OSPF Router ID & OSPF Router Types

Fig 1: OSPF Router ID & OSPF Router Types

OSPF Router ID

Each and every OSPF router selects a router ID that is also known as the RID that has to be unique in your in the network. OSPF stores the topology of network in the link state database and each router of the network is identified with it's unique router ID and if you have duplicate router IDs than you will face the reachability issues.

Following are the rules/criteria to select the router ID:

Manual configuration of the router ID.
Highest IP on the loopback interface.
Highest IP address on a non-loopback interface.

Explanation of the Router ID

First of all if you want to configure the router ID in the OSPF than you also configured it manually by the help of the command. And if you don't configure router ID manually that it will elect on the basis of the some rules. So in the above figure 1 we have a router on which there four interfaces two fast Ethernet interface and two loopback interfaces. First of all ignore the loopback interfaces now the election will occur in between the fast interfaces those interface which has the largest IP will become the router ID. And if in the election loopbacks interfaces participate than those loopback will win the election which has the large IP in between the loopback. The most important point is if any other interface of the router that has the large IP than the loopback than also election will win the loopback that will become the Router ID.

OSPF Router Types

OSPF works on the basis of the concepts of the areas and by default you will always have a single area normally this is area 0 and that area also known as the backbone area. Routers in the backbone area are called the backbone routers. And the router between the two areas are called the area border router that is mostly called as ABR. Router that run OSPF and that router connected to the other router that runs the other routing protocol that router is known as the autonomous system border router that mostly known as ASBR.