Route Summarization with Alternate Paths

Route summarization can be a good thing when done correctly. By summarizing a group of specific routes into one larger summarized (aggregate) route you can greatly improve the stability of the internetwork routing protocol. When a router advertises a summary it hides link failures and instabilities behind the summary from being seen by the rest of the routers in the network receiving the summary. This hiding behavior prevents uneccessary routing protocol convergence and keeps the internetwork stable if a routing loop or flapping link is affecting a peripheral area of the network. However, as with most good things, there are trade-offs. When we use summarization we are trading network stability for routing accuracy. If done incorrectly, route summarization can prevent a network from converging properly, create black holes, and cause sub-optimal routing conditions. Configuring route summarization can get particularly tricky where there are one or more alternate paths to a summarized destination.

Here are two common mistakes when implementing route summarization with alternate paths:

  1. Using alternate paths to bypass a point of summarization.
  2. Specfic routes leaking out of the summarized area from an alternate path.

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