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、operati on is similar to BGP
• D、supports two routing levels within an autonomous system