Cisco CCNP and CCIE Study Notes

Consider two routers connected back-to-back to each other via their serial interfaces. Both routers are running OSPF on their serial interfaces which are in the 10.10.10.0/24 subnet. OSPF is not running on any other interfaces and in fact no interface other than the serial interface is even up. In particular, no loopback interfaces are configured. The router-ids for the routers are 1.1.1.1 and 2.2.2.2.

The OSPF database for the routers shows the following -

Router1#show ip ospf database router 1.1.1.1

          OSPF Router with ID (1.1.1.1) (Process ID 100)

              Router Link States (Area 0)

LS age: 32
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 8000000A
Checksum: 0xFD50
Length: 48
Number of Links: 2

  Link connected to: another Router (point-to-point)
   (Link ID) Neighboring Router ID: 2.2.2.2
   (Link Data) Router Interface address: 10.10.10.1
    Number of TOS metrics: 0
     TOS 0 Metrics: 64

  Link connected to: a Stub Network
   (Link ID) Network/subnet number: 10.10.10.0
   (Link Data) Network Mask: 255.255.255.0
    Number of TOS metrics: 0
     TOS 0 Metrics: 64


Router1#

Why is a "stub network" showing up in the Router LSA in addition to the point-to-point link corresponding to the serial interface? What purpose is this stub network entry in the router LSA serving?

Assume that on a router with router-id 1.1.1.1 a loopback interface is configured with the IP address 80.80.80.80/24. In the area in which this router is connected, the OSPF database shows the following regarding this loopback interface -

R1#sh ip ospf database router 1.1.1.1
 :
 :
   Link connected to: a Stub Network
    (Link ID) Network/subnet number: 80.80.80.0
    (Link Data) Network Mask: 255.255.255.0
     Number of TOS metrics: 0
      TOS 0 Metrics: 1
 :
 :

What can you deduce about the loopback interface configuration from the above output?