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Module 2: Single Area OSPF

Module 2: Single Area OSPF. CCNA 3 version 3.0. Link-State Routing Review. Link-state routing algorithms, also known as shortest path first (SPF) algorithms… maintain a complex database of topology information

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Module 2: Single Area OSPF

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  1. Module 2: Single Area OSPF CCNA 3 version 3.0

  2. Link-State Routing Review • Link-state routing algorithms, also known as shortest path first (SPF) algorithms… • maintain a complex database of topology information • Link-state protocols flood routing information allowing every router to have a complete view of the network topology • Use triggered updates that allow efficient use of bandwidth and faster convergence • maintain full knowledge of distant routers and how they interconnect • Changes in the state of a link are sent to all routers in the network as soon as the change occurs. • Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols.

  3. Distance Vector vs. Link-State Distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers Link-state routing algorithm maintains full knowledge of distant routers and how they interconnect

  4. Distance-Vector Routing Example

  5. More on Link-State Routing… • Link-state routing protocols perform the following functions: • Respond quickly to network changes • Send triggered updates only when a network change has occurred • Send periodic updates known as link-state refreshes • Use a hello mechanism to determine the reachability of neighbors • Each router keeps track of the state or condition of its directly connected neighbors by multicasting hello packets • Each router also keeps track of all the routers in its network or area of the network by using link-state advertisements (LSAs).

  6. How Routing Information is Maintained • Link-state routing uses the following features: • Link-state advertisements (LSAs) • A topological database • The shortest path first (SPF) algorithm • The resulting SPF tree • A routing table of paths and ports to each network to determine the best paths for packets

  7. How Routing Information is Maintained - LSAs • After a failure occurs in the network (such as a neighbor becomes unreachable) • link-state protocols flood LSAs using a special multicast address throughout an area • Each link-state router takes a copy of the LSA and updates its link-state, or topological database • The link-state router will then forward the LSA to all neighboring devices • What do LSAs do? • LSAs cause every router within the area to recalculate routes and update their routing tables • Therefore, the number of link-state routers that can be in an area should be limited

  8. What is a “link” in Link-State Routing? • A “link” is the same as an interface on a router • The “state of the link” describes an interface and the relationship to its neighboring routers; includes… • the IP address of the interface • the subnet mask • the type of network to which it is connected • the routers connected to that network • Link-state routers advertise the states of their links to all other routers in the area so that each router can build a complete link-state database

  9. What is the database and how is the “best path” selected? • The collection of link-states forms a link-state database (also called a topological database) • The link-state database is used to calculate the best paths through the network • How is the best path found? • To find the “best path”, link-state routers apply the Dijkstra shortest path first (SPF) algorithm against the link-state database (used to build the shortest path first tree with the local router as the root) • The best paths are then selected from the SPF tree and placed in the routing table

  10. More on LSAs… • LSA exchange is triggered by an event in the network instead of periodic updates • This can greatly speed up the convergence process (no need to wait for timers to expire) • Routing updates occur only when the network changes • If there are no changes, the routing updates occur after a specific interval • If the network changes, a partial update is sent immediately (The partial update only contains information about links that have changed, not a complete routing table)

  11. Link-State Routing -Advantages vs. Disadvantages

  12. Link-State Routing vs. Distance-Vector Routing • D-V: View network topology from neighbor’s perspective • L-S: Gets common view of entire network topology • D-V: Adds distance vectors from router to router • L-S: Calculates the shortest path to other routers • D-V: Frequent, periodic updates = slow convergence • L-S: Event-triggered updates = faster convergence • D-V: Passes copies of routing tables to neighbors • L-S: Passes link-state routing updates to other routers

  13. RIP (D-V) vs. OSPF (L-S) • Network size • RIP is suitable for small networks • OSPF suitable for large networks • Best Path Selection • RIP – determined by number of hops • OSPF – determined by speed (cost) • Convergence • RIP is slow to converge (wait for timed updates) • OSPF faster to converge (event-triggered updates) • Network Topology • RIP uses a flat topology • OSPF uses a hierarchical topology (based on areas)

  14. More on Link-State Routing Protocols… • Link-state protocols support… • classless interdomain routing (CIDR) • variable-length subnet mask (VLSM) • This makes them a good choice for complex, scalable networks. • Link-state protocols generally outperform distance vector protocols on any size network • Link-state protocols are not implemented on every network because… • they can overwhelm slower equipment • They are quite complex and require well-trained administrators to correctly configure and maintain them

  15. Introduction to OSPF • Open Shortest Path First (OSPF) is a link-state routing protocol based on open standards • The Open in OSPF means that it is open to the public and is non-proprietary • OSPF is becoming the preferred IGP protocol when compared with RIP v1 and RIP v2 because it is scalable

  16. More on OSPF… • OSPF can be used and configured as a single area for small networks • can also be used for large networks if hierarchical network design principles are used (multi-area OSPF) • What does a hierarchical design mean? • Multiple areas connect to a distribution area, area 0, also called the backbone • Allows for extensive control of routing updates • Defining areas… • reduces routing overhead, speeds up convergence, confines network instability to an area and improves performance.

  17. Multi-Area OSPF NOTE: For CCNA 3 and the CCNA certification exam, you will only be responsible for Single Area OSPF configuration.

  18. OSPF Terms - Link

  19. OSPF Terms – Link-State

  20. OSPF Terms –Link-State Database Every router in the same OSPF area will have the same link-state database

  21. OSPF Terms - Area Border Router Internal Routers

  22. OSPF Terms – Cost

  23. OSPF Terms – Forwarding Database (Routing Table) The lowest cost path is added to the routing table

  24. OSPF Terms –Adjacencies Database

  25. OSPF Terms – Designated Router (DR) and Backup Designated Router (BDR) DROTHERs The DR and BDR serve as focal points for routing information exchange If a router is not a DR or BDR, it becomes a DROTHER.

  26. Establishing a Neighbor Relationship with Other Routers • A neighbor relationship is required for OSPF routers to share routing information • A router will try to become adjacent, or neighbor, to at least one other router on each IP network to which it is connected (Some routers may try to become adjacent to all their neighbor routers) • Other routers may try to become adjacent to only one or two neighbor routers • OSPF routers determine which routers to become adjacent to based on the type of network they are connected to • Once an adjacency is formed between neighbors, link-state information is exchanged

  27. Types of OSPF Networks

  28. Election of DR and BDR • The DR acts as the spokesperson for the segment • All other routers on the segment send their link-state information to the DR

  29. DR and BDR Election as it relates to network types • The DR sends link-state information to all other routers on the segment using the multicast address of 224.0.0.5 • Disadvantage of DR election…the DR represents a single point of failure…therefore: • A second router is elected as a backup (BDR) in case of DR failure • To ensure that both the DR and the BDR see the link states all routers send on the segment, the multicast address for all designated routers, 224.0.0.6, is used. • On point-to-point networks only two nodes exist and no DR or BDR is elected (routers become fully adjacent with each other)

  30. The Hello Protocol • When a router starts an OSPF routing process on an interface, it sends a hello packet and continues to send hellos at regular intervals • The rules that govern the exchange of OSPF hello packets are called the Hello protocol • Type field is set to 1 to indicate packet contains hello information

  31. More on Hello Packets… • Hello packet timing… • sent every 10 seconds by default on broadcast multi-access and point-to-point networks • 30 seconds by default on interfaces that connect to NBMA networks, such as Frame Relay

  32. Single Area OSPF Operations – Step 1: Neighbor Discovery

  33. Single Area OSPF Operations – Step 2: Elect DR & BDR on Multi-Access Networks We’ll get to what determines which router becomes the DR and which becomes the BDR later…

  34. Single Area OSPF Operations – Step 3: Selecting the Best Route

  35. Single Area OSPF Operations – Step 4: Maintaining Routing Information

  36. Basic OSPF Configuration • To enable OSPF routing, use the global configuration command syntax: • Router(config)#router ospfprocess-id • The process ID is a number that is used to identify an OSPF routing process on the router • Multiple OSPF processes can be started on the same router • Process ID can be any value between 1 and 65,535 • Most network administrators keep the same process ID throughout an autonomous system (not a requirement) • It is rarely necessary to run more than one OSPF process on a router

  37. Basic OSPF Configuration • IP networks are advertised as follows in OSPF: • Router(config-router)#networkaddress wildcard-mask area area-id • Each network must be identified with the area to which it belongs • The network address can be… • a whole network • a subnet • or the address of the interface • The wildcard mask represents the set of host addresses that the segment supports

  38. OSPF Priority & DR and BDR Election • A router with the highest OSPF priority will be selected as the DR • If the network type of an interface is broadcast, the default OSPF priority is 1 • The priorities can be set to any value from 0 to 255 • Router with the second highest priority will be the BDR • When OSPF priorities are the same, the OSPF election for DR is decided on the router ID (highest ID wins) • To modify the OSPF priority on an interface… • Router(config-if)#ip ospfpriority number • To verify OSPF operation… • Router#show ip ospf interfacetype number

  39. OSPF Router ID • When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF router ID • If there is no active interface, OSPF process will not start • If the active interface goes down, the OSPF process has no router ID and therefore ceases to function until the interface comes up again

  40. Loopback Interface • To ensure OSPF stability there should be an active interface for the OSPF process at all times. • A loopback interface, which is a logical rather than a physical interface, can be configured for this purpose • When a loopback interface is configured, OSPF uses this address as the router ID, regardless of the value • On a router that has more than one loopback interface, OSPF takes the highest loopback IP address as its router ID.

  41. Modifying the Cost Metric • OSPF uses cost as the metric for determining the best route • Cost is calculated using the formula: • 108/bandwidth (bandwidth is expressed in bps) • The Cisco IOS automatically determines cost based on the bandwidth of the interface • You must set the correct interface bandwidth by: • Router(config)#interface serial 0/0Router(config-if)#bandwidth 64 • The default bandwidth for Cisco serial interfaces is 1.544 Mbps, or 1544 kbps.

  42. More on OSPF Cost • Cost can be changed to influence the outcome of the OSPF cost calculation • Use the following interface configuration command to set the link cost: • Router(config-if)#ip ospf costnumber • The cost number can be between 1 and 65,535

  43. Configuring OSPF Authentication • Each OSPF interface can present an authentication key for use by routers sending OSPF information to other routers on the segment • The authentication key, known as a password, is a shared secret between the routers • This key is used to generate the authentication data in the OSPF packet header • Passwords can be sent as plain text or encryted

  44. For Simple Authentication – Plain Text • Use the following command 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 • Simple Authentication can be easily decoded if a packet sniffer captures an OSPF packet

  45. Encrypted Authentication • Use the interface configuration command mode syntax: • Router(config-if)#ip ospf message-digest-keykey-idmd5encryption-type key • The following is configured in router configuration mode: • Router(config-router)#areaarea-idauthentication message-digest

  46. OSPF Timers (Hello Interval and Dead Interval) • OSPF routers must have the same hello intervals and same dead intervals to exchange information (veryimportant!) • By default, the dead interval is four times the value of the hello interval • Means a router has four chances to send a hello packet before being declared dead • On broadcast OSPF networks… • the default hello interval is 10 seconds and the default dead interval is 40 seconds • On nonbroadcast networks… • the default hello interval is 30 seconds and the default dead interval is 120 seconds • These default values result in efficient OSPF operation and seldom need to be modified

  47. Configuring OSPF Timers • To configure the hello and dead intervals on an interface, use the following commands: • Router(config-if)#ip ospf hello-interval secondsRouter(config-if)#ip ospf dead-intervalseconds

  48. Configuring a Default Route • A configured default route is used by a router to generate a gateway of last resort: • Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address] • The following configuration statement will propagate this route to all the routers in a normal OSPF area: • Router(config-router)#default-information originate

  49. Verifying OSPF Configuration

  50. Debug & Clear Commands

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