Exploration routing chapter 10 & 11
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The document discusses link-state routing protocols and OSPF in particular. It describes how link-state protocols build a topological map of the network and use the SPF algorithm to independently calculate the shortest path to all networks. OSPF is introduced as an example link-state protocol, with explanations of its message encapsulation, packet types, hello protocol for neighbor discovery, link-state updates for exchanging routing information, and basic configuration including setting the router ID and verifying OSPF neighbors. The document also covers how OSPF uses cost based on bandwidth as its routing metric.
Exploration routing chapter 10 & 11
- 1. Link-State Routing Protocols - OSPF Routing Protocols and Concepts – Chapter 10 & 11 ITE I Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 1
- 2. Objectives Describe the basic features & concepts of link-state routing protocols. List the benefits and requirements of link-state routing protocols. Describe the background and basic features of OSPF Identify and apply the basic OSPF configuration commands Describe, modify and calculate the metric used by OSPF Describe the Designated Router/Backup Designated Router (DR/BDR) election process ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 2
- 3. Link-State Routing Link state routing protocols -Also known as shortest path first algorithms -These protocols built around Dijkstra’s SPF ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 3
- 4. Link-State Routing Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 4
- 5. Link-State Routing Link-State Routing Process How routers using Link State Routing Protocols reach convergence -Each routers learns about its own directly connected networks -Link state routers exchange hello packet to “meet” other directly connected link state routers. -Each router builds its own Link State Packet (LSP) which includes information about neighbors such as neighbor ID, link type, & bandwidth. -After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information. -Once all the routers have received all the LSPs, the routers then construct a topological map of the network which is used to determine the best routes to a destination ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 5
- 6. Link-State Routing Constructing a link state data base Routers use a database to construct a topology map of the network ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 6
- 7. Link-State Routing Shortest Path First (SPF) Tree Building a portion of the SPF tree Process begins by examining R2’s LSP information -R1 ignores 1st LSP Reason: R1 already knows it’s connected to R2 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 7
- 8. Link-State Routing Determining the shortest path The shortest path to a destination determined by adding the costs & finding the lowest cost ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 8
- 9. Link-State Routing Once the SPF algorithm has determined the shortest path routes, these routes are placed in the routing table. ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 9
- 10. Link-State Routing Protocols Advantages of a Link-State Routing Protocol Routing protocol Builds Topological map Router can independently determine the shortest path to every network. Distance vector No Link State Yes ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Convergence A periodic/ event driven routing updates Use of LSP No Slow Generally No No Yes Fast Generally Yes Yes Cisco Public 10
- 11. Link-State Routing Protocols Requirements for using a link state routing protocol Memory requirements Typically link state routing protocols use more memory Processing Requirements More CPU processing is required of link state routing protocols Bandwidth Requirements Initial startup of link state routing protocols can consume lots of bandwidth ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 11
- 12. Introduction to OSPF OSPF Message Encapsulation OSPF packet type There exist 5 types OSPF packet header Contains - Router ID and area ID and Type code for OSPF packet type IP packet header Contains - Source IP address, Destination IP address, & Protocol field set to 89 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 12
- 13. Introduction to OSPF OSPF Message Encapsulation Data link frame header Contains - Source MAC address and Destination MAC address ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 13
- 14. Introduction to OSPF OSPF Packet Types OSPF PACKET TYPE DESCRIPTION 1: HELLO Discovers neighbors & builds ADJACENCY W/ NEIGHBORS 2: DATABASE DESCRIPTION Checks for database synchronization between routers 3: LINK STATE REQUEST REQUESTS SPECIFIC LINK STATE DATABASE information from router to router 4: LINK STATE UPDATE TRANSPORTS LINK STATE records 5: LINK STATE ACKNOWLEDGEMENT ACKNOWLEDGES RECEIPT OF other packet types ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 14
- 15. Introduction to OSPF Hello Protocol OSPF Hello Packet –Purpose of Hello Packet discover OSPF neighbors & establish adjacencies advertise guidelines on which routers must agree to become neighbors used by multi-access networks to elect a designated router and a backup designated router ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 15
- 16. Introduction to OSPF Hello Packets continued Contents of a Hello Packet router ID of transmitting router OSPF Hello Intervals –Usually multicast (224.0.0.5) –Sent every 30 seconds for NBMA segments OSPF Dead Intervals –This is the time that must transpire before the neighbor is considered down –Default time is 4 times the hello interval ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 16
- 17. Introduction to OSPF Hello protocol packets contain information that is used in electing -Designated Router (DR) DR is responsible for updating all other OSPF routers -Backup Designated Router (BDR) This router takes over DR’s responsibilities if DR fails ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 17
- 18. Introduction to OSPF OSPF Link-state Updates Purpose of a Link State Update (LSU) Used to deliver link state advertisements Purpose of a Link State Advertisement (LSA) Contains information about neighbors & path costs ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 18
- 19. Introduction to OSPF OSPF Algorithm OSPF routers build & maintain link-state database containing LSA received from other routers –Information found in database is utilized upon execution of Dijkstra SPF algorithm –SPF algorithm used to create SPF tree –SPF tree used to populate routing table ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 19
- 20. Introduction to OSPF OSPF Authentication –Purpose is to encrypt & authenticate routing information –This is an interface specific configuration –Routers will only accept routing information from other routers that have been configured with the same password or authentication information ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 20
- 21. Basic OSPF Configuration OSPF network command -Requires entering: network address wildcard mask - the inverse of the subnet mask area-id - area-id refers to the OSPF area. OSPF area is a group of routers that share link state information -Example: Router(config-router)#network network-address wildcard-ask area area-id ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 21
- 22. Basic OSPF Configuration Router ID –This is an IP address used to identify a router –3 criteria for deriving the router ID Use IP address configured with OSPF router-id command -Takes precedence over loopback and physical interface addresses If router-id command not used then router chooses highest IP address of any loopback interfaces If no loopback interfaces are configured then the highest IP address on any active interface is used ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 22
- 23. Basic OSPF Configuration OSPF Router ID Router ID & Loopback addresses -Highest loopback address will be used as router ID if router-id command isn’t used -Advantage of using loopback address the loopback interface cannot fail OSPF stability The OSPF router-id command –Introduced in IOS 12.0 –Command syntax Router(config)#router ospf process-id Router(config-router)#router-id ip-address Modifying the Router ID –Use the command Router#clear ip ospf process ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 23
- 24. Basic OSPF Configuration Verifying OSPF Use the show ip ospf command to verify & trouble shoot OSPF networks Command will display the following: Neighbor adjacency -No adjacency indicated by Neighboring router’s Router ID is not displayed A state of full is not displayed -Consequence of no adjacencyNo link state information exchanged Inaccurate SPF trees & routing tables ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 24
- 25. OSPF Metric OSPF uses cost as the metric for determining the best route -The best route will have the lowest cost -Cost is based on bandwidth of an interface Cost is calculated using the formula 108 / bandwidth -Reference bandwidth defaults to 100Mbps can be modified using auto-cost reference-bandwidth command ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 25
- 26. OSPF Metric COST of an OSPF route Is the accumulated value from one router to the next ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 26
- 27. OSPF Metric Usually the actual speed of a link is different than the default bandwidth –This makes it imperative that the bandwidth value reflects link’s actual speed Reason: so routing table has best path information The show interface command will display interface’s bandwidth -Most serial link default to 1.544Mbps ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 27
- 28. Basic OSPF Configuration Modifying the Cost of a link Both sides of a serial link should be configured with the same bandwidth –Commands used to modify bandwidth value Bandwidth command –Example: Router(config-if)#bandwidth bandwidth-kbps ip ospf cost command – allows you to directly specify interface cost -Example:R1(config)#interface serial 0/0/0 R1(config-if)#ip ospf cost 1562 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 28
- 29. Basic OSPF Configuration Modifying the Cost of the link Difference between bandwidth command & the ip ospf cost command –Ip ospf cost command Sets cost to a specific value –Bandwidth command Link cost is calculated ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 29
- 30. OSPF and Multi-access Networks Challenges in Multi-access Networks OSPF defines five network types: –Point-to-point –Broadcast Multiaccess –Nonbroadcast Multiaccess (NBMA) –Point-to-multipoint –Virtual links ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 30
- 31. OSPF in Multi-access Networks 2 challenges presented by multi-access networks –Multiple adjacencies –Extensive LSA flooding ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 31
- 32. OSPF in Multi-access Networks Extensive flooding of LSAs For every LSA sent out there must be an acknowledgement of receipt sent back to transmitting router. consequence: lots of bandwidth consumed and chaotic traffic ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 32
- 33. OSPF in Multi-access Networks Solution to LSA flooding issue is the use of –Designated router (DR) –Backup designated router (BDR) DR & BDR selection –Routers are elected to send & receive LSA Sending & Receiving LSA –DRothers send LSAs via multicast 224.0.0.6 to DR & BDR –DR forward LSA via multicast address 224.0.0.5 to all other routers ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 33
- 34. OSPF in Multi-access Networks DR/BDR Election Process DR/BDR elections DO NOT occur in point to point networks ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 34
- 35. OSPF in Multi-access Networks DR/BDR elections will take place on multi-access networks as shown below ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 35
- 36. OSPF in Multi-access Networks Criteria for getting elected DR/BDR 1. DR: Router with the highest OSPF interface priority. 2. BDR: Router with the second highest OSPF interface priority. 3. If OSPF interface priorities are equal, the highest router ID is used to break the tie. ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 36
- 37. OSPF in Multi-access Networks Timing of DR/BDR Election –Occurs as soon as 1st router has its interface enabled on multiaccess network When a DR is elected it remains as the DR until one of the following occurs -The DR fails. -The OSPF process on the DR fails. -The multiaccess interface on the DR fails. ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 37
- 38. OSPF in Multi-access Networks Manipulating the election process -If you want to influence the election of DR & BDR then do one of the following Boot up the DR first, followed by the BDR, and then boot all other routers, OR Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers. ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 38
- 39. OSPF in Multi-access Networks OSPF Interface Priority Manipulating the DR/BDR election process continued –Use the ip ospf priority interface command. –Example:Router(config-if)#ip ospf priority {0 - 255} Priority number range 0 to 255 –0 means the router cannot become the DR or BDR –1 is the default priority value ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 39
- 40. More OSPF Configuration Redistributing an OSPF Default Route Topology includes a link to ISP –Router connected to ISP Called an autonomous system border router Used to propagate a default route –Example of static default route R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1 –Requires the use of the default-information originate command –Example of default-information originate command R1(config-router)#default-information originate ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 40
- 41. More OSPF Configuration Fine-Tuning OSPF Modifying OSPF timers –Reason to modify timers Faster detection of network failures –Manually modifying Hello & Dead intervals Router(config-if)#ip ospf hello-interval seconds Router(config-if)#ip ospf dead-interval seconds –Point to be made Hello & Dead intervals must be the same between neighbors ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 41
- 42. Summary Link State Routing protocols are also known as Shortest Path First protocols Summarizing the link state process -Routers 1ST learn of directly connected networks -Routers then say “hello” to neighbors -Routers then build link state packets -Routers then flood LSPs to all neighbors -Routers use LSP database to build a network topology map & calculate the best path to each destination ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 42
- 43. Summary Link An interface on the router Link State Information about an interface such as -IP address -Subnet mask -Type of network -Cost associated with link -Neighboring routers on the link Link State Packets -After initial flooding, additional LSP are sent out -when a change in topology occurs Examples of link state routing protocols -Open shortest path first -IS-IS ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 43
- 44. Summary RFC 2328 describes OSPF link state concepts and operations OSPF Characteristics –A commonly deployed link state routing protocol –Employs DRs & BDRs on multi-access networks DRs & BDRs are elected DR & BDRs are used to transmit and receive LSAs –Uses 5 packet types: 1: HELLO 2: DATABASE DESCRIPTION 3: LINK STATE REQUEST 4: LINK STATE UPDATE 5: LINK STATE ACKNOWLEDGEMENT ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 44
- 45. Summary OSPF Characteristics –Metric = cost Lowest cost = best path Configuration –Enable OSPF on a router using the following command R1(config)#router ospf process-id –use the network command to define which interfaces will participate in a given OSPF process Router(config-router)#network network-address wildcard-mask area area-id ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 45
Editor's Notes
- #6: Once LSP are created they are forwarded out to neighbors. -After receiving the LSP the neighbor continues to forward it throughout routing area.
- #8: Building a portion of the SPF tree -R1 uses 2nd LSP Reason: R1 can create a link from R2 to R5. This information is added to R1’s SPF tree R1 uses 3rd LSP Reason: R1 learns that R2 is connected to 10.5.0.0/16. This link is added to R1’s SPF tree.