Chapter 3: Developing an Optimum Design for Layer 3 (Part07)
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Summary

Add a note here In summary, we’ve looked at elements of advanced routing design, and we also touched on the merits of a well-planned IP addressing scheme. The IP addressing scheme is the foundation for greater efficiency in operating and maintaining a network. Without proper planning in advance, networks might not be able to benefit from route summarization features inherent to many routing protocols.

Add a note hereThe general advanced routing design discussion can be encapsulated in the following key points that were discussed previously:

  • Add a note hereRoute summarization and default routing are important in scaling routing designs.

  • Add a note hereRoute filtering can be used to manage traffic flows in the network, avoiding inappropriate transit traffic and as a defense against inappropriate routing updates.

  • Add a note hereRedistribution can be useful for manipulating and managing routing updates, but needs to be designed properly to prevent routing loops or other problems.

Add a note hereEIGRP converges quickly as long as it has a feasible successor. With no feasible successor, EIGRP sends queries out to its neighbors. To limit the scope of these queries, use route summarization and filtering. By limiting EIGRP query scope, you can speed up EIGRP convergence and increase stability. In addition, large numbers of neighbors should be avoided for any one router. Multiple autonomous systems may be used with EIGRP providing that you understand that they do not directly limit EIGRP query scope. You would use them to support migration strategies, different administrative groups, or very large network design.

Add a note hereOSPF scaling depends on summarization and controlling how much LSA flooding is needed. Simple, stub, summarized designs scale most effectively. Several techniques speed up convergence for OSPF, including fast hellos, iSPF, and BFD.

Add a note hereFinally, IBGP requires a full mesh of all IBGP routers, but full-mesh peering does not scale gracefully. Route reflectors pass along routing information to and from their clients. The route reflector clients are relieved of the burden of most IBGP peering. Confederations allow an autonomous system to be divided into sub-autonomous systems, where the sub-autonomous system border routers peer with each other and then pass along routes on behalf of the other sub-autonomous system routers. Confederation sequences are used to prevent information loops. Sub-autonomous systems can have different BGP polices from each other.

Add a note hereKey points to remember include the following:

  • Add a note hereIP address design allows for route summarization that supports network scaling, stability, and fast convergence.

  • Add a note hereRoute summarization, route filtering, and appropriate redistribution help minimize routing information in the network.

  • Add a note hereEIGRP converges quickly as long as it has a feasible successor. Multiple autonomous systems with EIGRP may be used, with care, to support special situations, including migration strategies and very large network design.

  • Add a note here Simple, stub, summarized OSPF designs scale most effectively. Several techniques speed up convergence for OSPF, including fast hellos, iSPF, and BFD.

  • Add a note hereIBGP designs can be scaled using route reflectors to pass routing information to and from their clients and confederations to allow an autonomous system to be divided into sub-autonomous systems.


References

  • Add a note hereCisco Systems, Inc. “Designing Large-Scale IP Internetworks,” at http://www.cisco.com/en/US/docs/internetworking/design/guide/nd2003.html

  • Add a note hereCisco Systems, Inc. “Cisco IOS IP Routing Protocols Command Reference,” at http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_book.html

  • Add a note hereThe Internet Engineering Task Force. RFC 1793: Extending OSPF to Support Demand Circuits, at http://www.ietf.org/rfc/rfc1793.txt

  • Add a note hereThe Internet Engineering Task Force. RFC 2328: OSPF Version 2, at http://www.ietf.org/rfc/rfc2328.txt

  • Add a note hereThe Internet Engineering Task Force. RFC 2796: BGP Route Reflection—An Alternative to Full Mesh IBGP, at http://www.ietf.org/rfc/rfc2796.txt

  • Add a note hereThe Internet Engineering Task Force. RFC 3065: Autonomous System Confederations for BGP, at http://www.ietf.org/rfc/rfc3065.txt

  • Add a note hereThe Internet Engineering Task Force. RFC 4136: OSPF Refresh and Flooding Reduction in Stable Topologies, at http://www.ietf.org/rfc/rfc4136.txt


Review Questions

Add a note hereAnswer the following questions, and then refer to Appendix A, “Answers to Review Questions,” for the answers.

Add a note here 1.

Add a note hereWhich two address blocks are summarizable? (Choose two.)

  1. Add a note here172.16.20.0 to 172.16.27.0

  2. Add a note here172.16.20.0 to 172.16.23.0

  3. Add a note here10.16.0.0 to 10.31.0.0

  4. Add a note here10.16.0.0 to 10.47.0.0

  5. Add a note here10.96.0.0 to 10.159.0.0

Add a note here Image from book

Add a note here 2.

Add a note here Which two can bit-splitting techniques be used for? (Choose two.)

  1. Add a note hereOSPF area design

  2. Add a note hereSummarizable address blocks with convenient role-based subnets

  3. Add a note hereAccess list convergence

  4. Add a note hereDetecting summarizable address blocks

  5. Add a note hereManual route summarization

Add a note here Image from book

Add a note here 3.

Add a note hereWhich is the recommended design approach?

  1. Add a note hereConfigure a static default route everywhere for predictability.

  2. Add a note hereConfigure static default routes using recursive routing for consistency.

  3. Add a note hereOriginate the default at the edge and redistribute it into dynamic routing.

  4. Add a note hereMake the OSPF backbone area 0 stubby.

  5. Add a note hereDo not use additional parameters with the originate default command.

Add a note here Image from book

Add a note here 4.

Add a note hereWhich two statements best describe redistribution? (Choose two.)

  1. Add a note hereRedistribution works poorly with an arbitrary mix of routing protocols anywhere.

  2. Add a note hereRedistribution seldom requires route filters.

  3. Add a note hereRedistribution is not useful after a merger.

  4. Add a note hereRedistribution works well with a limited number of redistribution points.

  5. Add a note hereRedistribution prevents summarization.

Add a note here Image from book

Add a note here 5.

Add a note hereSelect the best statement concerning EIGRP and OSPF routing design.

  1. Add a note hereRouting design needs to be done most carefully for small networks.

  2. Add a note hereOSPF should not be used for small networks.

  3. Add a note hereRouting design needs to be done most carefully for large networks.

  4. Add a note hereRoute summarization must be used in all network designs.

  5. Add a note hereOSPF works best with a full mesh.

Add a note here Image from book

Add a note here 6.

Add a note hereWhich three factors are the biggest influences on OSPF scalability? (Choose three.)

  1. Add a note hereFlooding paths and redundancy

  2. Add a note hereAmount of routing information in the OSPF area or routing domain

  3. Add a note hereNumber of routers capable of Cisco Express Forwarding

  4. Add a note hereNumber of adjacent neighbors

  5. Add a note hereOther routing protocols in use

Add a note here Image from book

Add a note here 7.

Add a note here Which statement best describes basic IBGP?

  1. Add a note hereIBGP is a link-state protocol.

  2. Add a note hereIBGP requires a full mesh of peers because it has no other way to prevent looping of routing information.

  3. Add a note hereIBGP inherently handles all full-mesh scalability issues.

  4. Add a note hereIBGP uses split horizoning to prevent looping of routing information.

  5. Add a note hereIBGP uses the autonomous system path to prevent looping of routing information.

Add a note here Image from book

Add a note here 8.

Add a note hereA route reflector reflects routes from a route reflector client to which three types of IBGP routers? (Choose three.)

  1. Add a note hereNonclient routers

  2. Add a note hereSub-autonomous system members

  3. Add a note hereOther route reflector client routers

  4. Add a note hereEBGP peers

  5. Add a note hereIBGP peers configured for EIGRP or OSPF routing

Add a note here Image from book

Add a note hereAnswers

Add a note here 1.

Add a note hereB, C.

Add a note here 2.

Add a note hereA, B.

Add a note here 3.

Add a note hereC.

Add a note here 4.

Add a note hereA, D.

Add a note here 5.

Add a note hereC.

Add a note here 6.

Add a note hereA, B, D.

Add a note here 7.

Add a note hereB.

Add a note here 8.

Add a note hereA, C, D.

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