What does the attach bit in an IS-IS PDU indicate?

A.
The L1/L2 router is exporting a default route into the area.

B.
The L1/L2 router has reachability information to another IS-IS area.

C.
The L1/L2 router is sending information from Level 2 towards Level 1.

D.
The L1/L2 router is sending information from Level 1 towards Level 2.

Explanation:
In ISO terminology, packets are referred to as Protocol Data Units (PDUs). There are 3 categories of IS-IS packets:
IS-IS Hello Packets (IIHs): These packets establish and maintain adjacencies between IS-IS neighbors.
Link State PDUs (LSPs): These packets are responsible to distribute routing information between IS-IS nodes.
Sequence Number PDUs (SNPs): These packets control the distribution of LSPs. SNPs provide mechanism to synchronize LSDBs between routers in the same area.
Each packet category consists of various types of packets; each type is assigned a Type Number.
All IS-IS packets are multicast on LAN. For Level-1 ISs, the packets are sent to 01-80-C2-00-00-14, and for Level-2 ISs, the packets are sent to 01-80-C2-00-00-15.

The following table shows IS-IS packet types: