Home > Articles > Cisco Certification > CCNP > Intermediate System-to-Intermediate System (IS-IS) Basics for the Cisco BSCI Exam

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

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 13, 2003.


  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.

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020