The only requirement is that the interface is active and in the up state. This means that the interface address does not need to be included in one of the OSPF network commands for the router to use that IP address as the router ID. If the router uses the highest IPv4 address for the router ID, the interface does not need to be OSPF-enabled. This is the least recommended method because it makes it more difficult for administrators to distinguish between specific routers.
If no loopback interfaces are configured, then the router chooses the highest active IPv4 address of any of its physical interfaces. This is the next best alternative to assigning a router ID. If the router ID is not explicitly configured, the router chooses the highest IPv4 address of any of configured loopback interfaces. This is the recommended method to assign a router ID. The rid value is any 32-bit value expressed as an IPv4 address. The router ID is explicitly configured using the OSPF router-id rid router configuration mode command. The routing device with the second highest router ID is elected the BDR.īut how does the router determine the router ID? As illustrated in the figure, Cisco routers derive the router ID based on one of three criteria, in the following preferential order: Assuming there is no priority configured, or there is a tie, then the router with the highest router ID is elected the DR. When OSPF links become active, the routing device configured with the highest priority is elected the DR. Participate in the election of the DR - In a multiaccess LAN environment, the election of the DR occurs during initial establishment of the OSPF network. Uniquely identify the router - The router ID is used by other routers to uniquely identify each router within the OSPF domain and all packets that originate from them. The router ID is used by the OSPF-enabled router to: The router ID can be defined by an administrator or automatically assigned by the router. To verify if directly connected subnets are really advertised into the different area, we can use the show ip route ospf command on both R1 and R3:Ĭharacters IA in front of the routes indicate that these routes reside in different areas.Every router requires a router ID to participate in an OSPF domain. We can verify that by using the show ip ospf neighbor command: Now R2 should have neighbor relationship with both R1 and R3. We can do that using the following sequence of commands: To do that, we need to specify different area ID for each neighbor relationship, 0 for R1 and 1 for R2. What about R2? Well, because R2 is an ABR, we need to establish neighbor relationship with both R1 and R3. OSPF process will use that RID (router-id) when communicating with other OSPF neighbors.īecause R1 connects only to R2, we only need to establish a neighbor relationship with R2 and advertise directly connected subnet into OSPF.Ĭonfiguration of R3 looks similar, but with one difference, namely area number. We have used the router-id 1.1.1.1 command to manually specify the router ID of this router. To do that, the following configuration on R1 will be used: Our goal is to advertise the subnets directly connected to R1 and R3. Router 2 connects to both areas, which makes him an ABR (Area Border Router). As you can see from the network topology depicted above, routers R1 and R3 are in the area 0 and area 1, respectively. In this example we have two OSPF areas – area 0 and area 1. In this example, we will configure multiarea OSPF network and some other OSPF features.Ĭonsider the following multiarea OSPF network: Configuring OSPF 2Īlthough basic OSPF configuration can be very simple, OSPF provides many extra features that can get really complex. You can see that R1 has learned about the network 192.168.0.0/24 through OSPF. For example, here is the output of the command on R1: All routes marked with the character O are OSPF routes. To verify if the routing updated were exchanged, we can use the show ip route command. We can verify that the routers have become neighbors by typing the show ip ospf neighbors command on either router: The network commands entered on both routers include subnets directly connected to both routers. This can be done by using the following sequence of commands on both routers: Then we need to define what network will be advertised into OSPF. We will use the following network topology:įirst, we need to enable OSPF on both routers. Let’s get started with some basic OSPF configuration. The OSPF process number doesn’t have to be the same on all routers in order to establish a neighbor relationship, but the Area ID has to be the same on all neighboring routers in order for routers to become neighbors.
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