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OSPF over Broadcast Link Introduction Open Shortest Path First (OSPF) is a dynamic routing protocol for use in Internet Protocol (IP) networks. Specifically, it is a link-state routing protocol and falls into the group of interior gateway protocols, operating within a single autonomous system (AS). This sample configuration shows how OSPF can be configured over a broadcast link like ethernet etc. Network Diagram Configuration Router A Okapi(config)# interface FastEthernet 0
Okapi(config-if-eth)# ip address 10.20.51.5 255.255.255.0
Okapi(config)# interface FastEthernet 1
Okapi(config-if-eth)# ip address 192.168.81.5 255.255.255.0
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Router B
Okapi(config)# interface FastEthernet 0
Okapi(config-if-eth)# ip address 10.20.51.7 255.255.255.0
Okapi(config)# interface FastEthernet 1
Okapi(config-if-eth)# ip address 172.168.54.12 255.255.255.0
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Over Point-to-Point Link Introduction This sample configuration shows how OSPF can be configured over a point-to-point link. Network Diagram Configuration Router A Okapi(config)# interface Serial 0
Okapi(config-if-eth)# ip address 10.20.51.5 255.255.255.0
Okapi(config)# interface FastEthernet 0
Okapi(config-if-eth)# ip address 192.168.81.5 255.255.255.0
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Router B
Okapi(config)# interface Serial 0
Okapi(config-if-eth)# ip address 10.20.51.7 255.255.255.0
Okapi(config)# interface FastEthernet 0
Okapi(config-if-eth)# ip address 172.168.54.12 255.255.255.0
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Area Border Router(ABR) Introduction An ABR is a router that connects one or more OSPF areas. It is considered a member of all areas it is connected to. An ABR keeps multiple copies of the link-state database in memory, one for each area to which that router is connected.
Below is a configuration for an ABR. Network statements describe which interfaces we should include in ospf LSA(Link-state advertisement), and to which areas they correspond. Network Diagram Configuration Okapi(config)# interface Fastethernet 0
Okapi(config-if-tunnel)# ip address 192.168.54.2/30
Okapi(config)# interface Fastethernet 1
Okapi(config-if-eth)# ip address 192.168.27.1/24
Okapi(config)# router ospf
Okapi(config-router)# ospf router-id 192.168.27.1
Okapi(config-router)# network 192.168.54.0/30 area 1.1.1.1
Okapi(config-router)# network 192.168.27.0/24 area 2.2.2.2
Advertise default Route into an Area Introduction This sample configuration shows how OSPF can be configured to advertise default route into an area. Network Diagram Configuration Router A
Okapi(config)# interface Serial 0
Okapi(config-if-eth)# ip address 192.168.54.2 255.255.255.0
Okapi(config)# interface Serial 1
Okapi(config-if-eth)# ip address 192.168.27.1 255.255.255.0
Okapi(config)# router ospf
Okapi(config-router)# network 192.168.54.0/24 area 0.0.0.0
Okapi(config-router)# network 192.168.21.0/24 area 1.1.1.1
Okapi(config-router)# default-information originate always
Simple OSPF route summarization
This shows how to summarize routes in OSPF, area... range is the key here. It means all networks inside 2.3.0.0/16 will be suppressed and announced to other areas as 2.3.0.0/16 instead of whole bunch of smaller networks (e.g. 2.3.2.0/24). Okapi(config)# router ospf
Okapi(config-router)# ospf router-id 1.2.3.4
Okapi(config-router)# network 1.2.3.0/24 area 0.0.0.0
Okapi(config-router)# network 2.3.4.0/24 area 0.0.0.1
Okapi(config-router)# area 0.0.0.1 range 2.3.0.0/16 Enable OSPF Authentication
Network Diagram Plain Authentication: Configuration Router A
Okapi(config)# interface Serial 0
Okapi(config-if-eth)# ip address 10.20.51.5 255.255.255.0
Okapi(config-if-eth)# ip ospf authentication-key ABCDEFGHIJK
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Okapi(config-router)# area 1 authentication
Router B
Okapi(config)# interface Serial 0
Okapi(config-if-eth)# ip address 10.20.51.7 255.255.255.0
Okapi(config-if-eth)# ip ospf authentication-key ABCDEFGHIJK
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Okapi(config-router)# area 1 authentication MD5 Authentication
Configuration Router A
Okapi(config)# interface Serial 0
Okapi(config-if-eth)# ip address 10.20.51.5 255.255.255.0
Okapi(config-if-eth)# ip ospf authentication message-digest
Okapi(config-if-eth)# ip ospf message-digest-key 1 md5 ABCDEFGHIJK
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Okapi(config-router)# area 1 authentication message-digest
Router B
Okapi(config)# interface Serial 0
Okapi(config-if-eth)# ip address 10.20.51.7 255.255.255.0
Okapi(config-if-eth)# ip ospf authentication message-digest
Okapi(config-if-eth)# ip ospf message-digest-key 1 md5 ABCDEFGHIJK
Okapi(config)# router ospf
Okapi(config-router)# network 10.20.51.0/24 area 1
Okapi(config-router)# area 1 authentication message-digest
Filtering redistributed routes using Route-map This example shows how to redistribute routes from rip with a metric of 1 to ospf with metric of 5 and external type 1 metric. Okapi(config)# router ospf
Okapi(config-router)# redistribute rip route-map rip-ospf
Configure route-map: Okapi(config)# route-map rip-ospf permit
Okapi(config-route-map)# match metric 1
Okapi(config-route-map)# set metric 5
Okapi(config-route-map)# set metric-type type1
MD5 Filtering redistributed routes using distribute-list (ospf/kernel) We want only prefixes belonging to network 20.0.0.0/8 to be redistributed from kernel to ospf. E.g. 20.1.2.0/24 and 20.3.2.0/19 will be included in redistributed routes. Okapi(config)# router ospf
Okapi(config-router)# redistribute kernel
Okapi(config-router)# distribute-list KERNELOUT out kernel
Configure access-list: Okapi(config)# access-list KERNELOUT permit 20.0.0.0/8
OSPF Virtual Link
All the OSPF areas should be connected to the backbone area, but sometimes it is not possible. In such cases we can connect them using ospf virtual link. This sample configuration shows how to establish a ospf virtual link between 2 ospf areas. Network Diagram Configuration
Router A
Okapi(config)# router ospf
Okapi(config-router)# router-id 1.1.1.1
Okapi(config-router)# network 10.20.51.0/24 area 0
Okapi(config-router)# network 20.20.51.0/24 area 1
Okapi(config-router)# area 1 virtual-link 2.2.2.2
Router B
Okapi(config)# router ospf
Okapi(config-router)# router-id 2.2.2.2
Okapi(config-router)# network 30.20.51.0/24 area 1
Okapi(config-router)# network 40.20.51.0/24 area 2
Okapi(config-router)# area 1 virtual-link 1.1.1.1
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