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Switching Basics and Intermediate Routing CCNA 3 Chapter 2

Learn about link-state routing algorithms, LSAs, OSPF, IS-IS, benefits, hierarchy, and mechanisms for maintaining stable and high-speed routing in a network environment.

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Switching Basics and Intermediate Routing CCNA 3 Chapter 2

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  1. Switching Basics and Intermediate Routing CCNA 3Chapter 2

  2. Link-State Routing OverviewMaintaining Routing Information Via Link States • Link-state routing algorithms, also known as shortest path first (SPF) algorithms, build a complex database of topology information • The algorithms compute the shortest path between nodes • Maintains full knowledge of distant routers and how they interconnect

  3. Link-State Routing OverviewMaintaining Routing Information Via Link States • Link-state routing uses link-state advertisements (LSAs) • A basic building block that describes a router’s local topology and is distributed to all other routers in the area • Link-state routing uses a topological database (or link-state database) • The set of all links learned from the flooding of LSAs • Synchronized with all other routers in the area

  4. Link-State Routing OverviewMaintaining Routing Information Via Link States • OSPF and Intermediate System-to-Intermediate System (IS-IS) are link-state routing protocols • Collect routing information from all other routers in the area • Each router calculates all the best paths to all destinations in the network • Because each router calculates best paths, they are less likely to propagate incorrect information learned from a neighboring router

  5. Link-State Routing OverviewMaintaining Routing Information Via Link States • Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols • Respond quickly to network changes • Send only triggered updates • Send periodic updates at long intervals, such as every 30 minutes • A hello mechanism determines reachability of neighbors

  6. Link-State Routing OverviewMaintaining Routing Information Via Link States Link-State Routing Relies on Complex Mechanisms to Permit Stable, Synchronous and High-Speed Routing

  7. Link-State Routing OverviewMaintaining Routing Information Via Link States • When a failure occurs in a network: • Link-state protocols flood LSAs; use a special multicast address • Each link-state router takes a copy of the LSA, updates its topological database, and forwards the LSA to neighboring routers • All link-state routers in the area recalculate their routing tables using the Dijkstra SPF algorithm • A link is similar to an interface on a router • The state of the link is a description of the interface and its relation to its neighboring routers

  8. Link-State Routing OverviewMaintaining Routing Information Via Link States OSPF Uses a Two-Layer Hierarchy

  9. Link-State Routing OverviewMaintaining Routing Information Via Link States Two primary elements exist in the two-layer hierarchy • Area: A grouping of contiguous networks • Areas are logical subdivisions of the autonomous system • Each area must be connected directly to the backbone area (known as area 0) • Autonomous System (AS): A collection of networks under a common administration • Share a common routing strategy • Can be logically subdivided into multiple areas

  10. Link-State Routing OverviewMaintaining Routing Information Via Link States • The backbone area is the transition area • All other areas communicate through it • All non-backbone areas are connected to it • These can be configured as a stub area, a totally stubby area, or a not-so-stubby area (NSSA) (not covered in this curriculum) to reduce the sizes of the link-state database and the routing table

  11. Link-State Routing OverviewLink-State Routing Protocol Algorithms • Link-State Routing Protocol Algorithms: • Rely on SPF protocols to maintain a complex database of the network topology • Develop and maintain a full knowledge of the network routers and how they interconnect • Use LSAs to exchange information with other routers • Each router that has exchanged LSAs constructs a topological database • The SPF algorithm is used to compute reachability to destination networks • A routing table is built from this information, containing only lowest-cost routes

  12. Link-State Routing OverviewLink-State Routing Protocol Algorithms • (continued): • LSA exchanges are triggered events • Greatly speed up convergence process • No need to wait for a series of timers to expire before the networked routers can begin to converge

  13. Link-State Routing OverviewLink-State Routing Protocol Algorithms Cost Metric Determines Shortest Path for Link-State Routing Protocols

  14. Link-State Routing OverviewLink-State Routing Protocol Algorithms Next Hops and Costs for Destination Routes (Previous Slide)

  15. Link-State Routing Benefits ofLink-State Routing • Link-state protocols use cost metrics to choose paths • Cost metric reflects the capacity of the links • Routing updates are less frequent • Network can be segmented into area hierarchies • Limits the scope of route changes • Link-state protocols send only updates of a topology change • Use triggered, flooded updates which lead to faster convergence times

  16. Link-State Routing Benefits ofLink-State Routing • Each router has a complete and synchronized picture of the network • Difficult for routing loops to occur • LSAs are sequenced and aged • Routers always base their routing information on the most recent set of information • With careful design work, size of link-state databases can be minimized • Smaller Dijkstra calculations and faster convergence

  17. Link-State Routing Limitations ofLink-State Routing • In addition to a routing table, link-state protocols require: • A topological database • An adjacency database • Lists all the relationships formed between neighboring routers for the purpose of exchanging routing information • A forwarding table • A data structure of a stripped down association between network prefixes and next hops

  18. Link-State Routing Limitations ofLink-State Routing • Dijkstra’s algorithm requires CPU cycles to calculate best paths through the network • If the network is large or unstable, this can require a significant amount of CPU time • Not a problem for most modern routers • A strict hierarchical network design is required to divide the network into smaller areas • Reduces the excessive use of memory and CPU cycles • Reduces size of topology tables and Dijkstra calculations • Areas must be contiguous at all times

  19. Link-State Routing Limitations ofLink-State Routing • Although configuration of link-state networks is usually simple, configuring a large network can be challenging • Trouble-shooting is usually easier, as every router has a copy of the topology • However, interpreting the information requires a good understanding of link-state routing concepts • Link-state protocols usually scale to bigger networks than distance vector protocols

  20. Link-State Routing Limitations ofLink-State Routing • Link-state routing raises two concerns: • During the initial discovery process, link-state routing protocols flood the network with LSAs • Significantly decreases the network’s capability to transport data • This is temporary, but noticeable • Link-state routing is both memory- and processor-intensive • Greater demand requires higher-end routers that cost more

  21. Single-Area OSPF Concepts • OSPF was developed by the Interior Gateway Protocol (IGP) group of the Internet Engineering Task Force (IETF) • Created in mid 1990s because RIP was unable to serve large, heterogeneous networks • OSPF has two primary characteristics: • Protocol is an open standard, not proprietary • Based on the SPF algorithm

  22. Single-Area OSPF ConceptsComparing OSPF with Distance Vector Routing Protocols • OSPF is a link-state protocol, RIP and IGRP are distance vector protocols • Distance vector protocols send all, or a portion of, their routing table in updates to their neighbors • A link is an interface on a router • The state of the link describes the interface and its relationship to neighboring routers • Can include IP address, subnet mask, type of network • The collection of link states forms a link-state database

  23. Single-Area OSPF ConceptsComparing OSPF with Distance Vector Routing Protocols • An OSPF router sends LSA packets to periodically advertise its link states instead of sending routing table updates • Information about attached interfaces and metrics are included • LSAs are flooded to all routers in the area • As OSPF routers accumulate link-state information, they use the SPF algorithm to calculate the shortest path to each destination

  24. Single-Area OSPF ConceptsComparing OSPF with Distance Vector Routing Protocols • A topological (link-state) database is an overall picture of networks in relationship to routers • Contains the collection of LSAs received from all routers in the same area • Database is pieced together from the LSAs • Routers in the same area have identical topological databases

  25. Single-Area OSPF ConceptsComparing OSPF with Distance Vector Routing Protocols • OSPF can operate within a hierarchy • The largest entity is the Autonomous System (AS): • A collection of networks under a common administration that share a common routing strategy • An AS can be divided into several areas, which are groups of contiguous networks and attached hosts

  26. Single-Area OSPF ConceptsOSPF Hierarchical Routing • OSPF’s capability to separate a large network into multiple areas is known as hierarchical routing • Hierarchical routing enables you to separate a large internetwork (AS) into smaller internetworks called areas • Routing still occurs between areas • Many of the minute internal routing operations, such as recalculating the database, are kept within an area

  27. Single-Area OSPF ConceptsOSPF Hierarchical Routing OSPF Uses Areas to Provide Hierarchy

  28. Single-Area OSPF ConceptsOSPF Hierarchical Routing • OSPF’s hierarchical topology possibilities have the following advantages: • Reduced frequency of SPF calculations • Smaller routing tables • Reduced link-state update overhead

  29. Single-Area OSPF ConceptsDijkstra’s Algorithm • In Dijkstra’s algorithm, the best path is the lowest cost path • Named for Edsger Wybe Dijkstra, a Dutch computer scientist • Each link has a cost • Each node has a name • Each node has a complete topological database

  30. Single-Area OSPF ConceptsDijkstra’s Algorithm Dijkstra’s Algorithm Uses Cost Metric

  31. Single-Area OSPF ConceptsDijkstra’s Algorithm • Dijkstra’s algorithm places each router at the root of a tree • Calculates the shortest path to each node based on the cumulative cost to reach the destination • Each router has its own view of the topology • Each router uses the information in its topological database to calculate a shortest-path tree, with itself as the root • The router uses this tree to route network traffic

  32. Single-Area OSPF ConceptsDijkstra’s Algorithm • The cost, or metric, of an interface indicates the overhead that is required to send packets across that interface • The OSPF cost of an interface is inversely proportional to that interface’s bandwidth • Higher bandwidth equals lower cost • Cost = 100,000,000 / bandwidth in bps

  33. Single-Area OSPF ConceptsDijkstra’s Algorithm Shortest Path is Measured from Each Root Node to Build a Shortest Path Tree

  34. Single-Area OSPF Configuration Basic OSPF Configuration • The router ospf command takes a process identifier as an argument: • Router (config)# router ospfprocess-id • The process ID is a locally significant number between 1 and 65,535 that you select to identify the routing process • It does not need to match the OSPF process ID on other OSPF routers

  35. Single-Area OSPF Configuration Basic OSPF Configuration • The network command identifies which IP networks on the router are part of the OSPF network: • Router(config-router)#networkaddresswildcard-maskareaarea-id (all on one command line) Parameters of a network Command

  36. Single-Area OSPF Configuration Basic OSPF Configuration • The wildcard mask is sometimes called an inverse mask because it is the inverse of the subnet mask for the network • This is not required; many network administrators use the 0.0.0.0 option to match the interface Basis OSPF Network with Each Router in Area 0

  37. Single-Area OSPF Configuration Basic OSPF Configuration Using the network statement in OSPF

  38. Single-Area OSPF Configuration Basic OSPF Configuration • A router uses the OSPF hello protocol to establish neighbor relationships • Hello packets let other routers know they are still functional • On networks supporting more than two routers (multiaccess networks), such as Ethernet networks, the hello protocol elects: • A designated router (DR) • Generates LSAs • Manages link-state synchronization • A backup designated router (BDR) • Becomes the DR if the existing DR fails

  39. Single-Area OSPF Configuration Loopback Interfaces • The OSPF router ID is the number by which the router is known to OSPF • To modify the OSPF router ID to a loopback address use this command: • Router(config)#interface loopbacknumber • The highest IP address on an active interface of a router at startup can be overridden by using a loopback address • OSPF is more reliable if a loopback interface is configured because a loopback interface is always active

  40. Single-Area OSPF Configuration Modifying the OSPF Cost Metric • OSPF uses cost as the metric to determine the best route • Cost is associated with the output side of an interface • It is calculated with the formula cost = 100,000,000/bandwidth in bps • The lower the cost, the more likely the route is to be used

  41. Single-Area OSPF Configuration Modifying the OSPF Cost Metric OSPF Cost Values

  42. Single-Area OSPF Configuration Modifying the OSPF Cost Metric • It is essential for proper OSPF operation that the correct interface bandwidth is set: • Router(config)#interface serial 0 • Router(config-if)#bandwidth 56 • Cost can be changed to influence the outcome of OSPF cost calculation • When costs are from different vendors are unequal, might want to make change to match costs • Might need to change cost to account for Gigabit Ethernet • Use this command to change cost: • Router(config-if)#ip ospf costnumber

  43. Single-Area OSPF Configuration OSPF Authentication • A router trusts the information that is coming from a router that should be sending it the information • To guarantee this trust, routers in a specific area can be configured to authenticate each other with OSPF authentication • Each interface can present an authentication key that the router uses to send OSPF information to other routers on the segment • The key, known as a password, is a shared secret between the routers • The key can be up to eight characters long • The key generates the authentication data in the OSPF header

  44. Single-Area OSPF Configuration OSPF Authentication • Use the following syntax to configure OSPF authentication: • Router(config-if)#ip ospf authentication-keypassword • After the password is configured, authentication must be enabled: • Router(config-router)#areaarea-numberauthentication • With simple authentication, the password is sent as plain text (security risk) • Configure encryption of the password

  45. Single-Area OSPF Configuration OSPF Authentication • Authentication password encryption syntax: • Router(config-if)#ip ospf message-digest-keykey-id encryption-type md5key (all on one line!) • The key-id is an identifier with a value of between 1 and 255 • The encryption-type refers to the type of encryption, where 0 means none and 7 means proprietary • The following is configured in router configuration mode on a router with an interface in the area area-id • Router(config-router)#areaarea-idauthentication message-digest • MD5 creates a message digest, which is scrambled data based on the password and the message contents • If the digests match, the receiving router trusts the data

  46. Single-Area OSPF Configuration OSPF Network Types and OSPF Timers • OSPF interfaces automatically recognize three OSPFnetwork types: • Broadcast multiaccess, such as Ethernet • Point-to-point networks • Nonbroadcast multiaccess networks (NBMA), such as Frame Relay • An administrator can manually configure a fourth OSPF network type: point-to-multipoint • In a multiaccess network, it is not known in advance how many routers will be connected • In point-to-point networks, only two routers will be connected

  47. Single-Area OSPF Configuration OSPF Network Types and OSPF Timers • In a broadcast multiaccess network segment, many routers can be connected • If every router has to establish adjacency with every other router, [n * (n-1) / 2] adjacencies need to be formed • For 5 routers the formula would be 5*(5-1) / 2 = 5*4 / 2 = 20 / 2 = 10 adjacencies • Routers hold an election for a DR router • This router becomes adjacent to all other routers in the broadcast segment • All other routers send their link-state information to the DR • The DR sends link-state information to all other routers on the segment by using the 224.0.0.5 multicast address

  48. Single-Area OSPF Configuration OSPF Network Types and OSPF Timers • Despite the gain in efficiency that electing a DR provides, a disadvantage exists: • The DR is a single point of failure • A second router is elected the BDR to take over in case the DR fails • To make sure that both the DR and BDR see the link states that all routers send on the segment, the 224.0.0.6 multicast address is used • On point-to-point networks, no DR or BDR is elected; both routers become fully adjacent

  49. Single-Area OSPF Configuration OSPF Network Types and OSPF Timers OSPF Network Type, Characteristics, and DR Election

  50. Single-Area OSPF Configuration OSPF Network Types and OSPF Timers • OSPF uses: • Hello intervals • Default of 10 seconds on broadcast networks • Default of 30 seconds on nonbroadcast networks • Dead intervals (4 times the hellow interval by default) • Default of 40 seconds on broadcast networks • Default of 120 seconds on nonbroadcast networks • To change the default times: • Router(config-if)#ip ospf hello-intervalseconds • Router(config-if)#ip ospf dead-intervalseconds

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