OSPF in a Single Area

OSPF in a Single Area
· OSPF uses the Shortest Path First (SPF) algorithm developed by Edgar Dijkstra.
· OSPF is an open-standard, link state protocol.
· OSPF classless and supports hierarchical routing and route summarization.
· OSPF uses cost as a metric, which is the inverse of the bandwidth of a link.
· OSPF requires more memory and faster processors to handle its additional information.
· The administrative distance for OSPF is 110.
· Each OSPF router has an ID, which is either the highest IP address on a loopback interface, if one exists, or the highest IP address on an active interface.

Configure a loopback interface:
Router(config)# interface loopback interface-number
Router(config-if)# ip address ip-address subnet-mask

· Routers use LSAs to learn the topology of the network.
Link State Advertisement (LSAs): A packet that contains the status of a router's links or network interfaces.
· Each OSPF router has a router ID, which is either the highest IP address on a loopback interface or the highest IP address on an active interface.
· LSAs are used to develop neighbor relationships and are sent as multicasts every 10 seconds. For LAN segments, a DR and a BDR are elected (highest router ID) to disseminate routing information.
· Routers use 224.0.0.6 to send information to the DR/BDR.
· OSPF is connection-oriented in that any routing information sent to another router requires a responding ACK. When DRs share routing information to their neighbors, the multicast address used is 224.0.0.5

OSPF Packet Types
OSPF includes five packet types:
Hello packets— Establish communication with directly attached neighbors.
Database Descriptor (DBD)— Sends a list of router IDs from whom the router has an LSA and the current sequence number. This information is used to compare information about the network.
Link State Requests (LSR)— Follow DBDs to ask for any missing LSAs.
Link State update (LSU)— Replies to a link-state request with the requested data.
Link-state acknowledgements (LSAck)— Confirm receipt of link-state information.


Configuring OSPF in a Single Area
Router(config)#router ospf process-number
Router(config-router)#network network-number wildcard-mask area area-number

RTA(config)#router ospf 65
RTA(config-router)#network 192.168.10.0 0.0.0.255 area 0
RTA(config-router)#network 192.168.20.0 0.0.0.255 area 0

RTB(config)#router ospf 65
RTB(config-router)#network 192.168.10.0 0.0.0.255 area 0
RTB(config-router)#network 192.168.30.0 0.0.0.255 area 0

Configuring MD5 Authentication
Router(config-if)#ip ospf message-digest-key key-id md5 key
Router(config-if)# ip ospf authentication [message-digest null]
Router(config-router)#area area-id authentication [message-digest]

key-id ----> An identifier in the range from 1 to 255
key-----> Alphanumeric password of up to 16 bytes

Example:
RouterA(config)#int s0/0
RouterA(config-if)#ip ospf message-digest-key 1md5 thewayofccie
RouterA(config-if)#ip ospf authentication message-digest
RouterA(config-if)#exit
RouterA(config)#router ospf 65
RouterA(config)#area 0 authentication message-digest