Given the network diagram, which routers currently make up the IS-IS backbone?

A.
R3,R4,R6

B.
R2,R3,R6,R7

C.
R1 through R8

D.
R3,R4,R5,R6

E.
R2,R3,R4,R6,R7

Explanation:
An intermediate system can be a level 1 (L1) router, a level 2 (L2) router, or both (L1/L2). L1
routers are analogous to OSPF nonbackbone Internal Routers, L2 routers are analogous to
OSPF backbone routers, and L1/L2 routers are analogous to OSPF ABRs. The L1/L2
routers are connected to L1 routers and to L2 routers. These L1/L2 routers must maintain
both a level 1 link-state database and a level 2 link-state database, in much the same way
that an OSPF ABR must maintain a separate database for each area to which it is attached.
Cisco routers are configured as L1-only, L2-only, or L1/L2 with the command is-type. By
default, they are L1/L2.
Note: In actuality, routers R4 and R5 would also make up the part of the backbone, but
since they are not given as choices the best answer is E.