Cisco CCNP and CCIE Study Notes

Consider the following output from a router running OSPF

R1#show ip interface brief 
Interface           IP-Address      OK? Method Status  Protocol
Serial2             20.20.20.1      YES NVRAM  up      up      
Serial3             30.30.30.1      YES NVRAM  up      up      
Loopback0           1.1.1.1         YES manual up      up      

R1#sh running-config | begin ospf
router ospf 100
 log-adjacency-changes
 network 1.1.1.1 0.0.0.0 area 0
 network 20.20.20.0 0.0.0.255 area 0
 network 30.30.30.0 0.0.0.255 area 0
!
ip classless
no ip http server
!
line con 0
line aux 0
line vty 0 4
 login
!
end

R1#sh ip protocols 
Routing Protocol is "ospf 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 223.223.223.223
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    1.1.1.1 0.0.0.0 area 0
    20.20.20.0 0.0.0.255 area 0
    30.30.30.0 0.0.0.255 area 0
  Routing Information Sources:
    Gateway         Distance      Last Update
    223.223.223.223      110      00:00:38
  Distance: (default is 110)

R1#

Note that the OSPF router ID is seen to be 223.223.223.223 even though the only loopback interface in the system does not have that IP address and the router-id is not explicitly set under the router ospf command. How could it be possibly be that given this configuration the OSPF router id is 223.223.223.223?

Consider two routers connected back-to-back with each other via a pair of serial interfaces. The routers are running RIP to advertise their loopbacks to each other. Here is the config from the two routers

Router1

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface Serial2
 ip address 20.20.20.1 255.255.255.0
 no keepalive
!
interface Serial3
 ip address 30.30.30.1 255.255.255.0
 no keepalive

Router2

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!
interface Serial2
 ip address 20.20.20.2 255.255.255.0
 no keepalive
!
interface Serial3
 ip address 30.30.30.2 255.255.255.0
 no keepalive

Now consider the following output from "show ip route 2.2.2.2" on router R1

R1#show ip route 2.2.2.2
Routing entry for 2.0.0.0/8
  Known via "rip", distance 120, metric 1
  Redistributing via rip
  Last update from 20.20.20.2 on Serial2, 00:00:10 ago
  Routing Descriptor Blocks:
  * 20.20.20.2, from 20.20.20.2, 00:00:10 ago, via Serial2
      Route metric is 1, traffic share count is 1

Note that R1 sees only one route to 2.2.2.2 via 20.20.20.2 even though clearly it has another path with the same cost via 30.30.30.2.

Your task is to determine using the information provided what must have been configured under the rip router configuration to cause only one path to 2.2.2.2 to be installed in the routing table.