Why do I lose my OSPF routes?

Question

I created little network and set up OSPF protocol. When I took away one ethernet cable I was hoping to see change in routing table...

But what happened was that the router only showed directly connected networks and none of the OSPF.

Just a note: It still had other routers connected to it. All I removed was one cable out of the network.

This is R0 routing table when everything is connected.

     192.168.1.0/29 is subnetted, 1 subnets
C       192.168.1.0 is directly connected, FastEthernet0/1
     192.168.2.0/29 is subnetted, 1 subnets
C       192.168.2.0 is directly connected, FastEthernet0/0
     192.168.3.0/29 is subnetted, 1 subnets
O       192.168.3.0 [110/2] via 192.168.1.2, 00:00:49, FastEthernet0/1
     192.168.4.0/29 is subnetted, 1 subnets
O       192.168.4.0 [110/2] via 192.168.2.2, 00:00:49, FastEthernet0/0

This s R0 routing table when I disconnect R1 and R0 (All the ospf networks dissapear)

     192.168.1.0/29 is subnetted, 1 subnets
C       192.168.1.0 is directly connected, FastEthernet0/1

The question is why is it happening. Shouldn't it stay the same except for the one that is disconnected?

R0
Router(config)#router ospf 1
Router(config-router)#network 192.168.1.0 0.0.0.7 area 0
Router(config-router)#network 192.168.2.0 0.0.0.7 area 0

R1
Router(config)#router ospf 1
Router(config-router)#network 192.168.2.0 0.0.0.7 area 0
Router(config-router)#network 192.168.4.0 0.0.0.7 area 0

R2
Router(config)#router ospf 1
Router(config-router)#network 192.168.4.0 0.0.0.7 area 0
Router(config-router)#network 192.168.3.0 0.0.0.7 area 0

R3
Router(config)#router ospf 1
Router(config-router)#network 192.168.3.0 0.0.0.7 area 0
Router(config-router)#network 192.168.1.0 0.0.0.7 area 0

R0

Router#show run
Building configuration...

Current configuration : 692 bytes
!
version 12.4
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
ip cef
no ipv6 cef
!
spanning-tree mode pvst
!
interface FastEthernet0/0
 ip address 192.168.2.1 255.255.255.248
 duplex auto
 speed auto
!
interface FastEthernet0/1
 ip address 192.168.1.1 255.255.255.248
 duplex auto
 speed auto
!
interface Vlan1
 no ip address
 shutdown
!
router ospf 1
 log-adjacency-changes
 network 192.168.1.0 0.0.0.7 area 0
 network 192.168.2.0 0.0.0.7 area 0
!
ip classless
!
ip flow-export version 9

line con 0
!
line aux 0
!
line vty 0 4
 login
end

R1

Router#show run
Building configuration...

Current configuration : 692 bytes
!
version 12.4
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
!
!
!
!
!
!
!
ip cef
no ipv6 cef
!
!
!
!
!
!
!
!
!
!
!
!
spanning-tree mode pvst
!
!
!
!
!
!
interface FastEthernet0/0
 ip address 192.168.2.2 255.255.255.248
 duplex auto
 speed auto
!
interface FastEthernet0/1
 ip address 192.168.4.1 255.255.255.248
 duplex auto
 speed auto
!
interface Vlan1
 no ip address
 shutdown
!
router ospf 1
 log-adjacency-changes
 network 192.168.2.0 0.0.0.7 area 0
 network 192.168.4.0 0.0.0.7 area 0
!
ip classless
!
ip flow-export version 9
!
!
!
!
!
!
!
line con 0
!
line aux 0
!
line vty 0 4
 login
!
!
!
end

R2

Router#sh run
Building configuration...

Current configuration : 692 bytes
!
version 12.4
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
!
!
!
!
!
!
!
ip cef
no ipv6 cef
!
!
!
!
!
!
!
!
!
!
!
!
spanning-tree mode pvst
!
!
!
!
!
!
interface FastEthernet0/0
 ip address 192.168.3.2 255.255.255.248
 duplex auto
 speed auto
!
interface FastEthernet0/1
 ip address 192.168.4.2 255.255.255.248
 duplex auto
 speed auto
!
interface Vlan1
 no ip address
 shutdown
!
router ospf 1
 log-adjacency-changes
 network 192.168.4.0 0.0.0.7 area 0
 network 192.168.3.0 0.0.0.7 area 0
!
ip classless
!
ip flow-export version 9
!
!
!
!
!
!
!
line con 0
!
line aux 0
!
line vty 0 4
 login
!
!
!
end

R3

Router#sh run
Building configuration...

Current configuration : 692 bytes
!
version 12.4
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
!
!
!
!
!
!
!
ip cef
no ipv6 cef
!
!
!
!
!
!
!
!
!
!
!
!
spanning-tree mode pvst
!
!
!
!
!
!
interface FastEthernet0/0
 ip address 192.168.3.1 255.255.255.248
 duplex auto
 speed auto
!
interface FastEthernet0/1
 ip address 192.168.1.2 255.255.255.248
 duplex auto
 speed auto
!
interface Vlan1
 no ip address
 shutdown
!
router ospf 1
 log-adjacency-changes
 network 192.168.3.0 0.0.0.7 area 0
 network 192.168.1.0 0.0.0.7 area 0
!
ip classless
!
ip flow-export version 9
!
!
!
!
!
!
!
line con 0
!
line aux 0
!
line vty 0 4
 login
!
!
!
end

Answer 1

The problem you see is caused by the lack of a stable router ID.

OSPF requires a router ID to identify itself in routing updates. If you don't manually specify the router ID, the router will use the highest loopback address as the router ID. Since you haven't created a loopback interface, the router chooses the highest interface address as the router ID.

When you unplug a cable, the IP address on that interface is no longer valid (because the interface is down). So the OSPF process stops running. That's why you don't see any OSPF routes.

Good practice is to manually create a router ID:

router ospf 1
router-id 1.1.1.1

Note that although the router ID is written in dotted decimal like an IP address, it is not an address -- it's just a number. Pick any unique number you like.

Alternatively, you can create a loopback interface. The router will use this address as the router ID. Since it's a virtual interface, it never goes down.

interface Loopback 0
ip address 1.1.1.1 255.255.255.255