Cisco Certification 640-802: Open Shortest Path First – OSPF Part II

Now on to part II of our CCNA certification topic of OSPF.

OSPF however, can be a challenge in Multiaccess Networks. Creating adjacencies for every two routers in your network can result to a great number of unnecessary adjacencies and this leads to LSA flooding. To better understand the result, the formula to figure out how many adjacencies will be creating in this scenario is (n stands for the number of routers): n(n-1)/2. Making the calculations for a 5 router scenario this gives us 10. That’s not that much, but let’s do the math for 20 routers: 20(20-1)/2 = 190. Now try to imagine what’s happening when the routers are sending out LSAs. The solution for this problem is to elect a Designated Router (DR).

Designated Routers (DR) is a router elected to distribute the LSAs. This way, large scale networks can avoid LSA floods. After the election of the DR a Backup Designated Router (BDR) is also elected, in case of DR failure. With a DR present in your network, the other routers, instead of flooding LSAs around the network, they only send them to the DR and the BDR using the multicast address 224.0.0.6 ( also known as ALLDRouters). After the DR receives the LSA, it sends it to the other routers in the network using the 224.0.0.5 (also known as ALLSPFRouters).

The election process is a fairly simple one. The DR is the router with the highest OSPF interface priority, the BDR is the one with second highest OSPF interface priority. If their interface priorities are equal, the router with the highest router ID wins. The show ip ospf neighbor command shows the status of the routers.

Router# show ip ospf neighbor                                                      

   ID          Pri   State        Dead Time     Address         Interface       

10.199.199.137  1    FULL/DR       0:00:31    192.168.80.37      Ethernet0       

172.16.48.1     1    FULL/DROTHER  0:00:33    172.16.48.1        Fddi0          

172.16.48.200   1    FULL/DROTHER  0:00:33    172.16.48.200      Fddi0           

10.199.199.137  5    FULL/DR       0:00:33    172.16.48.189      Fddi0          

The DR router should be a powerful one to handle it’s job. In almost every network you will see many models, some newer, some older. To make sure the router you want will become DR, you can manually set the priority with the ip ospf priority priority interface command (priority is between 0 and 255).

In some cases you may find that the update interval of Hello and Dead packets it’s not appropriate for your network. In this case you can adjust them with the ip ospf hello-interval seconds and ip ospf dead-interval seconds interface commands.

One of the must-have skills for every network administrator is the ability to use the show commands. Every time when you are in doubt about the good operation of your OSPF network, the show ip ospf command is here to assist you. To check if two routers have become neighbors, you must use the show ip ospf neighbors command. Also, show ip ospf interface interface-name will give you information about Hello and Dead times.

1)   show ip protocols

Router#show ip protocols                                                        

Routing protocol is “ospf 1”                                                       

  Outgoing update filter list for all interfaces is not set                       

  Incoming update filter list for all interfaces is not set                          

  Router ID 192.168.0.1                                                         

  Number of areas in this router is 1. 1 normal 0 stub 0 nssa                   

  Maximum path: 4                                                               

  Routing for Networks:                                                         

    192.168.0.0 0.0.0.255 area 0                                                 

    10.0.0.0 0.0.0.31 area 0                                                    

  Reference bandwidth unit is 100 mbps                                          

    Routing Information Sources:                                                

      Gateway         Distance     Last Update                                  

      192.168.0.2          110     11:29:29                                     

      10.0.0.2             110     11:29:29                                     

    Distance: (default is 110)                                                  

2)   show ip ospf

Router# show ip ospf                                                              

Routing Process “ospf 1″ with ID 192.168.0.1                                    

 Supports only single TOS(TOS0) routes                                          

 Supports opaque LSA                                                            

 Supports Link-local Signaling (LLS)                                            

 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 0×0                                      

 Number of opaque AS LSA 0. Checksum Sum 0×0                                    

 Number of DCbitless external and opaque AS LSA 0                               

 Number of DoNotAge external and opaque AS LSA 0                                 

 Number of areas in this router is 0. 0 normal 0 stub 0 nssa                     

 External flood list length 0                                                    

     Area BACKBONE(0)                                                             

         Number of interfaces in this area is 2                                 

         Area has message digest authentication                                 

         SPF algorithm executed 4 times                                          

         Area ranges are                                                        

         Number of LSA 4. Checksum Sum 0×29BEB                                  

         Number of opaque link LSA 0. Checksum Sum 0×0                           

         Number of DCbitless LSA 3                                              

         Number of indication LSA 0                                             

         Number of DoNotAge LSA 0                                               

         Flood list length 0                                                     

3)   show ip ospf interface interface-name

Router# show ip ospf interface ethernet 0                                          

Ethernet 0 is up, line protocol is up                                                

Internet Address 192.168.0.202, Mask 255.255.255.0, Area 0.0.0.0                 

AS 201, Router ID 192.168.0.1, Network Type BROADCAST, Cost: 10                  

Transmit Delay is 1 sec, State OTHER, Priority 1                                   

Designated Router id 192.168.0.10, Interface address 192.168.0.10                

Backup Designated router id 192.168.0.28, Interface addr 192.168.0.28             

Timer intervals configured, Hello 10, Dead 60, Wait 40, Retransmit 5            

Hello due in 0:00:05                                                             

Neighbor Count is 8, Adjacent neighbor count is 2                                

  Adjacent with neighbor 192.168.0.28  (Backup Designated Router)                 

  Adjacent with neighbor 192.168.0.10  (Designated Router)                      

Every OSPF route in show ip route has an O at the beginning like in the following example:

Router# show ip route                                                                   

Codes: I – IGRP derived, R – RIP derived, O – OSPF derived,                       

       C – connected, S – static, E – EGP derived, B – BGP derived,               

       * – candidate default route, IA – OSPF inter area route,                    

       i – IS-IS derived, ia – IS-IS, U – per-user static route,                    

       o – on-demand routing, M – mobile, P – periodic downloaded static route,     

       D – EIGRP, EX – EIGRP external, E1 – OSPF external type 1 route,           

       E2 – OSPF external type 2 route, N1 – OSPF NSSA external type 1 route,   

       N2 – OSPF NSSA external type 2 route                                       

Gateway of last resort is 10.119.254.240 to network 10.140.0.0                                                                                                   

O E2 172.150.0.0 [160/5] via 10.119.254.6, 0:01:00, Ethernet2                     

E    172.17.10.0 [200/128] via 10.119.254.244, 0:02:22, Ethernet2                

O E2 172.70.132.0 [160/5] via 10.119.254.6, 0:00:59, Ethernet2                   

O E2 10.130.0.0 [160/5] via 10.119.254.6, 0:00:59, Ethernet2                       

E    172.30.0.0 [200/128] via 10.119.254.244, 0:02:22, Ethernet2                     

If OSPF is not correctly configured, you, as a network administrator, should be able to figure out what’s missing or what’s the reason it doesn’t work from the output of the show commands. Study the output of these commands carefully in a working environment. Try to break something, study the output again and compare it with the one before. This way you will be able to recognize errors faster.

We hope you found this CCNA topic covering OSPF useful. We are proud to deliver top notch Cisco CCNA certification articles for you making your CCNA certification exam easier.