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Intermediate System-to-Intermediate System (IS-IS) Basics for the Cisco BSCI Exam


  1. Intermediate System-to-Intermediate System (IS-IS)

Chapter Description

This Flash Cards excerpt gives you the basic facts about IS-IS you need to know for the Cisco BSCI exam.
Intermediate System-to-Intermediate System (IS-IS) is a routing protocol developed by the ISO

Intermediate System-to-Intermediate System (IS-IS)

Intermediate System-to-Intermediate System (IS-IS) is a routing protocol developed by the ISO. It is a link-state protocol and behaves much like Open Shortest Path First (OSPF). The two protocols have some significant differences, however.

IS-IS was developed as part of the Open System Interconnection (OSI) stack of protocols. It uses OSI protocols to deliver its packets and establish its adjacencies. IS-IS routers need to be assigned OSI addresses, which they use as a Router ID to create network structure.

IS-IS has been adapted to carry IP network informa-tion, and this form is called Integrated IS-IS. Integrated IS-IS has the most important characteristic necessary in a modern routing protocol: It supports VLSM and converges rapidly. It is also scalable to support very large networks.

What type of company typically uses IS-IS?

Large ISPs typically use IS-IS because it is scalable to very large networks.

Describe IS-IS Level 2 routing.

Level 2 routing is routing between areas. Level 2-capable routers comprise the IS-IS backbone and can be in separate areas. Any traffic bound for other areas must go through a Level 2-capable router. Level 2 routing is based on area ID.

Describe the differences in backbone require-ments between OSPF and IS-IS.

OSPF requires that an area be defined as a backbone area and that each other area border that backbone area. Special configuration (a virtual link) is required for any area that does not border the backbone area. IS-IS backbone routers can reside in any area. There merely must be an unbroken chain of Level 2 or Level 1/2 routers in order for the backbone to function.

A router has a Network Entity Title (NET) of 49.001a.1122.3344.5566.00. To what area does this router belong, and what is its system ID?

The area is 49.001a. The router's system ID is 1122.3344.5566. The easiest way to figure this out is to start from the right and work towards the left. The last two numbers of the NET are the NSEL; they are always 00 on a router. The next 12 numbers (separated into 3 groups of 4 numbers) are the system ID. On Cisco routers, the system ID is always this length—6 bytes. Anything to the left of the system ID is the area ID.

Which of the following is a valid router NET address, and why?

  • 2.49.0000.00c0.1234.00

  • 40.0000.00c0.1234.56

  • 1234.5678.90ab.cdef.0001.00

  • 2.49.0000.00c0.1234.00 is not a valid NET address because the first number in the area address has to be at least one byte (two numbers) long.

  • 40.0000.00c0.1234.56 is not a valid NET address because the last two numbers, the NSEL, must always be 00.

  • 1234.5678.90ab.cdef.0001.00 is a valid NET address. The area ID is 1234.5678, and the system ID is 90ab.cdef.0001. The NSEL is 00.

Describe the link-state databases maintained by a L1 router, a L2 router, and a L1/L2 router.

A L1 router maintains a database of all routers within the area and tags L1/L2 routers for use as default routes. A L2 router maintains a database of all the areas in the autonomous system and the closest next-hop L2 or L1/L2 router for each area. A L1/L2 router maintains two separate databases—a L1 database for intra-area routing and a L2 database for inter-area routing. It also advertises a default route into its area.

What are the four types of IS-IS protocol data units (PDUs), and their use?

  • Hellos—Establish and maintain adjacencies

  • LSP (Link State PDU)—Advertises link-state information

  • CSNP (Complete Sequence Number PDU)—An update containing the complete list of LSPs known to the router

  • PSNP (Partial Sequence Number PDU)—Used to acknowledge a routing update (LSP) on point-to-point links and to request missing information about a route after receiving a CSNP

If two Cisco routers are directly connected via an Ethernet link, belong to the same area, and both are L1/L2 routers, what types of adjacencies do they establish?

They establish both a L1 and a L2 adjacency, maintain a separate database for each level, and send each other both L1 and L2 types of hellos.

What criteria are used in electing the DIS?

An IS-IS DIS is elected based on highest priority value, and then on highest SNPA address (typically the MAC address). The priority is assigned to each interface and has a default value of 64. Priority can be configured; the range is 1–127. In case of a tie, the router with the highest SPNA address for that interface is elected the DIS. No backup DIS exists.

How often are hellos sent on an IS-IS broadcast link?

Hellos are exchanged every 10 seconds on a broadcast link by all routers except the DIS. The DIS sends a hello every 3.3 seconds.

What command displays the IS-IS adjacencies formed by the router?

show clns neighbors

Recall that IS-IS routers form adjacencies via CLNS.

What command gives a summary of the IS-IS process on the router?

show clns protocol

This command displays the router's system ID, its IS type, area ID, interfaces participating in IS-IS routing, routes being redistributed, the administrative distance for CLNS, and the type of metrics in use.

You have an IS-IS router that is performing both L1 and L2 routing and has both L1 and L2 neighbors. How would you optimize the router's operation to conserve bandwidth and router resources?

Configure each interface as either L1 or L2 circuit type, depending on the type of adjacency needed out that interface. The command to do this is, at the interface configuration mode, isis circuit-type [level-1 | level-1-2 | level-2-only]. This prevents unnecessary hellos from being sent out interfaces, which uses bandwidth and router resources.

What is a SNPA, and how it is derived?

SNPA stands for Subnetwork Point of Attachment. It identifies a point at which a device connects to a network. It is roughly equivalent to a Layer 2 address in the non-CLNS world. The SNPA for a local-area network (LAN) connection is the MAC address of the interface. The SNPA for a wide-area network (WAN) interface is the virtual circuit identifier. For example, the data-link connection identifier (DLCI) on a Frame Relay connection. If the WAN interface is using High-Level Data Link Control (HDLC) encapsulation, the SNPA is simply HDLC. For example:

R2# show clns neighbor
System Id Interface SNPA           State Holdtime Type Protocol
R1        Et0       0000.0c09.9fea Up    24       L1L2 IS-IS
R3        Se0       *HDLC*         Up    28       L1L2 IS-IS

What two types of network topology are supported by IS-IS?

Broadcast and point-to-point. Broadcast topology typically describes a LAN, but it might also be used with an NBMA network such as Frame Relay.