Intermediate system to intermediate system (IS-IS), is a protocol used by network devices
(routers) to determine the best way to forward datagrams through a packet-switched
network, a process called routing. Which three attributes are of the IS-IS routing protocol?

A.
supports VLSM

B.
link-state routing protocol

C.
operation is similar to BGP

D.
supports two routing levels within an autonomous system

Explanation:
The point of this question is bacis knowledge of IS-IS.
IS-IS is a link-state routing protocol, it supports VLSM and CIDR, it supports classless
routing, IS-IS supports level-1 and level-2 within an autonomous system. So option A C and
D are correct.
not router-by-router.