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The IS-IS Protocol. BSCI Module 4 Lesson 1 Introducing IS-IS and Integrated IS-IS Routing. Objectives. Describe IS-IS routing and some of the ways in which IS-IS is used Describe the features of integrated IS-IS routing Explain the principles and issues of integrated IS-IS design

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The is is protocol

The IS-IS Protocol

BSCI Module 4 Lesson 1

Introducing IS-IS and Integrated IS-IS Routing


  • Describe IS-IS routing and some of the ways in which IS-IS is used

  • Describe the features of integrated IS-IS routing

  • Explain the principles and issues of integrated IS-IS design

  • Describe the features of the ES-IS protocol

  • Describe how to differentiate among the four OSI routing levels

  • Explain the similarities and differences between IS-IS and OSPF

Purpose of this lesson
Purpose of this Lesson

  • Coverage of topics new to the “Introducing IS-IS and Integrated IS-IS Routing” section in the “IS-IS” module of BSCI.

  • What’s new in this module?

    • Describe the features of the ES-IS protocol

    • Explain the similarities and differences between IS-IS and OSPF

The is is protocol

Uses for IS-IS Routing

Large ISPs

  • Stable protocol

  • Originally deployed by ISPs because US government mandated internet support of OSI and IP

Is is routing
IS-IS Routing

  • IS = router

  • IS-IS was originally designed as the IGP for the Connectionless Network Service (CLNS), part of the OSI protocol suite.

  • The OSI protocol suite layer 3 protocol is the Connectionless Network Protocol (CLNP).

  • IS-IS uses CLNS addresses to identify routers and build the LSDB.

Is is features
IS-IS Features

  • Link-state routing protocol

  • Supports VLSM

  • Uses Dijkstra’s SPF algorithm; has fast convergence

  • Uses Hellos to establish adjacencies and LSPs to exchange link-state information

  • Efficient use of bandwidth, memory, and processor

  • Supports two routing levels:

    • Level 1: Builds common topology of system IDs in local area and routes within area using lowest cost path.

    • Level 2: Exchanges prefix information (area addresses) between areas. Routes traffic to area using lowest-cost path.

Is is link state operation
IS-IS Link-State Operation

  • Routers identified as Level 1, Level 2, or Level 1-2:

    • Level 1 routers use LSPs to build topology for local area.

    • Level 2 routers use LSPs to build topology between different areas.

    • Level 1-2 routers act as border routers between Level 1 and Level 2 routing domains.

Integrated is is routing
Integrated IS-IS Routing

  • Integrated IS-IS is IS-IS for multiple protocols:

    • For IP, CLNS, or both

  • Uses its own PDUs to transport IP routing information; updates are not sent in IP packets.

  • Requires CLNS addresses, even if only routing for IP.

Integrated is is design principles
Integrated IS-IS Design Principles

  • IP and CLNP addressesmust be planned.

  • Use two-level hierarchy for scalability:

    • Limits LSP flooding

    • Provides opportunity for summarization

  • Summarization:

    • Limits update traffic

    • Minimizes router memory and CPU usages

Issues with integrated is is
Issues with Integrated IS-IS

  • Default narrow metrics are limited to 6-bit interface and 10-bit path metric:

    • In Cisco IOS 12.0, wide metrics allow 24-bit interface and 32-bit path metric.

  • Cisco IOS software has default metric of 10 on all interfaces.

End system to intermediate system
End System-to-Intermediate System

  • ES-IS forms adjacencies between ESs and routers (ISs).

    IP end-systems don’t use ES-IS

  • ESs transmit ESHs to announce their presence to ISs.

  • ISs transmit ISHs to announce their presence to ESs.

  • ISs transmit IIHs to other ISs.

Four osi routing levels
Four OSI Routing Levels

Similarities between is is and ospf
Similarities Between IS-IS and OSPF

  • Integrated IS-IS and OSPF are both open standard link-state protocols with the following similar features:

    • Link-state representation, aging timers, and LSDB synchronization

    • SPF algorithms

    • Update, decision, and flooding processes

    • VLSM support

  • Scalability of link-state protocols has been proven (used in ISP backbones).

  • They both converge quickly after changes.

Integrated is is vs ospf area design
Integrated IS-IS vs. OSPF: Area Design

  • OSPF is based on a central backbone with all areas attached to it.

    In OSPF the border is inside routers (ABRs)

    Each link belongs to one area

Advantages of integrated is is
Advantages of Integrated IS-IS

  • Supports CLNP and IP

  • More extensible through Type, Length, Value (TLV) design

Advantages of ospf
Advantages of OSPF

  • OSPF has more features, including:

    Has three area types: normal, stub, and NSSA

    Defaults to scaled metric (IS-IS always 10)

  • OSPF is supported by many vendors.

  • Information, examples, and experienced engineers are easier to find.

Comparison of integrated is is and ospf
Comparison of Integrated IS-IS and OSPF

The following table summarizes the differences between OSPF and integrated IS-IS.


  • IS-IS is a popular routing protocol in the ISP industry.

  • IS-IS is a stable, fast converging IGP that is positioned to route IPv4, CLNS, or IPv6.

  • All IS-IS interfaces have a default metric of 10.

  • ES-IS (for CLNS routing only) provides discovery between host and routers using hello packets to form adjacencies. Hosts send ESHs, while routers send ISHs.

  • .

Summary cont
Summary (Cont.)

  • OSI defines routing levels 0 through 3. Level 0 is between ES and IS. Levels 1 and 2 are between IS and IS to support intradomain routing. Level 3 supports interdomain routing.

    • Level 1 is intra-area; Level 2 is interarea.

  • IS-IS and OSPF are both open standard link state routing protocols that support VLSM, scalability, and quick convergence.

  • .


  • List all of the features of both IS-IS and OSPF and compare the advantages and disadvantages of both.

Self check
Self Check

  • How many levels are supported by IS-IS and ES-IS?

  • Which routing protocol is most closely related to IS-IS?

  • What is the default metric for IS-IS?

  • What are advantages of using IS-IS?

Additional links
Additional Links




    • .