Abstract of CCNA study guide-30 - OSPF 4

Troubleshooting OSPF

This section will have you verify sample OSPF configurations and configuration outputs in order to troubleshoot, maintain, and fix OSPF-related issues.

Problem 1:

If you see a configuration as shown here, you must know that there is no way a router will accept this input because the wildcard is incorrect:

Router(config)#router ospf 1

Router(config-router)#network 10.0.0.0 255.0.0.0 area 0

This would be correct statement:

Router(config)#router ospf 1

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

Problem 2:

look at a figure and determine which of the routers will become the designated router of the area. 

 All the router OSPF priorities are at the default.

Notice the RIDs of each router. The routers with the highest RIDs are routers A and B, since they have the highest IP addresses. RouterB should be the DR and RouterA should be the BDR. Okay, now here’s the thing: Since elections do not occur on point-to-point links by default, RouterB is the best answer.

Problem 3:

Let’s use another command to verify an OSPF configuration: the show ip ospf interface command. Look at the following output for routers A and B and see if you can determine why the two directly connected routers cannot establish an adjacency:

RouterA#sh ip ospf interface e0/0

Ethernet0/0 is up, line protocol is up

Internet Address 172.16.1.2/16, Area 0

Process ID 2, Router ID 172.126.1.1, Network Type BROADCAST, Cost: 10

Transmit Delay is 1 sec, State DR, Priority 1

Designated Router (ID) 172.16.1.2, interface address 172.16.1.1

No backup designated router on this network

Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5

RouterB#sh ip ospf interface e0/0

Ethernet0/0 is up, line protocol is up

Internet Address 172.16.1.1/16, Area 0

Process ID 2, Router ID 172.126.1.1, Network Type BROADCAST, Cost: 10

Transmit Delay is 1 sec, State DR, Priority 1

Designated Router (ID) 172.16.1.1, interface address 172.16.1.2

No backup designated router on this network

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Everything in the two outputs looks pretty good, except that the Hello and Dead timers are not the same. RouterA has Hello and Dead timers of 5 and 20, and RouterB has Hello and Dead timers of 10 and 40, which are the default timers for OSPF.

Problem 4:

Take a look at the network shown in Figure 7.8 with four routers and two different routing protocols.

If all parameters are set to default and redistribution is not configured, which path do you think RouterA will use to reach RouterD? Since IGRP has an AD of 100 and OSPF has an AD of 110, RouterA will send packets to RouterD through RouterC.

Problem 5:

Study Figure 7.9 carefully. You are running OSPF on the routers and an ISDN link provides connectivity to the remote sales office.

What type of route should be configured on the Corporate router to connect to the sales office’s remote network while minimizing network overhead on the ISDN link as shown in Figure 7.9?

The only way we can do that is to create a static route on the Corporate router to connect to the remote network; anything else would be too bandwidth intensive.

Configuring EIGRP and OSPF Summary Routes

This section will provide you with the commands to summarize both EIGRP and OSPF.

Figure 7.10 shows six networks with four block sizes of 4 (WAN links) and two block sizes of 8 (LAN connections). The network address used is 192.168.10.64 and with a block size of 32, the mask would be 255.255.255.224. On the core router, for EIGRP we’ll place the summary route on Ethernet0, which will advertise our summary route out to the backbone network (10.10.10.0 network). This will stop all six of our networks from being advertised individually and instead advertise them as one route to the other routers in the internetwork. Here is the complete configuration of EIGRP on the core router:

Core#config t

Core(config)#router eigrp 10

Core(config-router)#network 192.168.10.0

Core(config-router)#network 10.0.0.0

Core(config-router)#no auto-summary

Core(config-router)#interface ethernet 0

Core(config-if)#ip summary-address eigrp 10 192.168.10.64 255.255.255.224

The above EIGRP configuration for autonomous system 10 advertises directly connected networks 192.168.10.0 and 10.0.0.0. Since EIGRP auto-summarizes at classful boundaries, you must use the no auto-summary command as well. The summary route we will advertise to the backbone network is placed on the interface connected to the backbone, not under the routing process. This summary route tells EIGRP to find all networks in the 192.168.10.64 network with a block size of 32 and advertise them as one route out interface E0. This means, basically, that any packet with a destination IP address of 192.168.10.64 through

192.168.10.95 will be forwarded via this summary route.

To summarize the contiguous network with OSPF we used with the EIGRP example we need to configure OSPF into multiple areas, as shown in Figure 7.11.

To summarize area 1 into the area 0 backbone, use the following command under the OSPF Process ID. Here is the complete OSPF configuration for the Core (backbone) router:

Core#config t

Core(config)#router ospf 1

Core(config-router)#network 192.168.10.64 0.0.0.3 area 1

Core(config-router)#network 192.168.10.68 0.0.0.3 area 1

Core(config-router)#network 10.10.10.0 0.0.0.255 area 0

Core(config-router)#area 1 range 192.168.10.64 255.255.255.224

The no auto-summary command is not needed since OSPF does not summarize at any boundary by default. The preceding OSPF configuration will summarize all the networks from area 1 to the backbone area as one entry of 192.168.10.64/27.