Network Security Notes: Network Protocols: OSPF Protocol on CISCO Routing Protocols and Concepts
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I have already posted about Understanding OSPF Protocol. And here this post, you can find out more about OSPF on CISCO Routing Protocols and Concepts Chapter 13...
Let me quote some:
Exploration Routing Protocols and ConceptsChapter 11 - Presentation Transcript
1. OSPF Routing Protocols and Concepts – Chapter 11
2. Objectives
* 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 in multiaccess networks.
* Describe the uses of additional configuration commands in OSPF.
3. Introduction
4. Introduction to OSPF
* Background of OSPF
o Began in 1987
o 1989 OSPFv1 released in RFC 1131
o This version was experimental & never deployed
o 1991 OSPFv2 released in RFC 1247
o 1998 OSPFv2 updated in RFC 2328
o 1999 OSPFv3 published in RFC 2740
5. Introduction to OSPF
* OSPF Message Encapsulation
o OSPF packet type
+ There exist 5 types
o OSPF packet header
+ Contains - Router ID and area ID and Type code for OSPF packet type
o IP packet header
+ Contains - Source IP address, Destination IP address, & Protocol field set to 89
6. Introduction to OSPF
* OSPF Message Encapsulation
o Data link frame header
o Contains - Source MAC address and Destination MAC address
7. Introduction to OSPF
* OSPF Packet Types
8. Introduction to OSPF
* Hello Protocol
* OSPF Hello Packet
o 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 d esignated r outer and a b ackup d esignated r outer
9. Introduction to OSPF
* Hello Packets continued
o Contents of a Hello Packet router ID of transmitting router
* OSPF Hello Intervals
o Usually multicast (224.0.0.5)
o Sent every 30 seconds for NBMA segments
* OSPF Dead Intervals
o This is the time that must transpire before the neighbor is considered down
o Default time is 4 times the hello interval
10. Introduction to OSPF
* Hello protocol packets contain information that is used in electing
o Designated Router (DR)
+ DR is responsible for updating all other OSPF routers
o Backup Designated Router (BDR)
+ This router takes over DR’s responsibilities if DR fails
11. Introduction to OSPF
* OSPF Link-state Updates
o Purpose of a Link State Update (LSU)
+ Used to deliver link state advertisements
o Purpose of a Link State Advertisement (LSA)
+ Contains information about neighbors & path costs
12. Introduction to OSPF
* OSPF Algorithm
* OSPF routers build & maintain link-state database containing LSA received from other routers
o Information found in database is utilized upon execution of Dijkstra SPF algorithm
o SPF algorithm used to create SPF tree
o SPF tree used to populate routing table
13. Introduction to OSPF
* Administrative Distance
o Default Administrative Distance for OSPF is 110
14. Introduction to OSPF
* OSPF Authentication
o Purpose is to encrypt & authenticate routing information
o This is an interface specific configuration
o Routers will only accept routing information from other routers that have been configured with the same password or authentication information
15. Basic OSPF Configuration
* Lab Topology
* Topology used for this chapter
o Discontiguous IP addressing scheme
o Since OSPF is a classless routing protocol the subnet mask is configured in
16. Basic OSPF Configuration
* The router ospf command
* To enable OSPF on a router use the following command
o R1(config)# router ospf process-id
o Process id
+ A locally significant number between 1 and 65535
+ This means it does not have to match other OSPF routers
17. Basic OSPF Configuration
* OSPF network command
o 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
o Example: Router(config-router)# network network-address wildcard-ask area area-id
18. Basic OSPF Configuration
* Router ID
o This is an IP address used to identify a router
o 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
19. Basic OSPF Configuration
* OSPF Router ID
* Commands used to verify current router ID
o Show ip protocols
o Show ip ospf
o Show ip ospf interface
20. Basic OSPF Configuration
* OSPF Router ID
* Router ID & Loopback addresses
o Highest loopback address will be used as router ID if router-id command isn’t used
o Advantage of using loopback address
+ The loopback interface cannot fail OSPF stability
* The OSPF router-id command
o Introduced in IOS 12.0
o Command syntax
+ Router(config)#router ospfprocess-id
+ Router(config-router)#router-idip-address
* Modifying the Router ID
o Use the command Router #clear ip ospf process
21. Basic OSPF Configuration
* Verifying OSPF
* Use the show ip ospf command to verify & trouble shoot OSPF networks
* Command will display the following:
o 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
22. Basic OSPF Configuration
* Verifying OSPF - Additional Commands
Displays hello interval and dead interval Show ip ospf interface Displays OSPF process ID, router ID , OSPF area information & the last time SPF algorithm calculated Show ip ospf Displays OSPF process ID, router ID , networks router is advertising & administrative distance Show ip protocols Description Command
23. Basic OSPF Configuration
* Examining the routing table
* Use the show ip route command to display the routing table
o An “O’ at the beginning of a route indicates that the router source is OSPF
o Note OSPF does not automatically summarize at major network boundaries
24. OSPF Metric
* OSPF uses cost as the metric for determining the best route
o The best route will have the lowest cost
o Cost is based on bandwidth of an interface
+ Cost is calculated using the formula
# 10 8 / bandwidth
o Reference bandwidth
+ Defaults to 100Mbps
+ Can be modified using
+ Auto-cost reference-bandwidth command
25. OSPF Metric
* COST of an OSPF route
o Is the accumulated value from one router to the next
26. OSPF Metric
* Usually the actual speed of a link is different than the default bandwidth
o 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
o Most serial link default to 1.544Mbps
27. Basic OSPF Configuration
* Modifying the Cost of a link
* Both sides of a serial link should be configured with the same bandwidth
o Commands used to modify bandwidth value
+ Bandwidth command
# Example: Router(config-if)# bandwidth bandwidth-kbp s
+ 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
28. Basic OSPF Configuration
* Modifying the Cost of the link
* Difference between bandwidth command & the ip ospf cost command
o Ip ospf cost command
+ Sets cost to a specific value
o Bandwidth command
+ Link cost is calculated
29. OSPF and Multiaccess Networks
* Challenges in Multiaccess Networks
* OSPF defines five network types:
o Point-to-point
o Broadcast Multiaccess
o Nonbroadcast Multiaccess (NBMA)
o Point-to-multipoint
o Virtual links
30. OSPF in Multiaccess Networks
* 2 challenges presented by multiaccess networks
o Multiple adjacencies
o Extensive LSA flooding
31. OSPF in Multiaccess Networks
* Extensive flooding of LSAs
o For every LSA sent out there must be an acknowledgement of receipt sent back to transmitting router
o Consequence: lots of bandwidth consumed and chaotic traffic
32. OSPF in Multiaccess Networks
* Solution to LSA flooding issue is the use of
o Designated router (DR)
o Backup designated router (BDR)
* DR & BDR selection
o Routers are elected to send & receive LSA
* Sending & Receiving LSA
o DR others send LSAs via multicast 224.0.0.6 to DR & BDR
o DR forward LSA via multicast address 224.0.0.5 to all other routers
33. OSPF in Multiaccess Networks
* DR/BDR Election Process
o DR/BDR elections DO NOT occur in point to point networks
34. OSPF in Multiaccess Networks
* DR/BDR elections will take place on multiaccess networks as shown below
35. OSPF in Multiaccess Networks
* Criteria for getting elected DR/BDR
o DR: Router with the highest OSPF interface priority
o BDR : Router with the second highest OSPF interface priority
o If OSPF interface priorities are equal , the highest router ID is used to break the tie
36. OSPF in Multiaccess Networks
* Timing of DR/BDR Election
o Occurs as soon as 1 st 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
37. OSPF in Multiaccess Networks
* Manipulating the election process
o 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
38. OSPF in Multiaccess Networks
* OSPF Interface Priority
* Manipulating the DR/BDR election process continued
o Use the ip ospf priority interface command.
o 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
39. More OSPF Configuration
* Redistributing an OSPF Default Route
* Topology includes a link to ISP
o 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
40. More OSPF Configuration
* Fine-Tuning OSPF
* Since link speeds are getting faster it may be necessary to change reference bandwidth values
o Do this using the auto-cost reference-bandwidth command
o Example:
+ R1(config-router)# auto-cost reference-bandwidth 10000
41. More OSPF Configuration
* Fine-Tuning OSPF
* Modifying OSPF timers
o Reason to modify timers
+ Faster detection of network failures
o Manually modifying Hello & Dead intervals
+ Router(config-if)# ip ospf hello-interval seconds
+ Router(config-if)# ip ospf dead-interval seconds
o Point to be made
+ Hello & Dead intervals must be the same between neighbors
42. Summary
* RFC 2328 describes OSPF link state concepts and operations
* OSPF Characteristics
o A commonly deployed link state routing protocol
o Employs DR s & BDR s on multi-access networks
+ DRs & BDRs are elected
+ DR & BDRs are used to transmit and receive LSAs
o Uses 5 packet types:
+ 1: HELLO
+ 2: D ATA B ASE D ESCRIPTION
+ 3: L INK S TATE R EQUEST
+ 4: L INK S TATE U PDATE
+ 5: L INK S TATE A CKNOWLEDGEMENT
43. Summary
* OSPF Characteristics
o Metric = cost
+ Lowest cost = best path
* Configuration
o Enable OSPF on a router using the following command
+ R1(config)# router ospf process-id
o 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
44. Summary
* Verifying OSPF configuration
o Use the following commands:
+ show ip protocol
+ show ip route
+ show ip ospf interface
+ show ip ospf neighbor
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