What could be the problem?

Refer to the topology and router output shown in the exhibit. A technician is troubleshooting host
connectivity issues on the switches. The hosts in VLANs 10 and 15 on Sw11 are unable to
communicate with hosts in the same VLANs on Sw12. Hosts in the Admin VLAN are able to
communicate. The port-to-VLAN assignments are identical on the two switches. What could be the
problem?

A.
The link connecting the switches has not been configured as a trunk.

B.
The Fa0/1 port is not operational on one of the switches.

C.
A router is required for hosts on SW11 in VLANs 10 and 15 to communicate with hosts in the
same VLAN on Sw12.

D.
Port FastEthernet 0/1 needs to be configured as an access link on both switches.

E.
At least one port needs to be configured in VLAN 1 for VLANs 10 and 15 to be able to
communicate.

Explanation:
One of the most remarkable features of VLANs is that they can span multiple interconnected
switches. VLAN traffic is carried from switch to switch over interfaces called trunks. These trunk
links must be at least 100Mbps because the traffic must carry all the VLAN traffic from the access
ports.
Just as access ports have a single VLAN assigned to them, trunk ports essentially have all VLANs
assigned to them. As frames traverse a trunk link, the VLAN identifier is added to the ethernet
frame. The receiving switch uses the VLAN identifier and sends the frames out only the access
ports that have that VLAN assigned to them. As the frame is sent out the interface, the VLAN
identifier is removed, which gives the illusion to the end devices that the entire process is
transparent.
The first step to configuring a trunk link is deciding which type of trunk you want to use. For
instance, if you are connecting to a non-Cisco switch, you have to use a standard trunk VLAN
tagging method such as IEEE 802.1q. In addition, certain models of Cisco switches (such as the
Catalyst 2950) support only 802.1q trunking, so make sure you research the capabilities of your

switch model before configuring the interfaces.
To configure a trunk port, navigate to the interface that is connected to the other switch. On
models that support ISL and 802.1q trunking, you must first specify which VLAN tagging you want
to use with the switchport trunk encapsulation command, as shown here:
Switch(config)#interface FastEthernet 0/24
Switch(config-if)#switchport trunk encapsulation dot1q
Notice the syntax starts with switchport trunk instead of switchport access (from the VLAN
configurations) because this interface is being configured as a trunk to carry all VLANs.
With the trunk encapsulation configured, you are ready to enable the interface to begin forwarding
all VLAN traffic. The port, however, is still operating as an access port until you specifically
configure the interface to switch to trunking mode. To set this interface into a permanent trunking
state, you must also type the following command :
Switch(config-if)#switchport mode trunk