Hierarchical routing is the most efficient basis for large scale network designs because it:

• Breaks one large problem into several smaller problems that can be solved separately

• Reduces the size of the area through which topology change information must be propagated

• Reduces the amount of information routers must store and process

• Provides natural points of route summarization and traffic aggregation

The three layers of a hierarchical network design are described in Table 1-1.

Table 1-1. Summary of Goals and Strategies of Layers and Hierarchical

Network Design

Table 1-1. Summary of Goals and Strategies of Layers and Hierarchical

Network Design





Switching speed

Full reachability:

No default routes to internal destinations and reduction of suboptimal routing

No policy implementation:

Access control, no policy routing, and reduction of processor and memory overhead


Topology change isolation

Controlling the routing table size

Route summarization:

Provides topology change isolation, hides detail from the network core, and hides detail from access layer devices

Traffic aggregation

Minimizing core interconnections:

Reduces switching decision complexity and provides natural summarization and aggregation points


Feed traffic into the network

Control access

Preventing through traffic Packet level filtering

Other edge services include flagging packets for QoS and tunnel termination

So when should you begin considering the hierarchy of your network? Now. It's important to impose hierarchy on a network in the beginning when it's small. The larger a network grows, the more difficult it is to change. Careful planning now can save many hours of correctional work later.

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