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Module 2

Module 2. Routing Principles. Objectives. Upon completion of this chapter, you will be able to perform the following tasks: Describe classful and classless routing protocols Compare classful vs. classless routing protocols Describe link-state routing protocols operations

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Module 2

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  1. Module 2 Routing Principles

  2. Objectives • Upon completion of this chapter, you will be able to perform the following tasks: • Describe classful and classless routing protocols • Compare classful vs. classless routing protocols • Describe link-state routing protocols operations • Compare distance vector vs. link-state routing protocols

  3. Classful Routing Review • Classful routing protocols do not include the subnet mask with the route advertisement • Within the same network, consistency of the subnet masks is assumed • Summary routes are exchanged between foreign networks • Example of classful routing protocols: • RIPv1 (RIP Version 1)* • IGRP* *RIPv1 and IGRP are discussed in the ICND course and will not be discussed in this course

  4. Classful Routes • Subnetwork routes are shared by devices within the same network • Summary routes are exchanged between foreign networks • Summary routes are automatically created at Class A, B, and C network boundaries 10.1.0.0 10.2.0.0 172.16.2.0 172.16.1.0 A B C 10.1.0.0 10.2.0.0 172.16.0.0 10.1.0.0 10.2.0.0 172.16.1.0 172.16.2.0 10.0.0.0 172.16.1.0 172.16.2.0

  5. Classful Subnetting Requirements • All router interfaces within the same network must have the same subnet mask (Fixed-length subnet masking) • This approach may not fully utilize available allocation of host addresses • All subnets of the same major network must be contiguous 192.168.5.129 /27 A requirement for only two host addresses– forced to allocate 30 host addresses E0 S1 192.168.5.98 /27 S0 192.168.5.97 /27 E0 E1 192.168.5.33 /27 192.168.5.65 /27

  6. Classless Routing Overview • Classless routing protocols include the subnet mask with the route advertisement • Classless routing protocols support Variable-length subnet masking (VLSM) • Summary routes can be manually controlled within the network • Example of classless routing protocols: • OSPF • EIGRP • RIPv2 (RIP Version 2) • IS-IS • BGP

  7. Classless Subnetting Requirements • Router interfaces within the same network can have different subnet masks • Variable-length subnet masking (VLSM) is supported • This approach maximizes allocation of available host addresses 192.168.5.129 /27 A requirement for only two host addresses– VLSM support accommodates this E0 S1 192.168.5.209 /30 S0 192.168.5.210 /30 E0 E1 192.168.5.33 /27 192.168.5.65 /27

  8. Classful and Classless Updates RIPv1 network 172.16.2.0/24 172.16.1.0/24 A B 172.16.2.0

  9. Classful and Classless Updates (cont.) RIPv1 network 172.16.2.0/24 172.16.1.0/24 192.168.5.0/24 A C B 172.16.0.0 172.16.2.0 Routing table 172.16.0.0/16

  10. B Classful and Classless Updates (cont.) RIPv1 network 172.16.2.0/24 172.16.1.0/24 192.168.5.0/24 A C B 172.16.0.0 172.16.2.0 Routing table 172.16.0.0/16 OSPF network 172.16.2.0/24 172.16.1.0/24 A 172.16.2.0/24

  11. B Classful and Classless Updates (cont.) RIPv1 Network 172.16.2.0/24 172.16.1.0/24 192.168.5.0/24 A C B 172.16.0.0 172.16.2.0 Routing Table 172.16.0.0/16 OSPF Network Routing Table 172.16.2.0/24 172.16.1.0/24 172.16.2.0/24 172.16.1.0/24 192.168.5.0/24 A A C B 172.16.2.0/24 172.16.2.0/24 172.16.1.0/24

  12. Distance Vector Routing Protocols Review RoutingUpdate C D A B RoutingTable RoutingTable RoutingTable RoutingTable • With Distance Vector routing protocols, routers broadcast their routing tables to its adjacent neighbors at periodic intervals.

  13. Link-State Routing Protocols Overview B A C D Link-State Advertisement (LSA) Link StateDatabase SPF Algorithm Routing Table Shortest Path First Tree

  14. Basic Terms of Link-State Routing Protocols • Link-state (LS) routers know much more about the network than their distance-vector relatives—LS routers cannot be fooled as easily into making wrong decisions • Link-state routers keep track of: • Their neighbors • All the routers in the network, or at least within the same area • Best paths toward a destination

  15. Basic Terms—Link-State Data Structures • Neighbor table, formally known as an adjacency database (list of neighbors we are aware of) • Topology table, typically referred to as a link-state database—LSDB (routers and links in the area/network) • Routing table, commonly named a forwarding database (list of best paths to destinations)

  16. Basic Terms—The Best Path Calculation • Routers find the best paths to destinations by applying the Dijkstra Shortest Path First (SPF) algorithm to the link-state database • Every router in the area places itself into the root of the tree that is built • Best path is calculated with respect to the lowest total cost of links to a specific destination • Best routes are put into the forwarding database

  17. x x B B A A C C D D E E H H F F G G Basic Terms—Link-State Environment Link-state database Shortest paths Dijkstra (SPF) algorithm Adjacency database Forwarding database A, B,C,D Neighbors of x: (routing table)

  18. Link-State Data Structure—Network Hierarchy • Link-state routing requires a hierachical network structure • Enforced by some LS protocols (for example, OSPF) • Some LS protocols are more tolerant (IS-IS) • Two level hierarchy—areas • Backbone or level-2 area • Non-backbone or level-1 area

  19. E C D F H G Link-State Data Structure—Network Hierarchy Example Backbone Area External Routing Domain A I B • Minimizes routing table entries • Localizes impact of a topology change within an area Area 1 Area 3 Area 2 Autonomous System

  20. LS Data Structures—Link-State Database • The foundation for best-path calculation is the LS database • LS database has to be identical on all the routers in the area—identical view • Routers know everything about their respective area • Routers know about the nearest exit point(s) to other areas or other routing domains

  21. LS Data Structures—Adjacency Database • Routers discover neighbors by exchanging Hello packets • Routers declare neighbors “UP” after checking some parameters/options in the Hello packet • Some routers become adjacent (tightly connected neighbors)—“good neighbors” • Adjacent routers exchange topology information

  22. LS Data Structures—Link-State Packets • Upon establishing neighbor relationship, routers exchange their pictures of the network • Full picture—link-state database (LSDB)—is built by link-state packets (LS packets) • LS packets report the states of links and routers • LS packets are flooded reliably throughout the area/network • LS packets are sequenced, aged and periodically refreshed

  23. LS Data Structures—Link-State Maintenance • Routers maintain the consistency of the link-state database • Routers check their neighbor relationship by sending and receiving periodic hello packets • Routers report changes in the network (immediately/depending on timers) • Receivers of a link-state packet normally flood it further • Routers periodically resend their part of the “network map” (even if no changes)

  24. LS Data Structures— LS Advertisement Operation LS Update Packet (LSU) Is entry inlink-statedatabase? LSA No Add to database Send LSAck Flood LSA Run SPF to calculate new routing table End

  25. LSU LSA LS Data Structures—LS Advertisement Operation (cont.) Is entry inlink-statedatabase? Is seq. # the same? Ignore LSA Yes Yes No Add to database Send LSAck Flood LSA Run SPF to calculate new routing table End

  26. LSU LSA LS Data Structures—LS Advertisement Operation (cont.) Is entry inlink-statedatabase? Is seq. # the same? Ignore LSA Yes Yes No No Add to database Is seq. # higher? Send LSAck No Send LSU with newer information to source Flood LSA Run SPF to calculate new routing table End End

  27. LSU LSA Go to A LS Data Structures—LS Advertisement Operation (cont.) Is entry inlink-statedatabase? Is seq. # the same? Ignore LSA Yes Yes No No Add to database A Is seq. # higher? Yes Send LSAck No Send LSU with newer information to source Flood LSA Run SPF to calculate new routing table End End

  28. Benefits of Link-State Routing • Benefits: • Fast convergence—changes reported immediately by the source affected • Robustness against routing loops: • Routers know the topology • LS packetsare sequenced and acknowledged • By careful (hierarchical) network design, resources can be utilized optimally

  29. Caveatsof Link-State Routing • Caveats: • Significant demands for resources: • Memory (three tables: adjacency, topology, forwarding) • CPU (Dijkstra algorithm can be intensive, especially when a lot of instabilities are present) • Requires very strict network design(when more areas—area routing) • Problems with partitioning of areas • Configuration generally simple but can be complex when tuning various parameters and when the design is complex • Troubleshooting is easier than in distance vector routing—we have more information at hand (three databases) but requires a good understanding of link-state concepts

  30. Examples of Link-State Protocols • OSPF (Open Shortest Path First), supports IP only; Internet standard • DECnet Phase V, supports Decnet/OSI • IS-IS (supports CLNP); ISO standard • Integrated IS-IS (supports CLNS and IP); Internet standard, RFC-1195 • NLSP (Netware Link Services Protocol), supports IPX only, based on IS-IS

  31. Link-State Routing Protocols Comparison Chart * CharacteristicOSPF IS-IS EIGRP Hierarchical topology—Required X X Retains knowledge of all possible routes X X X Route summarization—Manual X X X Route summarization—Automatic X Event-triggered announcements X X X Load balancing—Equal paths X X X Load balancing—Unequal paths X VLSM support X X X Routing algorithm Dijkstra Dijkstra DUAL Metric Cost Cost Comp Scalability Large VryLg Large • *EIGRP is an advanced Distance Vector routing protocol with some link-state features

  32. Routing Protocol Comparison Chart * CharacteristicRIPv1IGRPEIGRP IS-IS OSPF Distance vector X X X Link-state X X Automatic route summarization X X X Manual route summarization X X X VLSM support X X X Proprietary X X Convergence time Slow Slow VryFst Fast Fast • * EIGRP is an advanced distance vector protocol with some Link State features

  33. Summary • After completing this chapter, you should be able to perform the following tasks: • Describe classful and classless routing protocols • Compare classful vs.classless routing protocols • Describe link-state routing protocols operations • Compare distance vector vs. link-state routing protocols

  34. Review Questions • What characteristic defines the difference between classful and classless protocols? • Link State Routers find the best paths todestinations by applying which algorithm to the LS database? • Link State Routers discover neighbors by exchanging what type of packets? • Why LS Packets contain a sequence number? • Why LS routing protocols require significant amount of memory and CPU cycles?

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