which interfaces should root guard be configured to ensure that this happens

The service provider wants to ensure that switch S1 is the root switch for its own network and the network of the customer. On which interfaces should root guard be configured to ensure that this happens?

A.
interfaces 1 and 2

B.
interfaces 1, 2, 3, and 4

C.
interfaces 1, 3, 5, and 6

D.
interfaces 5 and 6

E.
interfaces 5, 6, 7, and 8

F.
interfaces 11 and 12

Explanation:

The traditional STP does not provide any means for the network administrator to securely enforce the topology of the switched Layer 2 network. This may become especially important in networks with shared administrative control. For example, one switched network controlled by different administrative entities or companies. Forwarding topology of the switched network is calculated, based among other parameters, on the root bridge position. Although any switch can be Root Bridge in the network, it is better to place the root bridge manually, (somewhere in the core layer) so the forwarding topology will be optimal. The standard STP does not allow the administrator to enforce the position of the root bridge. If a bridge is introduced into the network with lower bridge priority, it will take the role of the root bridge. The root guard ensures that the port on which it is enabled is the designated port (normally, root bridge ports are all designated, unless two or more ports of the root bridge are connected together). If the bridge receives superior STP Bridge Port Data Units (BPDUs) on a root guard enabled port, this port will be moved to a root-inconsistent STP state (effectively equal to listening state), and no traffic will be forwarded across this port. The position of the root bridge will be enforced.
Configuring Root Guard