Phase 2 is the addition of high-speed backbone technology and routing between switches...

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LAN
switches perform switch processing and provide dedicated bandwidth to the desktop and to
shared-media hubs. Backbone routers are attached to either Fast Ethernet or ATM switches. The
increase in backbone bandwidth matches the increase bandwidth in the wiring closet. Figure 2-25
shows an example of how you can add high-speed backbone technology and routing between
existing switches in your network.
Figure 2-25
Adding high-speed backbone technology and routing between switches.
LAN switch
ATM campus
switch
High-speed
Router
core switch
In Phase 3, routers are distributed between the LAN switches in the wiring closet and the high-speed core switch. The network backbone is now strictly a high-speed transport mechanism with all other devices, such as the distributed routers, at the periphery. Figure 2-26 illustrates such a network.
Figure 2-26
Distributing routers between high-speed core and LAN switches.
LAN switch
High-speed
switch
2-40
Cisco CCIE Fundamentals: Network Design
Summary
Phase 4 is the final phase—the end point. It involves end-to-end switching with integral VLANs and multilayer switching capability. By this point, Layer 2 and Layer 3 integrated switching is distributed across the network and is connected to the high-speed core. Figure 2-27 shows an example of this
final phase.
Figure 2-27
End-to-end switching with VLAN and multilayer switching capability.
Si
Si
LAN switch
High-speed
Router
core switch
Summary
Now that the basic internetworking devices and general design principles have been examined, the
remaining chapters in this part focus on the different technologies available when designing an
internetwork.
Internetworking Design Basics 2-41
Summary
2-42
Cisco CCIE Fundamentals: Network Design
C H A P T E R
3
Designing Large-Scale
IP Internetworks
This chapter focuses on the following design implications of the Enhanced Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF) protocols, and the Border Gateway Protocol
(BGP):
• Network Topology
• Addressing and Route Summarization
• Route Selection
• Convergence
• Network Scalability
• Security
Enhanced IGRP, OSPF, and BGP are routing protocols for the Internet Protocol (IP). An
introductory discussion outlines general routing protocol issues; subsequent discussions focus on design guidelines for the specific IP protocols.
Implementing Routing Protocols
The following discussion provides an overview of the key decisions you must make when selecting
and deploying routing protocols. This discussion lays the foundation for subsequent discussions
regarding specific routing protocols.
Network Topology
The physical topology of an internetwork is described by the complete set of routers and the
networks that connect them. Networks also have a logical topology. Different routing protocols
establish the logical topology in different ways.
Some routing protocols do not use a logical hierarchy. Such protocols use addressing to segregate specific areas or domains within a given internetworking environment and to establish a logical
topology. For such nonhierarchical, or flat, protocols, no manual topology creation is required.
Other protocols require the creation of an explicit hierarchical topology through establishment of a backbone and logical areas. The OSPF and Intermediate System-to-Intermediate System (IS-IS)
protocols are examples of routing protocols that use a hierarchical structure. A general hierarchical network scheme is illustrated in Figure 3-1. The explicit topology in a hierarchical scheme takes precedence over the topology created through addressing.
Designing Large-Scale IP Internetworks 3-1
Implementing Routing Protocols
Figure 3-1
Hierarchical network.
Backbone
Router
Router
Router
Area 1
Area 2
Area 3
If a hierarchical routing protocol is used, the addressing topology should be assigned to reflect the hierarchy. If a flat routing protocol is used, the addressing implicitly creates the topology. There are two recommended ways to assign addresses in a hierarchical network. The simplest way is to give
each area (including the backbone) a unique network address. An alternative is to assign address
ranges to each area.
Areas are logical collections of contiguous networks and hosts. Areas also include all the routers having interfaces on any one of the included networks. Each area runs a separate copy of the basic routing algorithm. Therefore, each area has its own topological database.
Addressing and Route Summarization
Route summarization procedures condense routing information. Without summarization, each
router in a network must retain a route to every subnet in the network. With summarization, routers can reduce some sets of routes to a single advertisement, reducing both the load on the router and the perceived complexity of the network. The importance of route summarization increases with
network size.
Figure 3-2 illustrates an example of route summarization. In this environment, Router R2 maintains one route for all destination networks beginning with B, and Router R4 maintains one route for all destination networks beginning with A. This is the essence of route summarization. Router R1 tracks all routes because it exists on the boundary between A and B.
3-2
Internetwork Design Guide
Addressing and Route Summarization
Figure 3-2