Foundation Summary

The Foundation Summary provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following lists and tables are a convenient way to review the day before the exam.

Table 9-4 outlines similarities between Integrated IS-IS and OSPF.

Table 9-4. Integrated IS-IS and OSPF Similarities
IS-ISComparable OSPF Tool
AreaStub area
Area IDArea ID
Level 2Area 0
IS (intermediate system)Router
ISO Routing DomainAutonomous system
Level 1 RouterInternal nonbackbone stub area router
Level 1-2 RouterArea border router (ABR)
Level 2 RouterBackbone router
LSP (link-state packet)LSA (link-state advertisement)
CSNP and PSNP (complete and partial sequence number PDUs)Link-state acknowledgement packet
PDU (protocol data unit)Packet
NET (network entity title)IP destination address (subnet and host), used in a similar way to router ID
NSAP (network service access point)IP destination address + IP protocol number
Routing technology = link stateRouting technology = link state
  • Classless routing protocol

  • Classless routing protocol

  • Address summarization between areas

  • Address summarization between areas

  • Uses a link-state database

  • Uses a link-state database

  • Acknowledges LSPs

  • Acknowledges LSAs

  • Shortest path is computed using Dijkstra's SPF algorithm

  • Shortest path is computed using Dijkstra's SPF algorithm

  • Hellos create and maintain adjacencies

  • Hellos create and maintain adjacencies

  • Hellos and holdtime may be configured

  • Hellos and holdtime can be configured

Subnet = data linkSubnet = IP network
SNPA (subnetwork point of attachment)Layer 2 address; for example, the Ethernet MAC address
System IDRouter ID
Virtual link (defined but not supported)Virtual link

Figure 9-5 shows the format of an ISO address.

Figure 9-5. An ISO Address
AFI (1 octet)IDIHigh Order DSPSystem ID (1-8 octets)NSEL (1 octet)

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

  • The maximum packet size (MTU).

  • A common routing level (Level 1 or Level 2) so that they can decode the Hellos sent by the other router.

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

  • A unique system ID.

  • 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— Hellos create and maintain neighbor relationships.

  • LSP— LSPs hold information on the neighbors connected to the router. There are two types of LSP, one for each routing level.

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

    - Complete SNP (CSNP)— Includes every LSP.

    - Partial SNP (PSNP)— Includes a subset of LSPs; used to request LSPs and to acknowledge receipt.

The routing process is divided into four stages:

  • The update process

  • The decision process

  • The forwarding process

  • The receive process

Some typical area designs include the following:

  • A totally flat network that uses only Level 1 routing.

  • A totally flat network using only Level 2 routing. As the network expands, Level 1 areas may be added.

  • A totally flat network using the Cisco default configuration of every router running Level 1-2 routing.

  • A hierarchical network where the core is running Level 2 routing with Level 1 areas connecting into the core. Level 1-2 routers are used to connect the areas.

Integrated IS-IS does not have a point-to-multipoint configuration option. The alternatives are as follows:

  • Configure the link as multipoint, creating a broadcast environment that requires the election of a DIS.

  • Configure the interfaces with subinterfaces that are point-to-point.