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Embed code for: 23.10 Configuring Single-Area OSPF
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23.10 Configuring Single-Area OSPF
Configuring SingleArea OSPF Section 23: Implementing Dynamic Routing Basic OSPF is configured in two steps: 1. Enable the OSPF routing process. 2. Identify the networks that you want to advertise. Branch(config)#router ospf 1 Branch(config-router)#network 10.0.0.0 0.255.255.255 area 0 Branch(config)#interface GigabitEthernet 0/1 Branch(config-if)#ip ospf 1 area 0 The router ospf command uses a process identifier as an argument. The process ID is a unique, arbitrary number that you select to identify the routing process. The process ID does not need to match the OSPF process ID on other OSPF routers. The network command identifies which IP networks on the router are part of the OSPF network. For each network, you must also identify the OSPF area to which the networks belong. The table defines the commands that are used to configure OSPF. Command and Variable Description Command and Variable Description router ospf p r o c e s s _ i d Enters into the OSPF routing configuration mode. The process ID is a number between 1 and 65,535 and it is chosen by the network administrator. The process ID is locally significant, which means that it does not have to match other OSPF routers to establish adjacencies with those neighbors. network i p - a d d r e s s w i l d c a r d _ m a s k area a r e a _ i d Uses a combination of the network address and wildcard mask and serves as criteria to match to identify the interfaces that are enabled to send and receive OSPF packets. The network address, along with the wildcard mask, identifies which IP networks are part of the OSPF network and are included in OSPF routing updates. The area ID identifies the OSPF area to which the network belongs. When all of the routers are within the same OSPF area, the network commands must be configured with the same area ID on all routers. Even if no areas are specified, there must be an area 0. In a singlearea OSPF environment, the area is always 0. ip ospf p r o c e s s _ i d area a r e a _ i d Alternatively to a network command, you can use this interface configuration mode command that enables OSPF explicitly on the selected interface. The network that is identified in the network command does not tell the router which network to advertise; instead, it indicates the interfaces on which OSPF will be enabled. The subnet on that interface is what will be advertised. For example, entering network 10.1.1.1 0.0.0.0 area 0 on the Branch router tells the router to enable the interface GigabitEthernet 0/0.1 for the routing process. The OSPF process advertises the network that is on this interface (10.1.1.0/24). Passive Interface Option The router automatically sends OSPF hello packets out of every interface that is included in the OSPF process. To limit the amount of unnecessary traffic in the LAN or to prevent other routers on a local network from learning about routes dynamically, you can stop sending routing updates out of the specific interface. This action is achieved by configuring the passive interface option for the selected interface on which OSPF adjacency is not desired. In the following figure, the GigabitEthernet 0/0.1 interface on the Branch router is configured as passive in the OSPF configuration mode with the passive-interface G i g a b i t E t h e r n e t 0 / 0 . 1 configuration command. Branch(config)#router ospf 1 Branch(config-router)#passive-interface GigabitEthernet 0/0.1 To be able to perform routing toward external networks or toward the Internet, the router must either know all the destination networks or have a default route. You can statically configure a default route, but it can also be learned dynamically via the OSPF routing protocol. The router that announces the default route needs to be configured with the default-information originate command in the routing protocols configuration mode. Up Next: Verifying the OSPF Configuration