Summarization, in general, provides many benefits to your network and its operation; some of these benefits are apparent and others have a smaller impact on the health of network. There are four primary reasons to implement proper summarization:
• Shrink the routing table—This benefit should be clear to you after the preceding discussion. The use of summarization allows a smaller number of route entries in your table, with the added benefits of decreasing the amount of memory used. Specifically, it is much faster for a router to look up a summarized single route to find a match than it is to look up smaller /24 routes, for example.
• Improve router operation—Using summarization means fewer routes in the routing table and thus fewer times that the router needs to run the SPF algorithm. Additionally, summarization of routes gives you a smaller link-state database, which also speeds things considerably.
• Reduce routing updates—In this case, when you are advertising a summary, say /22, and a /24 within that, /22 begins to flap. Other routers do not see the routing updates because the summary route, in essence, hides these changes in your network, thus reducing routing updates.
This benefit assumes that flapping is not a normal occurrence in a network; in general, that is true. Enter the depends rule! Dial-up interfaces, where users are accessing the network through modems, flap as part of their operation, with calls connecting and disconnecting constantly. By summarizing the networks, you in effect isolate all the flapping, whether it is as a result of an issue or by design. The disadvantage here is that the network might be down, and the other routers keep sending data to the router that is advertising the summarized network.
• Provide troubleshooting—Troubleshooting is a benefit of summarization. For example, if you summarize parts of your network either based on geography (for example, England) or perhaps on function (for example, dial-up), when a routing problem occurs or is suspected, you can easily track down the issue instead of dealing with many smaller subnets.
Are you now convinced that summarization is an excellent solution and something you cannot live without? Well, not so fast—there is a hidden cost to summarization that you must be aware of. Consider for a moment that you are summarizing based on geographical location. Therefore, you choose to summarize England. This is handy to do because it is an island, and links to it from any continent can get pricey; you definitely want the benefit of summarization. This results in a simplified routing table; however, the subnet that is associated with everyone in Scotland goes down, and that change does not leave the England network. There are two ways to get into England's network. One way is shorter to Scotland, the other longer. If you summarize, you might take the less-optimal entry point into England's network, which means that it takes longer to get to Scotland. This is a weakness of summarization and aggregation—loss of routing information. If both routes were still advertised, there would be a difference in OSPF metric, so the shortest path would be followed.
Therefore, you experience suboptimal routing because your packets had to go all the way to the destination to discover that it was down. Suboptimal routing is taking a longer-than-necessary path to a reachable destination. So, like many things in networking, a trade-off exists with regard to summarization. Being aware of this trade-off upfront allows you to understand and better deploy summarization effectively.
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