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Chapter 1

Scaling Large Internetworks

Segmentation with a Router

As you know, routers work at the Network layer and are used to route pack-
ets to destination networks. Routers use routing tables to make routing deci-
sions. However, in the routing tables, routers keep information on how to
get to networks in their tables, not to hosts, using that information to route
packets through an internetwork. Routers use logical network addresses
instead of hardware addresses when making their routing decisions. They
maintain a routing table for each protocol on the network--a Cisco router
will keep a routing table for AppleTalk, a different one for IPX, and still
another for IP, as shown in Figure 1.2.

F I G U R E 1 . 2

Routing tables are kept for each Network layer routing protocol.

Here are the pros regarding routers:

Manageability

Multiple routing protocols give the network manager

who's creating an internetwork a lot of flexibility.

Increased functionality

Cisco routers provide features that address the

issues of flow, error and congestion control, fragmentation, reassembly,
and control over a packet's lifetime.

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