Step 3: Check OSPF on a Stub Router

Action

To verify the OSPF configuration on a stub router in your network, enter the following commands:

user@host> show configuration

user@host> show ospf interface

Sample Output

The following sample output is for an OSPF configuration on R5, a stub router shown in Figure 11:

user@R5> show configuration

[...Output truncated...]

interfaces {

    so-0/0/2 {

        unit 0 {

            family inet {

                address 10.1.45.2/30;

            }

        }

    }

    lo0 {

        unit 0 {

            family inet {

                address 10.0.0.5/32;

            }

        }

    }

}

routing-options {

    router-id 10.0.0.5;

}

protocols {

    ospf {

        area 0.0.0.2 {

            stub;

            interface so-0/0/2.0;

            interface lo0.0 {

                passive;

            }

        }

    }

}

user@R5> show ospf interface 

Interface         State     Area            DR ID           BDR ID       Nbrs

lo0.0             DRother  0.0.0.2         0.0.0.0         0.0.0.0         0

so-0/0/2.0        PtToPt   0.0.0.2         0.0.0.0         0.0.0.0         1

What It Means

The sample output shows a basic OSPF configuration at the [edit protocols ospf] and [edit interfaces] hierarchy levels on R5, a stub router.

R5 has two interfaces included at the [edit protocols ospf] hierarchy level, and those interfaces have the family inet statement included at the [edit interfaces] hierarchy level. Both interfaces, so-0/0/2.0 and the loopback interface (lo0), are in the stub area (0.0.0.2).

R5 has the router ID configured manually to avoid possible problems when the OSPF router ID (RID) changes; for example, when multiple loopback addresses are configured. The RID uniquely identifies the router within the OSPF network. It is transmitted within the LSAs used to populate the link-state database and calculate the shortest-path tree. In a link-state network, it is important that two routers do not share the same RID value, otherwise IP routing problems may occur.

A stub area does not allow AS external advertisements to flood within that area. R5 relies on a default route (0.0.0.0/0) to reach destinations outside the AS. The default route can be statically configured on R5 or advertised by an ABR (R4). In this network, the default LSA is advertised by R4.

A stub area is useful if you want to reduce the size of the topological database and therefore the amount of memory required from the routers in the stub area. However, some restrictions apply to a stub area. You cannot create a virtual link through a stub area, and a stub area cannot contain an ASBR.