A router has a single Ethernet connection to a switch. In the router configuration, the Ethernet interface
has three sub-interfaces, each configured with ACLs applied to them and 802.1q trunks.
Which of the following is MOST likely the reason for the sub-interfaces?

A.
The network uses the subnet of 255.255.255.128.

B.
The switch has several VLANs configured on it.

C.
The sub-interfaces are configured for VoIP traffic.

D.
The sub-interfaces each implement quality of service.

Explanation:
A subinterface is a division of one physical interface into multiple logical interfaces. Routers commonly
employ subinterfaces for a variety of purposes, most common of these are for routing traffic between
VLANs. Also, IEEE 802.1Q is the networking standard that supports virtual LANs (VLANs) on an Ethernet
network.
Incorrect Answers:
A, C, D: Subnets, VoIP, and QoS do not make use of this standard.

http://en.wikipedia.org/wiki/Subinterface
http://en.wikipedia.org/wiki/IEEE_802.1Q