Upon completing this lesson, you will be able to: Explain the operation of variable-length subnet masks on Cisco routers | © 2002, Cisco Systems, Inc. All rights reserved. © 2002, Cisco Systems, Inc. All rights reserved. Variable-Length Subnet Masks Purpose: This chapter introduces the Cisco IOS™ CLI on the Catalyst® 1900 switch and router. Timing: This chapter should take about 2 hours to present. Note: The Catalyst 1900 switch only has a subset of the router Cisco IOS commands available. Contents: Introduction to Cisco IOS. Explain to the student what is IOS? Cisco Device startup procedures in general. IOS configuration source. General introduction to the IOS CLI. Cat 1900 switch startup procedures. Intro to Cat 1900 CLI. This part covers the basic configuration on the switch, like setting the IP address and hostname. More details about the various Cat 1900 switch configuration commands are explained in Chapter 6 and 7. Router startup procedures. More details on the router startup process is discussed in chapter 5. Router IOS CLI. Objectives Upon completing this lesson, you will be able to: Explain the operation of variable-length subnet masks on Cisco routers Slide 1 of 2 Purpose: This slide states the chapter objectives. Emphasize: Read or state each objective so that each student has a clear understanding of the chapter objectives. Note: Catalyst switches have different CLIs. The Catalyst 2900xl and the Catalyst 1900 has a Cisco IOS CLI. The Cisco IOS CLI commands available on the 2900xl is different from the 1900. The Catalyst 5000 family has no Cisco IOS CLI, and use the set commands instead. This class only covers the configuration on the Catalyst 1900 switch. What Is a Variable-Length Subnet Mask? Subnet is divided into smaller subnets: Subnet with one mask (/27) Then further subnet one of the unused /27 subnets into multiple /30 subnets Calculating VLSMs A Working VLSM Example What Is Route Summarization? Routing protocols can summarize addresses of several networks into one address Summarizing Within an Octet Summarizing Addresses in a VLSM-Designed Network Implementation Considerations Multiple IP addresses must have the same highest-order bits. Routing decisions are made based on the entire address. Routing protocols must carry the prefix (subnet mask) length. Route Summarization Operation in Cisco Routers Supports host-specific routes, blocks of networks, default routes Routers use the longest match /32 Host /27 Subnet /24 Network /16 Block of Networks /0 Default Summarizing Routes in a Discontiguous Network RIPv1 and IGRP do not advertise subnets, and therefore cannot support discontiguous subnets. OSPF, EIGRP, and RIPv2 can advertise subnets, and therefore can support discontiguous subnets. Summary When an IP network is assigned more than one subnet mask, it is considered a network with variable-length subnet masks, overcoming the limitation of a fixed number of fixed-size subnetworks imposed by a single subnet mask. In large internetworks, hundreds or even thousands of network addresses can exist. In these environments, it is often not desirable for routers to maintain many routes in their routing table. Route summarization, also called route aggregation or supernetting, can reduce the number of routes that a router must maintain by representing a series of network numbers in a single summary address. Purpose: This slide discuss the initial configurations on the routers and switches. Note: There is no setup mode on the Catalyst 1900 switch.
