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

[xiangzou2]

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?

[Marvin Rhoads · Accepted solution]

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]

Hi Marvin,

    Thanks for your reply. It really helps!

[linda.yuan]

Hi, Marvin:

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

[Marvin Rhoads]

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.