© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicBSCI Module 4 Lesson 1 1

The IS-IS Protocol

BSCI Module 4 Lesson 1

Introducing IS-IS and Integrated IS-IS Routing

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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

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

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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

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Uses for IS-IS Routing

Large ISPs

Stable protocol

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

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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.

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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.

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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.

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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.

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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

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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.

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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.

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Four OSI Routing Levels

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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.

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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

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Advantages of Integrated IS-IS

Supports CLNP and IP

More extensible through TLV design

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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.

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Comparison of Integrated IS-IS and OSPF

OSPF Integrated IS-IS

Area border inside routers (ABRs) Area border on links

Each link in only 1 area Each router in only 1 area

More complex to extend backbone Simple extension of backbone

Many small LSAs sent Fewer LSPs sent

Runs on top of IP Runs on top of data-link layer

Requires IP addresses Requires IP and CLNS addresses

Default metric is scaled by interface bandwidth

Default metric is 10 for all interfaces

Not easy to extend Easy to support new protocols with new TLV tuples

Equipment, personnel, and information more readily available

Equipment, personnel, and information not as easily available

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

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Summary

. 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.

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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.

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Activity

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

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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?

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Additional Links

http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800c5bc1.html

http://www.certificationzone.com/cisco/newsletter/SL/IE-ISIS-WP2-F02_BIF.html

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfisis.htm

.

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Q and A

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