to port 1 on module 0 (or the base unit).

A logical interface can be a subinterface on a routed port and will have a dot (".") preceding the subinterface number (ex. GigabitEthernet 0/1.1). It can also be a loopback or virtual interface (on a router this could also include interfaces like tunnel and virtual tunnel or VTI types). A switch can also have VLAN type logical interfaces (e.g. interface vlan 1) which are used as layer 3 virtual interfaces.

xiangzou2 Tue, 09/09/2014 - 18:39
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Hi Marvin,

    Thanks for your reply. It really helps!

linda.yuan Tue, 09/09/2014 - 19:40
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Hi, Marvin:

For logical interface, is there any way to know the relationship between physical interface and logical interface?  

Marvin Rhoads Tue, 09/09/2014 - 20:23
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There may or may not be a relationship.

If it is a subinterface, then the logical subinterface will have the same name (in the part preceding the dot) as the physical interface.

If its a virtual interface type then it's independent of the device's physical interfaces. In fact, that's the primary purpose of using loopback interface types - they can be always up and reachable independent of any particular physical interface.

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