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Icnd23 s03l07

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  • 1. Determining IP Routes Implementing Variable-Length Subnet Masks © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-1
  • 2. Outline • Overview • VLSM Benefits • VLSM Calculations • Route Summarization with VLSM • Route Summarization Implementation Considerations • Route Summarization Management • Summary © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-2
  • 3. What Is a Variable-Length Subnet Mask? • Subnet 172.16.14.0/24 is divided into smaller subnets – Subnet with one mask (/27) – Then further subnet one of the unused /27 subnets into multiple /30 subnets © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-3
  • 4. Calculating VLSMs © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-4
  • 5. A Working VLSM Example © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-5
  • 6. What Is Route Summarization? • Routing protocols can summarize addresses of several networks into one address. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-6
  • 7. Summarizing Within an Octet © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-7
  • 8. Summarizing Addresses in a VLSM-Designed Network © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-8
  • 9. 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. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-9
  • 10. Route Summarization Operation in Cisco Routers 192.16.5.33 192.16.5.32 192.16.5.0 192.16.0.0 0.0.0.0 /32 /27 /24 /16 /0 Host Subnet Network Block of Networks Default • Supports host-specific routes, blocks of networks, and default routes • Routers use longest match © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-10
  • 11. 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. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-11
  • 12. Summary • VLSMs provide more efficient use of IP addresses and have greater capability to use route summarization. • VLSMs can provide more network addresses and fewer hosts per network. • Route summarization enables a router to summarize several addresses into a single network number. • Route summarization reduces memory use on routers and routing protocol network traffic. • Cisco routers manage route summarization by performing automatic summarization and by selecting routes from route summaries. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-12
  • 13. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-13