Open Shortest Path First (OSPF)

1.   Link state routing protocol.

2.   Interior gateway protocol.

3.   Non-proprietary routing protocol/Open standard

4.   Classless protocol (shares information of subnet mask).

5.   Supports VLSM, CIDR, Summarization.

6.   Auto –summary disabled (default).

7.   Unlimited hop counts.

8.   Administrative distance 110.

9.   Maximum path 32, default 4.

10.                 Metric is cost, and cost is 100/Bandwidth.

11.                 Performs equal cost load balancing.

12.                 Multicast update packet at 224.0.0.5 & 224.0.0.6

13.                 Uses SPF (Shortest Path First) algorithm to decide best path and backup path.

14.                 Uses concept of process id (1-65535). It is used to uniquely identify OSPF process in a network.

15.                Authentication: NULL, Plain text & MD5 authentication.

16.                OSPF versions:

·     OSPF v2(IPv4)

·     OSPF v3(IPv6)

17.                OSPF timers:

·     Hello: 10 secs.

·     Dead: 40 secs (always 4 times that of hello).

18.                OSPF tables:

·     Neighbor table: contains neighbor information.

·     Database table: contains LSA’s.

·     Routing table: contains best path information.

19.                OSPF states:

·     Down: all process are down. No OSPF neighbors detected at present.

·     Initialize: Hello packet is sent. Hello is sent every 10 secs. Neighbor discovery, neighbor ship formation, as a keep alive check. Dead timer is 40 secs. (4 times).

·     2 way: hello packet is exchanged.

·     Ex-start: elects master and slave on the basis of priority (default = 1). If priority matches then on the basis of router-id. DR BDR election will take place here.

·     Ex-change: exchanges summary database. DBD’s are sent.

·     Loading: exchange of database info between neighbors. LSR’s & LSU’s are sent.

·     Full: full state or fully synchronized.

20.                OSPF packets:

·     Hello: in every 10 secs.

·     Database descriptor: summary of database table is sent.

·     Link state request

·     Link state update

·     Link state acknowledge.

21.                 OSPF network:

·     Single area OSPF network.

·     Multi area OSPF network.

22.                 OSPF area: logical grouping of OSPF router is OSPF area.

23.                 A router can participate in more than one area.

24.                OSPF Areas:

·     Area 0: backbone area or transit area. All other area must be connected with area 0.

·     Regular area: area other than area 0 are regular area.

·     Stub area: area where external routes are not allowed.

25.                 OSPF router types:

·     Backbone router: Router who’s at-least one interface is in area 0.

·     Area border router: Router whose different interfaces are in different areas.

26.                 DR (Designated Router) & BDR(Backup Designated Router)

·     In an OSPF running network, adjacency is in the form of full mesh form. Formulae = n (n-1)/2.

·     To reduce the adjacency count and LSA flooding, we elect DR & BDR router on broadcast link but not on point-to-point link.

NOTE: Serial link is considered as point-to-point but considers fast Ethernet link as broadcast or multi-access link.

27.                Election of DR & BDR router

·     First highest priority (0-255), default is 1 is considered.

·     Router with highest Router-Id will be elected as DR.

·     Highest loopback interface ip address will be considered.

·     Highest ip address on all active Physical interface.

·     If DR fails, BDR will take its place as DR.

·     Now communication will be done via DR & BDR to reduce LSA flooding.

·     In DR/BDR case, adjacency = (2n-3)

·     In only DR case, adjacency =(n-1)

28.                OSPF Configurations commands:

·     To configure ospf on a router:

Router(conf )#router ospf 1(process id)

Router(config-router)#network mask area

·     To enable ospf on multiple interfaces simultaneously:

Router(conf )#int range fa0/0-1

Router(conf-if )#ip ospf 1 area 0

·     To check routing table:

Router#show ip route

·     To check only ospf routes in routing table:

Router#show ip route ospf

·     To check ospf neighbor table:

Router#show ip ospf neighbor

·     To check ospf database table:

Router#show ip ospf database

·     To check current running routing protocols:

Router#show ip protocols

·     Debug ospf packets:

Router#debug ip ospf packets

·     To refresh ospf process:

Router#clear ip ospf process

·     To check for border routers:

Router#show ip ospf border-routers

·     To check ospf statistics:

Router#show ip ospf statistics

·     To change administrative distance of OSPF bound router:

Router(config)#router ospf 1

Router(config-router)#distance OSPF external

Router(config-router)#distance OSPF intra-area

Router(config-router)#distance OSPF inter-area

·     To change maximum supported path for load balancing:

Router(config)#router ospf 1

Router(config-router)#maximum-paths 4

·     TO change router-id:

Router(config)#router ospf 1

Router(config-router)#router-id 1.0.0.1 (clear ospf process once

·     To enable authentication :

Router(config)#int fa0/0

Router(config-if)#ip ospf authentication message-digest

Router(config-if)#ip ospf message-digest-key 1 md5 ccna

·     To define a virtual link:

Router(config)#router ospf 1

Router(config-router)#area (in-between area) virtual-link (router-id of other router)