![]() ![]() Route summarization is most effective within a subnetted environment when the network addresses are in contiguous blocks in powers of 2. Route summarization is possible only when a proper addressing plan is in place. Therefore, router E does not need to continually modify its routing table as a result of this flapping activity.įlapping is a common term used to describe intermittent interface or link failures. For example, in Figure 1-16, if a specific link (such as 172.16.13.0/24) is flapping (going up and down rapidly), the summary route (172.16.12.0/22) does not change. However, if there were other subnets of 172.16.12.0/22 elsewhere in the network (for example, if 172.16.12.0 were discontiguous), summarizing in this way might not be valid.Īnother advantage of using route summarization in a large, complex network is that it can isolate topology changes from other routers. Router D in Figure 1-16 is advertising that it can route to network 172.16.12.0/22, including all subnets of that network. Router E also saves CPU resources, because it evaluates packets against fewer entries in its routing table.įigure 1-16 Routers Can Summarize to Reduce the Number of RoutesĪ summary route is announced by the summarizing router as long as at least one specific route in its routing table matches the summary route.Router E needs to maintain only one route and therefore saves memory.Bandwidth is saved on the link between routers D and E.If router D summarizes the information into a single network number entry, the following things happen: Route summarization (also called route aggregation or supernetting) can reduce the number of routes that a router must maintain, because it is a method of representing a series of network numbers in a single summary address.įor example, in Figure 1-16, router D can either send four routing update entries or summarize the four addresses into a single network number. It is often problematic for routers to maintain this volume of routes in their routing tables. In large internetworks, hundreds, or even thousands, of network addresses can exist. With VLSM, you break a block of addresses into smaller subnets in route summarization, a group of subnets is rolled up into a summarized routing table entry. This section describes summarization CIDR is covered in the later section "Classless Interdomain Routing." The relationship between summarization and VLSM is also examined. With a thorough understanding of route summarization and CIDR, you can implement a scalable network. Route summarization and CIDR techniques can manage this corporate growth much like Internet growth has been managed. This growth taxes CPU resources, memory, and bandwidth used to maintain the routing table. As the result of corporate expansion and mergers, the number of subnets and network addresses in routing tables is increasing rapidly. ![]()
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