Chapter 3

1:	How many levels of hierarchy does the IS-IS routing protocol support and what is their significance?
A:	Two levels, Level 1 and Level 2, are specified by ISO 10589 and adopted by RFC 1195. Hierarchical routing allows control over the spread of routing updates, which is a key requirement for network stability and scalability.
2:	What is the reason for suboptimal interarea routing in the ISO 10589 architecture?
A:	In the ISO 10589 architecture, the Level 1 routing areas are stubs and do not have a complete view of the domain that is necessary for making optimal exit decisions out of their local area. Instead, Level 1 routers point defaults to the nearest Level 2 router, which might not necessarily be on the best path to a specific destination outside the area.
3:	Name the two categories of IS-IS protocol functions and describe the services they provide.
A:	The two categories are the subnetwork-dependent functions and the subnetwork-independent functions. The former is primarily responsible for the adjacency formation process, and the latter for managing the routing information update and maintenance processes.
4:	What is the general layout of IS-IS packet formats?
A:	All IS-IS packets are composed of a header and variable-length TLV fields.
5:	List the TLVs specified by ISO 10589.
A:	See Table 3.1.
6:	List the TLVs specified by RFC 1195 and describe their significance.
A:	See Table 3.2. The TLVs specified by RFC 1195 allow the IS-IS protocol, which was originally designed for routing ISO CLNP packets, to be used for IP routing.
7:	How does the TLV Type 133 specified by RFC 1195 differ from the original authentication TLV Type 10 specified by IS0 10589?
A:	TLV Type 133 does not place any restriction on the length of the authentication TLV (255 octets in TLV 10). Also, it does not specify authentication Type 255 for routing domain private authentication as in TLV 10.
8:	Describe any differences between the adjacency formation processes on point-to-point and broadcast links.
A:	Point-to-point adjacency formation is preceded by detection of ISHs; also, reliability is not built into the process because of the lack of an IS Neighbor TLV field in point-to-point hellos. A reliable three-way adjacency for point-to-point links has been proposed in the IETF for standardization.
9:	Describe the three-way adjacency formation process that has been proposed in the IETF to enhance the method for forming adjacencies on point-to-point links.
A:	The method allows for introduction of a new TLV field (240), which will include IS neighbor information in point-to-point hellos. This information can then be used for a three-way handshake in the adjacency formation process.
10:	What is the relevance of the pseudonode functionality?
A:	The pseudonode functionality is provided on broadcast media as a way to control flooding in such environments. LSPs are multicast to neighbors on such links, and the DIS that acts as the pseudonode facilitates database synchronization over the LAN by periodically multicasting summaries of all known LSPs. This allows use of a less reliable but periodic mechanism to alleviate the challenges of reliably advertising LSPs among the many possible adjacencies on multiaccess or broadcast links.
11:	Briefly describe the DIS election and replacement process.
A:	DIS election is by highest interface priority with the highest SNPA (MAC address) as the tie breaker. Election also is pre-emptive and any newly connected router can take over the DIS functions if it better qualifies to be the DIS, according to this criteria.
12:	Explain why no backup DIS exists.
A:	A backup DIS is unnecessary because of the periodic database synchronization on LAN media and the shorter hello interval used by the DIS, which allows for fast detection of any failures and subsequent replacement.