Foundation Summary

     

The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam.

Figure 10-7 shows the format of a CLNS address.

Figure 10-7. An ISO Address

For routing purposes, ISO 10589 specifies three parts to the address, namely how to get to the area, how to find the host, and how to find the application within the host. However, there are only two elements in the address used to find a destination host: the address that locates the area and the host within that area. Therefore, Integrated IS-IS has two levels of hierarchy, as follows :

• IDP The IDP is used to route to the domain, or autonomous system. The IDP is given by the ISO and identifies the body responsible for assigning the format for the rest of the address by defining the DSP structure. The following two parts comprise the IDP:

  - AFI (authority and format identifier) AFI is the first octet of the address. It is defined as one octet. It states the authority in charge of issuing addresses and the format of the rest of the address, within the constraints of IDI and DSP, to describe Area, ID, and SEL.
  
  

  - IDI (initial domain identifier) IDI is the suborganization to the AFI. (As an analogy, the U.S. government is the AFI and the General Services Agency is the IDI, otherwise known as GOSIP.)
  
  

• DSP DSP is used to route within the autonomous system. The authority specified by the IDP may further subdivide the DSP and may assign subauthorities responsible for parts of the DSP. The DSP can be broken down into the following fields:

  - High Order DSP High Order DSP is typically the area within the autonomous system.
  
  

  - System ID System ID can have a value between one and eight octets. It must have the same value throughout all systems in the autonomous system. Cisco uses six octets, which is a common solution because it allows the use of the MAC address to autoconfigure the system.
  
  

  - NSEL NSEL identifies the particular service at the network layer to which to hand the packet.
  
  

Characteristics of a Level 1 router include the following:

• An intra-area router

• Similarity to an OSPF stub router

• Knowledge of the network limited to the area

• A link-state database with all the routing information for the area

• The address of the closest Level 2 router to which to send traffic destined for another area

• Neighbors must be in the same area

• A DIS that is elected on LANs

Characteristics of a Level 2 router include the following:

• An interarea router

• Similarity to a backbone internal router in OSPF

• Level 2 routers must be contiguous (that is, the area cannot be fractured)

• Neighbors can be in different areas

• A Level 2 database with all the information for interarea routing

Characteristics of a Level 1-2 router include the following:

• An intra-area router and an interarea router

• Similar to an area boundary router (ABR) in OSPF

• Neighbors can be in different areas

• A Level 2 database with all the information for interarea routing

• A Level 1 database for each area to which it is connected

• Informs Level 1 routers that it is a Level 2 router and can forward traffic to other areas

• Informs other Level 2 routers of the areas to which it is connected

For an adjacency to be formed and maintained , both interfaces must agree on the following:

• The maximum packet size (MTU) that can be transmitted by the interface must be the same.

• Each router needs to be configured as the same level of routingthat is, either Level 1 or Level 2so that they can decode the Hellos sent by the other router.

• If the routers are both Level 1, they must be in the same area.

• Level 1 routers form adjacencies with each other, and Level 2 routers form adjacencies with other Level 2 routers. For a Level 1 router to communicate with a Level 2 router, one of the routers needs to be configured as a Level 1-2 router. Therefore, to connect to another area, at least one of the routers must be configured as a Level 1-2, allowing the Level 2 router to receive the packets destined for another area from the Level 1 router.

• The system ID must be unique to each router.

• If authentication is configured, it must be configured identically on both routers.

There are three Integrated IS-IS packets, as the following list describes:

• Hello These packets create and maintain neighbor relationships and adjacencies. There are three types of Integrated IS-IS Hello packet. The type of packet is defined in the fixed header under the Type field and allows the packet to be handed off to the appropriate process. The different types are as follows:

  - LAN Level 1 Generated by Level 1 and Level 1-2 routers
  
  

  - LAN Level 2 Generated by Level 2 and Level 1-2 routers
  
  

  - Point-to-point Generated by Level 1, Level 2, and Level 1-2 routers
  
  

• LSP LSPs hold information on the neighbors connected to the router. There are two types of LSP, as follows:

  - Level 1 Generated by Level 1 and Level 1-2 routers
  
  

  - Level 2 Generated by Level 2 and Level 1-2 routers
  
  

• Sequence number packet (SNP) SNPs describe the LSPs in the transmitting router's link-state database. The information is condensed and is never flooded but only sent between neighbors. SNPs ensure link-state databases synchronization by:

  - Distributing groups of LSPs on a LAN without explicit individual acknowledgements

  - Acknowledging individual LSPs

  - Requesting LSPs at startup

  There are two types of SNP for each level of routing, as follows:

  - Complete SNP (CSNP) Includes every LSP in the database:
  
  

  - Level 1

  Level 2

  - Partial SNP (PSNP) Includes a subset of LSPs, used to request individual LSPs and to acknowledge receipt of these LSPs:
  
  

  - Level 1

  Level 2

There are three different Hellos, as follows:

• Point-to-point Hello

• LAN Level 1 Hello

• LAN Level 2 Hello