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Link-State Routing
Protocols - OSPF

Routing Protocols and Concepts – Chapter 10 & 11

ITE I Chapter 6

© 2006 Cisco Syste...
Objectives


Describe the basic features & concepts of link-state
routing protocols.



List the benefits and requiremen...
Link-State Routing
 Link state routing protocols
-Also

known as shortest path first algorithms

-These protocols built a...
Link-State Routing
Dikjstra’s algorithm also known as the shortest path first
(SPF) algorithm

ITE 1 Chapter 6

© 2006 Cis...
Link-State Routing
Link-State Routing Process
 How routers using Link State Routing Protocols reach
convergence
-Each rou...
Link-State Routing
Constructing a link state data base
 Routers use a database to construct a topology map of
the network...
Link-State Routing
Shortest Path First (SPF) Tree
 Building a portion of the SPF tree
Process begins by examining R2’s LS...
Link-State Routing
 Determining the shortest path
The shortest path to a destination determined by adding the
costs & fin...
Link-State Routing
 Once the SPF algorithm has determined the shortest
path routes, these routes are placed in the routin...
Link-State Routing Protocols
Advantages of a Link-State Routing Protocol

Routing
protocol

Builds
Topological
map

Router...
Link-State Routing Protocols
Requirements for using a link state routing protocol
 Memory requirements
Typically link sta...
Introduction to OSPF
OSPF Message Encapsulation
 OSPF packet type
There exist 5 types
 OSPF packet header
Contains - Rou...
Introduction to OSPF
OSPF Message Encapsulation
 Data link frame header
Contains - Source MAC address and Destination
MAC...
Introduction to OSPF
OSPF Packet Types
OSPF PACKET TYPE

DESCRIPTION

1: HELLO

Discovers neighbors & builds
ADJACENCY W/ ...
Introduction to OSPF
Hello Protocol
 OSPF Hello Packet
–Purpose of Hello Packet
 discover OSPF neighbors & establish adj...
Introduction to OSPF
 Hello Packets continued
Contents of a Hello Packet
router ID of transmitting router

 OSPF Hello I...
Introduction to OSPF
 Hello protocol packets contain information that is used
in electing
-Designated Router (DR)
 DR is...
Introduction to OSPF
OSPF Link-state Updates
 Purpose of a Link State Update (LSU)
Used to deliver link state advertiseme...
Introduction to OSPF
OSPF Algorithm
 OSPF routers build &
maintain link-state
database containing LSA
received from other...
Introduction to OSPF
 OSPF Authentication
–Purpose is to encrypt & authenticate routing
information
–This is an interface...
Basic OSPF Configuration
 OSPF network command
-Requires entering: network address
wildcard mask - the inverse of the sub...
Basic OSPF Configuration
 Router ID
–This is an IP address used to identify a router
–3 criteria for deriving the router ...
Basic OSPF Configuration
OSPF Router ID
 Router ID & Loopback addresses
-Highest loopback address will be used as router ...
Basic OSPF Configuration
Verifying OSPF
 Use the show ip ospf command to verify & trouble
shoot OSPF networks
Command wil...
OSPF Metric
 OSPF uses cost as the metric for determining the
best route
-The best route will have the lowest cost
-Cost ...
OSPF Metric
 COST of an OSPF route
Is the accumulated value from one router to the next

ITE 1 Chapter 6

© 2006 Cisco Sy...
OSPF Metric
 Usually the actual speed of a link is different than the
default bandwidth
–This makes it imperative that th...
Basic OSPF Configuration
Modifying the Cost of a link
 Both sides of a serial link should be configured with the
same ban...
Basic OSPF Configuration
Modifying the Cost of the link
 Difference between bandwidth command & the ip ospf
cost command
...
OSPF and Multi-access Networks
Challenges in Multi-access Networks
 OSPF defines five network types:
–Point-to-point
–Bro...
OSPF in Multi-access Networks
 2 challenges presented by multi-access networks
–Multiple adjacencies
–Extensive LSA flood...
OSPF in Multi-access Networks
 Extensive flooding of LSAs
For every LSA sent out there must be an acknowledgement of
rece...
OSPF in Multi-access Networks
 Solution to LSA flooding issue
is the use of
–Designated router (DR)
–Backup designated ro...
OSPF in Multi-access Networks
DR/BDR Election Process
 DR/BDR elections DO NOT occur in point to point
networks

ITE 1 Ch...
OSPF in Multi-access Networks
 DR/BDR elections will take place on multi-access
networks as shown below

ITE 1 Chapter 6
...
OSPF in Multi-access Networks
 Criteria for getting elected DR/BDR
1. DR: Router with the highest OSPF
interface priority...
OSPF in Multi-access Networks
 Timing of DR/BDR Election
–Occurs as soon as 1st router has its interface enabled on multi...
OSPF in Multi-access Networks
 Manipulating the election process
-If you want to influence the election of DR & BDR then ...
OSPF in Multi-access Networks
OSPF Interface Priority
 Manipulating the DR/BDR election process continued
–Use the ip osp...
More OSPF Configuration
Redistributing an OSPF Default Route
 Topology includes a link to ISP
–Router connected to ISP
C...
More OSPF Configuration
Fine-Tuning OSPF
 Modifying OSPF timers
–Reason to modify timers
Faster detection of network fai...
Summary
 Link State Routing protocols are also known as Shortest
Path First protocols
 Summarizing the link state proces...
Summary
 Link
An interface on the router
 Link State
Information about an interface such as
-IP address
-Subnet mask
-Ty...
Summary
 RFC 2328 describes OSPF link state concepts and
operations
 OSPF Characteristics
–A commonly deployed link stat...
Summary
 OSPF Characteristics
–Metric = cost
Lowest cost = best path

 Configuration
–Enable OSPF on a router using the...
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Exploration routing chapter 10 & 11

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Exploration routing chapter 10 & 11

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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 adjacencyNo link state information exchanged Inaccurate SPF trees & routing tables ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 24
  25. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

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