EIGRP到OSPF迁移实战指南：轻松迈向开放路由标准

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《日志一天：从EIGRP迁移到OSPF》是一本专为想要深入研究Juniper网络的学生编写的实用指南，它属于Juniper Networks Day One书籍系列，旨在帮助读者在最短的时间内提升网络技术水平，过渡到开放路由标准。该书由经验丰富的专家Jack W. Parks IV撰写，专注于指导读者逐步执行从传统的EIGRP协议向OSPF协议的迁移过程。在生产网络中更换内部网关协议（IGP），如从EIGRP迁移到OSPF，可能会被视作一项艰巨的任务。然而，通过精心的前期规划和系统实施策略，这种迁移可以顺利进行，不会造成意外中断。本书详细阐述了如何进行这样的迁移，强调了成功的关键要素，如选择OSPF的原因，因为它在企业、政府和教育网络中广泛应用，因其卓越的工程知识、设备兼容性和网络规模适应性而备受青睐。 OSPF作为当今最流行的IGP，其优势在于它的灵活性和可靠性。在书中，作者不仅会介绍如何分析现有网络架构，还会提供实用步骤，包括但不限于： 1. **评估网络现状**：了解当前EIGRP的配置、拓扑和性能，识别可能的迁移难点。 2. **规划迁移策略**：设计一个分阶段的迁移计划，确保平稳过渡期间的业务连续性。 3. **技术准备**：学习和理解OSPF的基本原理、路由计算机制以及邻居关系管理。 4. **配置调整**：逐步更新路由器配置，从区域划分、路由汇总到动态路由协议的设置。 5. **测试与验证**：在模拟环境或隔离区域进行迁移测试，确认新协议的正确运作。 6. **监控和优化**：实施后持续监控，根据实际性能数据调整和优化路由策略。通过阅读这本书，读者将获得实际操作中的宝贵经验和最佳实践，以确保从EIGRP到OSPF的迁移项目顺利完成，并为未来的网络管理奠定坚实的基础。同时，利用Day One网站提供的完整资源库，读者可以进一步深化对各种网络技术的理解，提升专业技能。

12 Day One: Migrating EIGRP to OSPF

Table 1.3 OSPF LSA Matrix

LSA Type Name Description Flooding Scope

Type-1 Router LSA

The router originates a Type-1 LSA to describe

all the interfaces attached to that router.

(*except those that have elected a DR/DBR)

Intra Area

Type-2 Network LSA

The router originates a Type-2 LSA to describe

all the NBMA and broadcast networks attached

to it. Type-2 LSAs have a listing of all routers

attached to these speciﬁc networks.

Intra-Area

Type-3 Summary LSA

A router that exists on the boundary between

two areas converts the Type-1 & 2 LSAs into a

summary LSA. This summary transitions area

boundaries and can be shared with other areas.

Inter-Area

Type-4 Summary LSA

A Type-4 LSA is a special summary that describes

routers in the OSPF autonomous system that has

routes external to the OSPF. It only describes the

router not the routes

Inter-Area

Type-5 External LSA

Routers that have routing knowledge beyond the

OSPF autonomous system share this routing

information with a Type-5 LSA

Inter-Area

NOTE There are additional LSA types but their scope is outside this book.

Link State Database

After the routers form adjacencies using hello packets and share

routing information between one another with LSA packets, there

needs to be a place to store all of that information. That place is the

link state database (LSDB) and its function is to keep the collective

network knowledge of all OSPF routers. The SPF algorithm runs

against the information contained in the LSDB and the routing table is

created.

NOTE The LSDB is important during the network validation phase of the

EIGRP to OSPF migration in this book.

Chapter 1: Network Preparation 13

Areas

While this book doesn’t demonstrate the use of multiple OSPF areas,

understanding how areas help the network scale is important for you

to know. Most Enterprise networks only need to use a single area.

Simply stated, OSPF areas are used to partition the network into

smaller pieces. Partitioning limits the scope of the routing information

contained within each area, which in turn allows OSPF to scale in most

massively large networks. (The computing power of routers has grown

exponentially over the past few years and Dijkstra is less of a burden

on the CPU than was in the past; still, scaling is critical.)

TIP The area number is actually represented in dotted decimal notation,

but it is common to use the shorthand representation. Area 0 is

actually area 0.0.0.0

There may be situations in Enterprise networks where hub-and-spoke

Wide Area Network (WAN) aggregation points could warrant the use

of a dedicated area, or areas, to limit the propagation of topology

information over low-speed links. (Branch routers and Small Ofﬁce

Home Ofﬁce (SOHO) routers have limited scaling capacity and

minimal computing power.) The spokes on a WAN network are by

design a one-way in and out stub network so remote stub networks

warrant little more than the propagation of a single default route

rather than the entire network topology. But the ﬁnite resources of

smaller routers, the simplistic topology of remote networks, and the

centralized aggregation of branch ofﬁces often become prime candi-

dates for the use of OSPF areas.

This book uses a single area during the migration. There is a speciﬁc

rule, however, that must be adhered to when working with multiple

OSPF areas: there is but a single backbone area in OSPF and that area

is area 0. Multi-area deployments must use area 0 and area 0 may not

be segmented in a discontiguous fashion in the network. As the term

backbone implies, all other areas must connect to and transit area 0.

Two areas may only connect through area 0.

Different Area types to be aware of are also listed in Table 1.4. Differ-

ent area types affect the ﬂooding scope of the LSAs and subsequently

affect the routing tables of the routes themselves.

Chapter 1: Network Preparation 15

proprietary nature of EIGRP that decreases its business value to the

network since integrating best-of-breed or more cost-effective vendors

is limited.

NOTE In all fairness, EIGRP is a multiprotocol IGP supporting legacy

protocols such as AppleTalk and IPX. Unfortunately, both Novell and

Apple now rely on IP as the network transport so the multiprotocol

capabilities of EIGRP are more of a liability than a beneﬁt.

Common Characteristics

EIGRP and OSPF share common characteristics with each other even

though EIGRP at its core is a Distance Vector protocol and OSPF is a

Link-State protocol. Under the covers, each protocol is fundamentally

different from the other, but the effect of both protocols is similar for

the distribution of routing information throughout the network. This

section focuses on the similarities between EIGRP and OSPF in order

to bridge your existing understanding of protocol operation while

highlighting any signiﬁcant differences between the two.

NOTE EIGRP has been referred to as a hybrid IGP that contains Distance Vec-

tor protocol and Link States protocol properties. This is not accurate.

Link State protocols use a topology database to derive reachability

information. EIGRP relies on vector calculations to build routing

information. Since no topology database exists, EIGRP cannot be or

contain Link State information.

Classless Interdomain Routing

Classless Interdomain Routing (CIDR) is the ability of a routing

protocol to support variable length subnet masks (VLSM). Older IGPs

did not support this functionality and required the use of classful

subnetting on the speciﬁc bit boundaries. CIDR is the current method

for subnetting and allows preﬁx lengths like /30, /18, and /15, regard-

less of the leading bits of the IP address. Before CIDR, subnetting was

ﬁxed and based on the “class” of the IP address range. Because of the

ﬁxed preﬁx length requirement, point-to-point WAN links had to use

/24’s at a minimum! Table 1.5, The Classful Address Table, is shown

here for a little trip back in history.

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