Which two statements are true about IS-IS? (Choose two.)

Which two statements are true about IS-IS? (Choose two.)
A. IS-IS DIS election is nondeterministic.
B. IS-IS SPF calculation is performed in three phases.
C. IS-IS works over the data link layer, which does not provide for fragmentation and reassembly.
D. IS-IS can never be routed beyond the immediate next hop.
Correct Answer:CD
Layer 3 Technologies Explanation Explanation/Reference:
IS-IS runs directly over the data link alongside IP. On Ethernet, IS-IS packets are always 802.3 frames, with LSAPs 0xFEFE while IP packets are either Ethernet II frames or SNAP frames identified with the protocol number 0x800. OSPF runs over IP as protocol number 89. IS-IS runs directly over layer 2 and hence:
– cannot support virtual links unless some explicit tunneling is implemented – packets are kept small so that they don’t require hop-by-hop fragmentation
– uses ATM/SNAP encapsulation on ATM but there are hacks to make it use VcMux encapsulation
– some operating systems that support IP networking have been implemented to differentiate Layer 3 packets in kernel. Such Oss require a lot of kernel modifications to support IS-IS for IP routing.
– can never be routed beyond the immediate next hop and hence shielded from IP spoofing and similar Denial of Service attacks.