Why most of CE routers have routing configuration on sub interfaces instead of physical interfaces?

[chauhans2]

Hi All,

I'm working in an ISP company and i have seen that most of the CE routers have physical port 'not configured' and sub-interface of that port configured for routing or link termination with PE. Here in case of Ethernet connections, i found that encapsulation dot1q <vlan id> command to be configured on sub-interface. Now i understand that we need encapsulation to tag the frame with vlan id coming from or going to layer 2 switch in path. My question is why we can't configure encapsulation on router physical interface? Is it ios limitation or something else? I'm talking about normal routers physical interfaces not the routers which have switching module installed. And in this case does router receives layer 2 frames and checks for vlan tag or is there any way i can confirm that this port is trunking or not? I checked 'sh interface trunk' or 'sh trunk' doesn't work on router. Now router is a layer 3 device and so what functions it performs at layer 2 in this case?

And in NBMA environments like frame relay or ATM on cisco router serial interfaces, do we use sub-interface when we need point to multi-point connection only or any other scenarios?

My understanding was that we use sub-interface when physical interfaces are not sufficient or available on router.

I tried searching online a lot but couldn't get satisfactory answer.

Please help or provide any link which can help to explain.

Thanks,

Regards:

Rashmikant

[Richard Burts]

Rashmikant

Cisco IOS routers treat their Ethernet interfaces as normal host interfaces (or as access ports if we use the terminology of switches). As a standard NIC it processes Ethernet frames without tagging. Cisco IOS routers implement support for processing tagged Ethernet frames in the subinterface mode. So in a sense yes it is an IOS limitation. If you want the IOS router to process tagged frames you must configure the subinterface on the Ethernet interface.

You are correct that the commands show interface trunk or show trunk do not work on IOS routers. These commands are for switches and do not work on IOS routers. On an IOS router if it is configured with subinterfaces on the Ethernet then it is configured for trunking. And if the router Ethernet interface does not have subinterfaces then it is not configured for trunking.

HTH

Rick

[chauhans2]

Thanks Rick !

[Richard Burts]

Rashmikant

You are welcome.

HTH

Rick