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Configure, verify, and troubleshoot OSPF

Open Shortest Path First (OSPF) is an open standard routing protocol that’s been implemented by a wide variety of network vendors, including Cisco. If you have multiple routers and not all of them are Cisco (what!), then you can’t use EIGRP, can you? So, your remaining CCNA

objective options are basically RIP, RIPv2, and OSPF.

OSPF works by using the Dijkstra algorithm. First, a shortest path tree is constructed, and then the routing table is populated with the resulting best paths. OSPF converges quickly, although perhaps not as quickly as EIGRP, and it supports multiple, equal-cost routes to the same destination. Like EIGRP, it does support both IP and IPv6 routed protocols.

OSPF provides the following features:

 Consists of areas and autonomous systems

 Minimizes routing update traffic

 Allows scalability

 Supports VLSM/CIDR

 Has unlimited hop count

 Allows multi-vendor deployment (open standard)

OSPF is the first link-state routing protocol that most people are introduced to, so it’s useful to see how it compares to more traditional distance-vector protocols such as RIPv2 and RIPv1. Table 4.6 gives you a comparison of these three protocols.

OSPF has many features beyond the few I’ve listed in Table 4.6, and all of them contribute to a fast, scalable, and robust protocol that can be actively deployed in thousands of production networks.

OSPF is supposed to be designed in a hierarchical fashion, which basically means that you can separate the larger internetwork into smaller internetworks called areas. This is the best design for OSPF.

The following are reasons for creating OSPF in a hierarchical design:

 To decrease routing overhead

 To speed up convergence

 To confine network instability to single areas of the network

This does not make configuring OSPF easier, but more elaborate and difficult. Figure 4.8 shows a typical OSPF simple design. Notice how each router connects to the backbone—called area 0, or the backbone area. OSPF must have an area 0, and all other areas should connect to this area. Routers that connect other areas to the backbone area within an AS are called Area Border Routers (ABRs). Still, at least one interface of the ABR must be in area 0.

FIGURE 4.8 OSPF design example

OSPF runs inside an autonomous system, but it can also connect multiple autonomous systems. The router that connects this ASs is called an Autonomous System Boundary Router (ASBR). Ideally, you would create other areas of networks to help keep route updates to a minimum and to keep problems from propagating throughout the network.

Configuring OSPF Areas

After identifying the OSPF process, you need to identify the interfaces that you want to activate OSPF communications on as well as the area in which each resides. This will also configure the networks you’re going to advertise to others. OSPF uses wildcards in the configuration—which are also used in access-list configurations.

Here’s an OSPF basic configuration example for you:

Lab_A#config t

Lab_A(config)#router ospf 1

Lab_A(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

Lab_A(config-router)#network 10.0.0.0 0.255.255.255 area 0

Remember, the OSPF Process ID number is irrelevant. It can be the same on every router on the network, or it can be different—doesn’t matter. It’s locally significant and just enables the OSPF routing on the router.

We’re going to start by taking a quick look at the routing table of the Corp router: So, let’s issue a show ip route command on the Corp router:

10.0.0.0/24 is subnetted, 12 subnets

O 10.1.10.0 [110/65] via 10.1.5.2, 00:01:31, Serial0/2/0 O 10.1.9.0 [110/74] via 10.1.4.2, 00:01:31, Serial0/1/0 O 10.1.8.0 [110/65] via 10.1.4.2, 00:01:31, Serial0/1/0 O 10.1.12.0 [110/66] via 10.1.5.2, 00:01:31, Serial0/2/0 C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.2.0 is directly connected, Serial0/0/0 C 10.1.1.0 is directly connected, FastEthernet0/1

O 10.1.7.0 [110/74] via 10.1.3.2, 00:01:32, Serial0/0/1 [110/74] via 10.1.2.2, 00:01:32, Serial0/0/0

O 10.1.6.0 [110/74] via 10.1.3.2, 00:01:32, Serial0/0/1 [110/74] via 10.1.2.2, 00:01:32, Serial0/0/0

C 10.1.5.0 is directly connected, Serial0/2/0 C 10.1.4.0 is directly connected, Serial0/1/0

The Corp router shows the found routes for all 12 of our networks, with the O representing OSPF internal routes (the Cs are obviously our directly connected networks). It also found the dual routes to networks 10.1.6.0 and 10.1.7.0. I removed the bandwidth and delay commands from under the interface, so the defaults are being used to determine the metric. But remember, OSPF only uses bandwidth to determine the best path to a network.

The show ip ospf Command

The show ip ospf command is used to display OSPF information for one or all OSPF processes running on the router. Information contained therein includes the Router ID, area information, SPF statistics, and LSA timer information. Let’s check out the output from the Corp router:

Corp#sh ip ospf

Routing Process "ospf 132" with ID 10.1.5.1

Start time: 04:32:04.116, Time elapsed: 01:27:10.156 Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS) Supports area transit capability

Router is not originating router-LSAs with maximum metric Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs Maximum wait time between two consecutive SPFs 10000 msecs Incremental-SPF disabled

Minimum LSA interval 5 secs Minimum LSA arrival 1000 msecs LSA group pacing timer 240 secs

Interface flood pacing timer 33 msecs Retransmission pacing timer 66 msecs

Number of external LSA 0. Checksum Sum 0x000000 Number of opaque AS LSA 0. Checksum Sum 0x000000 Number of DCbitless external and opaque AS LSA 0 Number of DoNotAge external and opaque AS LSA 0

External flood list length 0 Area BACKBONE(0)

Number of interfaces in this area is 5 Area has no authentication

SPF algorithm last executed 00:14:52.220 ago SPF algorithm executed 14 times

Area ranges are

Number of LSA 6. Checksum Sum 0x03C06F

Number of opaque link LSA 0. Checksum Sum 0x000000 Number of DCbitless LSA 0

Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0

Notice the Router ID (RID) of 10.1.5.1, which is the highest IP address configured on the router. The show ip ospf database Command

Using the show ip ospf database command will give you information about the number of routers in the internetwork (AS) plus the neighboring router’s ID (this is the topology database I

mentioned earlier). Unlike the show ip eigrp topology command, this command shows the “OSPF routers,” not each and every link in the AS as EIGRP does.

The output is broken down by area. Here’s a sample output, again from Corp:

Corp#sh ip ospf database

OSPF Router with ID (10.1.5.1) (Process ID 132) Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count 10.1.5.1 10.1.5.1 72 0x80000002 0x00F2CA 9 10.1.7.1 10.1.7.1 83 0x80000004 0x009197 6 10.1.9.1 10.1.9.1 73 0x80000001 0x00DA1C 4 10.1.11.1 10.1.11.1 67 0x80000005 0x00666A 4 10.1.12.1 10.1.12.1 67 0x80000004 0x007631 2 Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

10.1.11.2 10.1.12.1 68 0x80000001 0x00A337

You can see all five routers and the RID of each router (the highest IP address on each router). The router outputs shows the link ID—remember that an interface is also a link—and the RID of the router on that link under the ADV router, or advertising router.

The show ip ospf interface command displays all interface-related OSPF information. Data is displayed about OSPF information for all interfaces or for specified interfaces. (I’ll bold some of the important things.)

Corp#sh ip ospf interface f0/1

FastEthernet0/1 is up, line protocol is up Internet Address 10.1.1.1/24, Area 0

Process ID 132, Router ID 10.1.5.1, Network Type BROADCAST, Cost: 1 Transmit Delay is 1 sec, State DR, Priority 1

Designated Router (ID) 10.1.5.1, Interface address 10.1.1.1 No backup designated router on this network

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 oob-resync timeout 40

Hello due in 00:00:01

Supports Link-local Signaling (LLS) Index 1/1, flood queue length 0 Next 0x0(0)/0x0(0)

Last flood scan length is 0, maximum is 0

Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 0, Adjacent neighbor count is 0 Suppress hello for 0 neighbor(s)

The following information is displayed by this command:

 Interface IP address  Area assignment  Process ID  Router ID  Network type  Cost  Priority

 DR/BDR election information (if applicable)

 Hello and Dead timer intervals

 Adjacent neighbor information

The reason I used the show ip ospf interface f0/1 command is that I knew that there would be a designated router elected on the FastEthernet broadcast multi-access network. We’ll get into DR and DBR elections in detail in a minute.

The show ip ospf neighbor Command

The show ip ospf neighbor command is super-useful because it summarizes the pertinent OSPF information regarding neighbors and the adjacency state. If a DR or BDR exists, that information will also be displayed. Here’s a sample:

Corp#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface 10.1.11.1 0 FULL/ - 00:00:37 10.1.5.2 Serial0/2/0 10.1.9.1 0 FULL/ - 00:00:34 10.1.4.2 Serial0/1/0 10.1.7.1 0 FULL/ - 00:00:38 10.1.3.2 Serial0/0/1 10.1.7.1 0 FULL/ - 00:00:34 10.1.2.2 Serial0/0/0

This is a super-important command to understand because it’s extremely useful in production networks. Let’s take a look at the R3 and 871W routers outputs:

R3#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface 10.1.5.1 0 FULL/ - 00:00:39 10.1.5.1 Serial0/0/1

10.1.11.2 1 FULL/BDR 00:00:31 10.1.11.2 FastEthernet0/1 871W#sh ip ospf nei

Neihbor ID Pri State Dead Time Address Interface 10.1.11.1 1 FULL/DR 00:00:30 10.1.11.1 Vlan1

Since there’s an Ethernet link (broadcast multi-access) on the Corp router, there’s going to be an election to determine who will be the designated router and who will be the non-designated router. You can see that the 871W became the designated router, and it won because it had the highest IP address on the network. You can change this, but that’s the default.

The reason that the Corp connections to R1, R2, and R3 don’t have a DR or BDR listed in the output is that by default, elections don’t happen on point-to-point links. But you can see that the Corp router is fully adjacent to all three routers (and on both connections to R1) from its output.