配置IS-IS协议详解与实战

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"配置IS-IS协议" 本章节主要讲解了中间系统到中间系统（IS-IS）协议的相关知识，这是构建可扩展的Cisco网络中的一个重要部分。IS-IS协议是OSI模型中的一种路由协议，用于在局域网和广域网环境中实现数据包的传递和路由选择。章节内容包括以下部分： 1. **OSI协议和IS-IS路由概述**：这部分内容介绍了开放系统互连（OSI）模型以及IS-IS路由协议在该模型中的地位和作用。IS-IS协议是OSI第二层（数据链路层）上的协议，主要处理CLNS（连接定位服务）/CLNP（连接定位协议）的数据包，同时也支持第三层（网络层）的IP路由。 2. **IS-IS在CLNS/CLNP中的操作**：详细阐述了IS-IS如何在CLNS协议栈上工作，包括数据包的封装、路由信息的交换以及路由决策过程。IS-IS通过L1（Level 1）和L2（Level 2）两种层次来组织网络，分别负责区域内路由和区域间路由。 3. **集成IS-IS的IP和OSI路由**：讨论了IS-IS如何同时处理IP和OSI路由，实现网络层的统一。集成IS-IS允许在一个单一的协议框架下同时进行OSI和IP的路由，提高了网络的效率和灵活性。 4. **基本的集成IS-IS路由器配置**：这部分提供了关于如何配置路由器以运行IS-IS的指导，包括启动IS-IS进程、设置区域ID、宣告网络和接口等基础配置步骤。 5. **在集成IS-IS中的WAN网络建模**：讨论了如何在广域网环境中应用IS-IS，包括网络拓扑的设计、路由选择策略的制定以及如何优化WAN性能。 6. **总结**：对章节主要内容的回顾，强调了IS-IS的关键概念和技术要点。 7. **配置练习：配置多区域IS-IS网络**：提供了一个实践性的配置案例，帮助读者理解如何在多区域环境下部署IS-IS。 8. **配置练习答案：配置多区域IS-IS网络**：给出了配置练习的解决方案，以便读者检查自己的理解和操作。 9. **复习问题**：列出了相关的问题，帮助读者巩固所学知识，并在阅读完章节后自我检测。此外，本章节还配套有在线补充材料，包括对原书附录D、G、H和I的更新，提供IS-IS命令的总结列表、复习问题的答案、配置练习设备需求及主干路由器配置命令等，以辅助学习者更深入地理解和应用IS-IS协议。这些补充材料旨在增强读者对IS-IS协议的实战技能和理论理解。

Operation of IS-IS for CLNS/CLNP 16

the LSPs to identify the router, build the topology table, and build the underlying IS-IS

routing tree.

The NSAPs contain the following:

• The OSI address of the device

• The link to the higher-layer process

NOTE The NSAP address can be thought of as the equivalent of a combination of IP address and

upper-layer protocol in an IP header.

Dijkstra’s algorithm, which is used for both IS-IS and OSPF, requires a point-to-point

connection between devices. OSPF uses the router ID to represent each router for this

connection, while IS-IS uses the NET (a subset of the NSAP discussed in the following

section).

NSAP Address Structure

Cisco routers can route CLNS data that uses addressing conforming to the ISO 10589

standard. The NSAP structure is illustrated in Figure S-6.

Figure S-6 Structure of NSAP Addresses

An OSI NSAP address can be up to 20 octets long and consists of the following parts, as

shown in Figure S-6:

• The authority and format ID (AFI) speciﬁes the format of the address and the

authority that assigned that address. The AFI is 1 byte.

• The interdomain ID (IDI) identiﬁes this domain. The IDI can be up to 10 bytes.

• The AFI and IDI together make up the interdomain part (IDP) of the NSAP address.

This can loosely be equated to an IP classful major network.

• The high-order domain-speciﬁc part (DSP) (HODSP) is used for subdividing the

domain into areas. This can be considered loosely as the OSI equivalent of a subnet

in IP.

IDP DSP

AFI IDI High Order DSP System ID NSEL

Variable Length Area Adress 6 Bytes 1 Byte

2283newchapf.fm Page 16 Monday, December 16, 2002 12:58 PM

17 Chapter : Conﬁguring IS-IS Protocol

• The system ID identiﬁes an individual OSI device (an ES or an IS). In OSI, a device

has an address, just as it does in the DECnet protocol. This is different from IP, in

which an interface has an address. OSI does not specify a ﬁxed length for the system

ID, but it does specify that it be consistent for all devices. Cisco IOS Software ﬁxes

the system ID as the 6 bytes preceding the 1-byte NSAP selector (NSEL).

NOTE A Media Access Control (MAC) address is often used for the system ID.

• The NSEL (also known as the N-selector, the service identiﬁer or the process ID)

identiﬁes a process on the device. It is a loose equivalent of a port or socket in IP. The

NSEL is 1 byte long. It is not used in routing decisions. When the NSEL is set to 00,

the address identiﬁes the device itself—its network level address. In this case, the

NSAP is known as a network entity title (NET).

• The HODSP, system ID, and NSEL together make up the domain-speciﬁc part (DSP)

of the NSAP address.

IS-IS Versus ISO-IGRP NSAPs

IS-IS and ISO-IGRP interpret NSAPs differently, as shown in Figure S-7.

Figure S-7 How IS-IS and ISO-IGRP Interpret NSAP Addresses

IS-IS uses a two-layer architecture, joining the IDP and HODSP ﬁelds and treating them as

its area address (Level 2), with the remaining system ID used for Level 1 routing. When

used with IS-IS, therefore, the NSAP is divided into three parts, as shown in Figure S-7: 1

octet for NSEL, 6 octets for the system ID, and from 1 to 13 octets for the area address or

area ID ﬁeld. An NSAP has a variable length of 8 to 20 octets. It is usually longer than 8

bytes to permit some granularity in the allocation of areas.

IDP DSP

AFI IDI High Order DSP System ID NSEL

Variable Length Area Adress 6 Bytes 1 Byte

Area-ID or Area Address

Min 1 Byte, Max 13 Bytes

System ID

6 Bytes

NSEL

1 Byte

Domain

Min 1 Byte,

Max 11

Area-ID or Area Address

2 Bytes

System ID

6 Bytes

Unused

1 Byte

IS-IS

ISO-IGRP

2283newchapf.fm Page 17 Monday, December 16, 2002 12:58 PM

Operation of IS-IS for CLNS/CLNP 18

NOTE Figure S-7 shows ﬁeld length in bytes (or octets), where the NSAP address usually is

represented in hexadecimal. NSAP addresses, like ATM addresses, are 160 bits long:

ISO-IGRP routes are based on a three-level architecture: domain (using the AFI and IDI

ﬁelds for Level 3), area (using the HODSP ﬁeld for Level 2), and system ID (for Level 1).

What IS-IS treats simply as the area ID, ISO-IGRP splits into a domain and an area. ISO-

IGRP sets the 2 bytes to the left of the system ID as the area ID or area address ﬁeld,

allowing for a theoretical 65,535 areas in an ISO-IGRP network. Everything else (a

maximum of 11 bytes) is treated as a domain ID. Therefore, the minimum length for an

ISO-IGRP NSAP is 10 bytes (1-byte NSEL, 6-byte system ID, 2-byte area, and minimum

1-byte domain).

ISO-IGRP sends routing information based on domain (variable length), area (length ﬁxed

by the protocol at 2 bytes), and ﬁnally system ID (ﬁxed at 6 bytes). The NSEL is not used

by ISO-IGRP.

Network Entity Title

As discussed earlier, if the NSEL ﬁeld is 00, the NSAP refers to the device itself—that is,

it is the equivalent of the Layer 3 OSI address of that device.

This address with the NSEL set to 00 is known as the NET. The NET is used by routers to

identify themselves in the LSPs. Therefore, it forms the basis for the OSI routing

calculation.

NET Is NSAP with NSEL = 00

A key point is that an NSAP address in which the NSEL is set to 00 is called the NET.

NETs and NSAPs are speciﬁed in all hexadecimal digits and must start and end on a byte

boundary.

Ofﬁcial NSAP preﬁxes are required for CLNS routing. Addresses starting with the value of

49 (AFI = 49) are considered to be private addresses (analogous to RFC 1918 for IP

addresses). These addresses are routed by IS-IS; however, this group of addresses should

not be advertised to other CLNS networks.

40 Hex Digits ×

4 bits

Hex Digits

8 bits

octets

= 20 octets ×

= 160 bits

2283newchapf.fm Page 18 Monday, December 16, 2002 12:58 PM

19 Chapter : Conﬁguring IS-IS Protocol

Addresses starting with AFI values 39 and 47 represent the ISO data country code and ISO

international code designator, respectively.

As shown in Figure S-7 and the examples in the next section, the Cisco IOS Software IS-

IS routing process interprets the NSAP address as follows (from the right, or least-

signiﬁcant digit, end):

• The last byte is the NSEL ﬁeld and must be speciﬁed as a single byte, with two hex

digits, preceded by a period (.). In a NET, this N-selector ﬁeld is set to (00).

• The preceding 6 bytes (this length is ﬁxed by Cisco IOS Software) are the system ID.

It is customary to use either a MAC address from the router or (for Integrated IS-IS)

an IP address (for example, the IP address of a loopback interface) as part of the

system ID.

• The IS-IS routing process of Cisco IOS Software treats the rest of the address as the

area ID, or area address, as follows:

— It is 1 to 13 bytes long.

— Using a 1-byte ﬁeld for area limits the scope for area deﬁnitions. Thus, the

customary area ID consists of 3 bytes, with an AFI of 1 byte and 2 additional

bytes for the area ID. For example, in the address

49.0001.0000.0c12.3456.00, the AFI is 49 and the additional 2 bytes are

0001, for an effective area ID of 49.0001.

— Cisco IOS Software attempts to summarize the area ID as much as possible.

For example, if an IS-IS network is organized with major areas subdivided

into minor areas, and this is reﬂected in the area ID assignments, then:

Between the minor areas, Cisco IOS Software will route based on the

whole area ID.

Between the major areas, Cisco IOS Software will summarize the area

ID portion up to the major area boundary.

NSAP Examples

The following examples illustrate how an NSAP address is interpreted by IS-IS and ISO-IGRP.

The NSAP 49.0001.aaaa.bbbb.cccc.00 consists of the following:

• For IS-IS:

— Area = 49.0001

— System ID = aaaa.bbbb.cccc

— N-selector = 00

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