Summary
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.
| Layers | Goals | Strategies |
|---|---|---|
| Core | 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 |
| Distribution | Topology change isolation
Controlling the routing table size Traffic aggregation | Route summarization:
Provides topology change isolation, hides detail from the network core, and hides detail from access layer devices Minimizing core interconnections: Reduces switching decision complexity and provides natural summarization and aggregation points |
| Access | 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.