How to Keep Your Routing Tables Manageable

by Tony DeSimone

One of the keys to effective routing is keeping the routing tables at a manageable size. A large organization could potentially have a triple-digit number of routes in its routing tables which significantly adds to its routers’ overhead – cpu, ram, bandwidth, etc. The primary tool in the fight against enormous routing tables is route summarization, but proper design will make the most of your summarization.

The these five keys to keep your routing tables small:

  • Assign IP subnets of a specific geographic location in a contiguous fashion. Randomly assigned subnets may require too many summarizations and thereby, defeat the purpose.
  • Understand how a routing protocol like EIGRP (Enhanced Interior Gateway Routing Protocol) may possibly do automatic route summarization on classful boundaries with the auto-summary command.
  • Keep in mind OSPF (Open Shortest Path First) requires a multiple area design and summarization can only be configured between OSPF areas or between OSPF and another autonomous system. Huge routing tables can be an indicator you have outgrown your single OSPF area.
  • Use route filtering with distribute lists, prefix lists, and route maps to control where your routing protocol advertises routes.
  • Determine locations in your network where default routes could be used in place of having many specific routes. For example, use default routes at remote locations to get to your core. OSPF stub areas are an easy way to implement default routes.

Training Resources:
Implementing Cisco Enterprise Advanced Routing and Services (ENARSI)

 

 

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