Chapter_11_OSPF

OSPF
www.Athena.Edu.Vn
Routing Protocols and Concepts – Chapter 11
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.
Introduction
Introduction to OSPF
Background of OSPF
Began in 1987
1989 OSPFv1 released in RFC 1131
This version was experimental & never deployed
1991 OSPFv2 released in RFC 1247
1998 OSPFv2 updated in RFC 2328
1999 OSPFv3 published in RFC 2740
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
Introduction to OSPF
OSPF Message Encapsulation
Data link frame header
Contains - Source MAC address and Destination MAC address
Introduction to OSPF
OSPF Packet Types
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
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
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
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
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
Introduction to OSPF
Administrative Distance
Default Administrative Distance for OSPF is 110
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
Basic OSPF Configuration
Lab Topology
Topology used for this chapter
Discontiguous IP addressing scheme
Since OSPF is a classless routing protocol the subnet mask is configured in
Basic OSPF Configuration
The router ospf command
To enable OSPF on a router use the following command
R1(config)#router ospf process-id
Process id
A locally significant number between 1 and 65535
This means it does not have to match other OSPF routers
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
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
Basic OSPF Configuration
OSPF Router ID
Commands used to verify current router ID
Show ip protocols
Show ip ospf
Show ip ospf interface
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 ospfprocess-id
Router(config-router)#router-idip-address
Modifying the Router ID
Use the command Router#clear ip ospf process
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
Basic OSPF Configuration
Verifying OSPF - Additional Commands
Basic OSPF Configuration
Examining the routing table
Use the show ip route command to display the routing table
An “O’ at the beginning of a route indicates that the router source is OSPF
Note OSPF does not automatically summarize at major network boundaries
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
OSPF Metric
COST of an OSPF route
Is the accumulated value from one router to the next
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
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)#bandwidthbandwidth-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

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
OSPF and Multiaccess Networks
Challenges in Multiaccess Networks
OSPF defines five network types:
Point-to-point
Broadcast Multiaccess
Nonbroadcast Multiaccess (NBMA)
Point-to-multipoint
Virtual links
OSPF in Multiaccess Networks
2 challenges presented by multiaccess networks
Multiple adjacencies
Extensive LSA flooding
OSPF in Multiaccess 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
OSPF in Multiaccess 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
DR others 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
OSPF in Multiaccess Networks
DR/BDR Election Process
DR/BDR elections DO NOT occur in point to point networks
OSPF in Multiaccess Networks
DR/BDR elections will take place on multiaccess networks as shown below
OSPF in Multiaccess Networks
Criteria for getting elected DR/BDR
DR: Router with the highest OSPF interface priority
BDR: Router with the second highest OSPF interface priority
If OSPF interface priorities are equal, the highest router ID is used to break the tie
OSPF in Multiaccess 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
OSPF in Multiaccess 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
OSPF in Multiaccess 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
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
More OSPF Configuration
Fine-Tuning OSPF
Since link speeds are getting faster it may be necessary to change reference bandwidth values
Do this using the auto-cost reference-bandwidth command
Example:
R1(config-router)#auto-cost reference-bandwidth 10000
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
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
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
Summary
Verifying OSPF configuration
Use the following commands:
show ip protocol
show ip route
show ip ospf interface
show ip ospf neighbor