Saeed Darvish Pazoki – MCSE, CCNA Abstracted From: Cisco Press – ICND 2 – 5 VLSM and Route Summarization 1

VLSM VLSM occurs when an internetwork uses more than one mask in different subnets of a single Class A, B, or C network. VLSM allows engineers to reduce the number of wasted IP addresses in each subnet, allowing more subnets and avoiding having to obtain another registered IP network number from regional IP address assignment authorities. A common mistake occurs when people think that VLSM means “using more than one mask,” rather than “using more than one mask in a single classful network.” 2

VLSM and Route Summarization VLSM 3

VLSM and Route Summarization Classless and Classful Routing Protocols For a routing protocol to support VLSM, the routing protocol must advertise not only the subnet number but also the subnet mask when advertising routes. Additionally, a routing protocol must include subnet masks in its routing updates to support manual route summarization. Each IP routing protocol is considered to be either classless or classful, based on whether the routing protocol does (classless) or does not (classful) send the mask in routing updates. No commands exist to enable or disable whether a particular routing protocol is a classless or classful routing protocol. 4

VLSM and Route Summarization Classless and Classful Routing Protocols 5

VLSM and Route Summarization Overlapping VLSM Subnets The subnets chosen to be used in any IP internetwork design must not overlap their address ranges. With a single subnet mask in a network, the overlaps are somewhat obvious; however, with VLSM, the overlapping subnets might not be as obvious. 6

VLSM and Route Summarization VLSM Configuration Routers do not enable or disable VLSM as a configuration feature. From a configuration perspective, VLSM is simply a side effect of the ip address interface subcommand. Routers configure VLSM by virtue of having at least two router interfaces, on the same router or among all routers in the internetwork, with IP addresses in the same classful network but with different masks. 7

VLSM and Route Summarization Manual Route Summarization Small networks might have only a few dozen routes in their routers’ routing tables. The larger the network, the larger the number of routes. In fact, Internet routers have more than 100,000 routes in some cases. As routing tables grow, they consume more memory in a router. Route summarization causes some number of more-specific routes to be replaced with a single route that includes all the IP addresses covered by the subnets in the original routes. Route summarization 1. Reduces the size of routing tables while maintaining routes to all the destinations in the network. 2. Improves convergence time, because the router that summarizes the route no longer has to announce any changes to the status of the individual subnets. The term manual refers to the fact that manual route summarization only occurs when an engineer configures one or more commands. Auto- summarization occurs automatically without a specific configuration command. 8

VLSM and Route Summarization Manual Route Summarization 9 Routing table with Auto-Summary enabled

VLSM and Route Summarization Manual Route Summarization 10 Routing table with Manual-Summary enabled

VLSM and Route Summarization Route Summarization Strategies Manual route summarization works best when the network engineer plans his choice of subnet numbers anticipating route summarization. 11

VLSM and Route Summarization Route Summarization Strategies The following list describes a generalized binary process by which you can find a best summary route for a group of subnets: 12

VLSM and Route Summarization Route Summarization Strategies 13

VLSM and Route Summarization Auto-summarization Auto-summarization can be characterized as follows: When advertised on an interface whose IP address is not in network X, routes related to subnets in network X are summarized and advertised as one route. That route is for the entire Class A, B, or C network X. 14

VLSM and Route Summarization Discontiguous Classful Networks Autosummarization does not cause any problems as long as the summarized network is contiguous rather than discontiguous. To better understand what the terms contiguous and discontiguous mean in networking, refer to the following two formal definitions when reviewing the example of a discontiguous classful network that follows: Contiguous network: A classful network in which packets sent between every pair of subnets can pass only through subnets of that same classful network, without having to pass through subnets of any other classful network. Discontiguous network: A classful network in which packets sent between at least one pair of subnets must pass through subnets of a different classful network. 15

VLSM and Route Summarization Discontiguous Classful Networks 16

VLSM and Route Summarization Discontiguous Classful Networks 17

VLSM and Route Summarization Auto-summarization Support and Configuration Classful routing protocols must use auto-summarization. Some classless routing protocols support auto-summarization, defaulting to use it, but with the ability to disable it with the no auto- summary router subcommand. Other classless routing protocols, notably Open Shortest Path First (OSPF), simply do not support auto-summarization. 18