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Chapter 1

Scaling Large Internetworks

Adaptability

Another important goal for an administrator is to design an internetwork
that responds well to change. To achieve this goal, internetworks need to be
able to

Pass both routable and nonroutable network protocols Examples
would be TCP/IP, which is routable, and Microsoft's NetBEUI (NetBIOS
Extended User Interface), which is not routable, only bridgeable.

Create islands of networks using different protocols This allows you to
add protocols used by the network islands to Core layer routers or use
tunneling in the backbone to connect the islands, which keeps you from
having to add unwanted protocols to the core backbone.

Balance between multiple protocols in a network Each protocol has
different requirements, and the internetwork must be able to accommo-
date the specific issues of each one.

The Cisco IOS also has many different features that contribute to network

adaptability:

EIGRP Cisco's proprietary EIGRP allows you to use multiple protocols
within one routing algorithm. EIGRP supports IP, IPX, and AppleTalk.

Redistribution Allows you to exchange routing information between
networks that use different routing protocols. For example, you can
update a routing table from a network running IGRP on a router partic-
ipating in an RIP network.

Accessibility and Security

Access routers must be both accessed and used to access a variety of WAN
services, while maintaining security to keep hackers out.

The Cisco IOS features that support these requirements are as follows:

Dedicated and switched WAN support You can create a direct connec-
tion with Cisco routers using basic or digital services (a T1, for example).
Cisco routers also support many different switched services, such as
Frame Relay, SMDS, X.25, and ATM, to give you options to meet cost,
location, and traffic requirements.

Exterior protocol support Both Exterior Gateway Protocol (EGP) and
Border Gateway Protocol (BGP) are supported by the Cisco IOS. BGP
(discussed in detail in Chapters 7 through 9) is used primarily by Internet
Service Providers (ISPs) and has mostly replaced EGP.

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