Instructor & Todd Lammle Sybex CCENT 100-101 Chapter 9: Open Shortest Path First (OSPF) Instructor & Todd Lammle

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

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

OSPF best feature summary

OSPF vs. RIPv2 vs. RIPv1

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.

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

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

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

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: 224.0.0.5

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

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

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

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

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

Table 9.2: LSA update multicast addresses Network Type Multicast Address Description Point-to-point 224.0.0.5 AllSPFRouters Broadcast 224.0.0.6 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.

Configuring OSPF Enabling OSPF Configuring OSPF areas

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 ? <1-65535> Process ID A value in the range from 1 to 65,535 identifies the OSPF process ID.

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 10.0.0.0 0.255.255.255 area ? <0-4294967295> OSPF area ID as a decimal value A.B.C.D OSPF area ID in IP address format Router(config-router)#network 10.0.0.0 0.255.255.255 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.

Configuring OSPF Areas 2 The network command has 2 arguments Network number (10.0.0.0) Wildcard mask (0.255.255.255) Always one less than the block size OSPF will use this command to find any interface on the router configured in the 10.0.0.0 network and will place any interface it finds into area 0

Wildcard Example In this scenario, you have a router with these four subnets connected to four different interfaces: 192.168.10.64/28 192.168.10.80/28 192.168.10.96/28 192.168.10.8/30 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 192.168.10.0 0.0.0.255 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 192.168.10.64 0.0.0.15 area 0 Test(config-router)#network 192.168.10.80 0.0.0.15 area 0 Test(config-router)#network 192.168.10.96 0.0.0.15 area 0 Test(config-router)#network 192.168.10.8 0.0.0.3 area 0

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 192.168.10.64 0.0.0.7 area 0 Lab_A(config-router)#network 10.255.255.80 0.0.0.3 area 0 The Lab_A router is using a /29, or 255.255.255.248, 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 255.255.255.252—block size of 4, with a wildcard of 3. Notice that I typed in the subnet number, not the interface IP address.

Figure 9.4: Our new network layout

Corp router’s configuration 0.0.0.0 means that the IP address must precisely match each octet

SF router’s configuration

LA router’s configuration

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.

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

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

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.

Configuring Loopback Interfaces

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

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

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