1. Instructor & Todd Lammle
Sybex CCENT
Chapter 9: Open Shortest Path First (OSPF)
Instructor & Todd Lammle

2. Chapter 9 Objectives IP Routing Technologies
The CCENT Topics Covered in this chapter include:
IP Routing Technologies
Configure and verify OSPF (single area)
Benefit of single area
Configure OSPF v2
Router ID
Passive interface 2

3. Open Shortest Path First (OSPF)
OSPF uses Dijkstra algorithm to construct a shortest path tree
Then, OSPF populates the routing table with the resulting shortest path
It supports multiple, equal-cost routes to the same destination

4. OSPF best feature summary

5. OSPF vs. RIPv2 vs. RIPv1

6. Figure 9.1: OSPF design example.
An OSPF hierarchical design minimizes routing table entries and keeps the impact of any topology changes contained within a specific area.

7. OSPF terminology Link Router ID
A link is a network or router interface assigned to any given network
When an interface is added to the OSPF process, it’s considered to be a link
This link has up or down state information and one or more IP addresses
Router ID
RID is an IP address used to identify the router
The highest IP address of all configured loopback interfaces is selected as RID
Or the highest IP address of all active physical interface is selected as RID

8. OSPF terminology 2 Neighbor Two router that are connected a PPP link
OSPF neighbors must have the same configuration to establish a neighborhood relationship
Area ID
Stub area flag
Authentication password (if using one)
Hello and Dead intervals

9. OSPF terminology 2 Adjacency Designated Router (DR)
A relationship between 2 OSPF routers that permits direct exchange of route updates
Not all neighbors become adjacency
Designated Router (DR)
A DR is elected whenever OSPF routers are connected to the same broadcast network
To minimize the number of adjacencies formed
To publicize received routing information to and from the remaining routers
The router with the highest priority is the winner

10. OSPF terminology 3 Backup Designated Router (BDR) Hello protocol
A hot standby for the DR
BDR receives OSPF updates from all adjacent routers but doesn’t disperse Link State Advertisement(LSA) updates
Hello protocol
Provides dynamic neighbor discovery and maintain neighbor relationships
Hello packets and LSAs build and maintain the topological database
Hello packets are addressed to multicast address:

11. OSPF terminology 4 Neighborship database Topological database
A list of all OSPF routers for which Hello packets have been seen
A variety of details are maintained on each router in the DB
Topological database
Contains information from all of the LSA packets
The information is used as input to Dijkstra algorithm

12. OSPF terminology 5 OSPF areas
A grouping of contiguous networks and routers
Enhances scalability of OSPF
All of the routers within the same area have the same topology table
All routers in the same area share a common area ID
The area ID is associated with specific interfaces on the router
A router can be a member of multiple areas at a time
Area 0 is the backbone area
Broadcast (multi-access)
In OSPF, a DR and BDR must be elected for each broadcast network

13. OSPF operation
OSPF operation is basically divided into three categories:
Neighbor and adjacency initialization
Hello protocol
LSA flooding
SPF tree calculation

14. Figure 9.2: The Hello protocol
The Hello protocol is used to discover neighbors, establish adjacencies, and maintain relationships with other OSPF routers. Hello packets are periodically sent out each enabled OSPF interface and in environments that support multicast.
Hello packets are periodically sent out of each enabled OSPF interface and in environments that support multicast

15. LSA flooding The method of OSPF uses to share routing information
LSA information containing link-state data is shared with all OSPF routers within an area
A network topology is created from the LSA updates
Efficient flooding is achieved through the use of a reserved multicast address

16. Table 9.2: LSA update multicast addresses
Network Type
Multicast Address
Description
Point-to-point
AllSPFRouters
Broadcast
AllDRouters
Point-to-multipoint NA
The network type determines the multicast address used for sending updates. Table 9.2 contains the multicast addresses associated with LSA flooding. Point-to-multipoint networks use the adjacent router’s unicast IP address.

17. Configuring OSPF
Enabling OSPF
Configuring OSPF areas

18. Enabling OSPF
The easiest and also least scalable way to configure OSPF is to just use a single area. Doing this requires a minimum of two commands.
The first command is used to activate the OSPF routing process is as follows:
Router(config)#router ospf ?
< > Process ID
A value in the range from 1 to 65,535 identifies the OSPF process ID.

19. Configuring OSPF Areas
Here’s an example of basic OSPF basic configuration for you, showing our 2nd minimum command needed, the network command:
Router#config t
Router(config)#router ospf 1
Router(config-router)#network area ?
< > OSPF area ID as a decimal value
A.B.C.D OSPF area ID in IP address format
Router(config-router)#network area 0
The areas can be any number from 0 to 4.2 billion. Don’t get these numbers confused with the process ID, which ranges from 1 to 65,535.

20. Configuring OSPF Areas 2
The network command has 2 arguments
Network number ( )
Wildcard mask ( )
Always one less than the block size
OSPF will use this command to find any interface on the router configured in the network and will place any interface it finds into area 0

21. Wildcard Example
In this scenario, you have a router with these four subnets connected to four different interfaces:
/28
/28
/28
/30
All interfaces need to be in area 0, so it seems to me the easiest configuration would look like this:
Test#config t
Test(config)#router ospf 1
Test(config-router)#network area 0
Let’s create a separate network statement for each interface using the subnet numbers and wildcards. Test#config t
Test(config)#router ospf 1
Test(config-router)#network area 0
Test(config-router)#network area 0
Test(config-router)#network area 0
Test(config-router)#network area 0

22. Figure 9.3: Sample OSPF wildcard configuration
Here’s the OSPF configuration using wildcards:
Lab_A#config t
Lab_A(config)#router ospf 1
Lab_A(config-router)#network area 0
Lab_A(config-router)#network area 0
The Lab_A router is using a /29, or , mask on the Fa0/0 interface. This is a block size of 8, which is a wildcard of 7. The G0/0 interface is a mask of —block size of 4, with a wildcard of 3. Notice that I typed in the subnet number, not the interface IP address.

23. Figure 9.4: Our new network layout

24. Corp router’s configuration
means that the IP address must precisely match each octet

25. SF router’s configuration

26. LA router’s configuration

27. Figure 9.5: Adding a non-OSPF network to LA router
We can use the same command that we did under that routing process here as well! Take a look:
LA(config)#router ospf 100
LA(config-router)#passive-interface fastEthernet 0/1
I added this command as an example on interface Fa0/1, which happens to be an interface we’re not using in this network because I want OSPF to work on my other router’s interfaces.

28. Corp router’s configuration cont.
Configure Corp router to advertise a default route to the SF and LA routers

29. OSPF and Loopback Interfaces
Loopback interfaces are logical interfaces
Vital to configure loopback interfaces when using OSPF
Ensure that an interface is always active and available for OSPF process
Come in handy for diagnostic purposes
The highest IP address of any logical interface becomes the router’s RID

30. Figure 9.6: OSPF router ID (RID)
The RID is not only used to advertise routes, it’s also used to elect the designated router (DR) and the backup designated router (BDR). These designated routers create adjacencies when a new router comes up and exchanges LSAs to build topological databases.

31. Configuring Loopback Interfaces

32. Configuring Loopback Interfaces cont.
Any IP address can be used as long as the addresses are never the same on any two routers
The /32 mask is called a host mask and works fine for loopback interfaces
Also allows us to save subnets
Create logical interfaces before you start OSPF routing
The loopback interface would always become your RID

33. Verifying OSPF Configuration
Show ip route, show ip int brief
Show ip ospf
Displays OSPF information for one or all OSPF processes running on a router
Show ip ospf database
Gives you information about the number of routers in the AS plus the neighboring router’s ID—the topology database
Show ip ospf interface
Reveals all interface related information
Show ip ospf neighbor
Summarizes the pertinent OSPF information regarding neighbors and the adjacency state
Show ip protocols
Provides an overview of the actual operation of all currently running protocols

34. Written Labs and Review Questions
Read through the Exam Essentials section together in class
Open your books and go through all the written labs and the review questions.
Review the answers in class. 34