Troubleshooting OSPF - LAB 1

Here is a quick lab I put together to really test your knowledge about OSPF Path selection and some of the hidden rules behind the path selection.

Simple topology below.

Objective: 

• R3 should choose R2 over Gi1.23 to reach R1 loopback 150.1.1.1/32. Imagine that this is a 40Gbit link where as Gi1.13 is just a 100Mb link. Currently R3 is choosing Gi1.13 to reach R1 loopback 150.1.1.1/32. 

Restrictions:

• Do not configure any new routing protocols.
• Do not change Area numbers.
• Do not create any NEW areas. 
• Do not change OSPF cost values
• R3 should choose R1 with OSPF as the routing protocol so no STATIC routes etc. 

Configuration excerpts below so you can configure it in your lab.

 R1#  
 !  
 interface Loopback0  
  ip address 150.1.1.1 255.255.255.255  
 !  
 interface GigabitEthernet1.12  
  encapsulation dot1Q 12  
  ip address 12.12.12.1 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 0  
 !  
 interface GigabitEthernet1.13  
  encapsulation dot1Q 13  
  ip address 13.13.13.1 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 1  
 !  
 router ospf 1  
  network 150.1.1.1 0.0.0.0 area 0  

 R2#  
 !  
 interface Loopback0  
  ip address 150.1.2.2 255.255.255.255  
 !  
 interface GigabitEthernet1.12  
  encapsulation dot1Q 12  
  ip address 12.12.12.2 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 0  
 !  
 interface GigabitEthernet1.23  
  encapsulation dot1Q 23  
  ip address 23.23.23.2 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 1  
 !  
 router ospf 1  
  area 1 virtual-link 150.1.3.3  

 R3#  
 !  
 interface Loopback0  
  ip address 150.1.3.3 255.255.255.255  
 !  
 interface GigabitEthernet1.13  
  encapsulation dot1Q 13  
  ip address 13.13.13.3 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 1  
 !  
 interface GigabitEthernet1.23  
  encapsulation dot1Q 23  
  ip address 23.23.23.3 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 1  
 !  
 interface GigabitEthernet1.34  
  encapsulation dot1Q 34  
  ip address 34.34.34.3 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 2  
 !  
 router ospf 1  
  area 1 virtual-link 150.1.2.2  

 R4#  
 !  
 interface Loopback0  
  ip address 150.1.4.4 255.255.255.255  
 !  
 interface GigabitEthernet1.34  
  encapsulation dot1Q 34  
  ip address 34.34.34.4 255.255.255.0  
  ip ospf hello-interval 1  
  ip ospf 1 area 2  
 !  
 router ospf 1  

 R3#show ip route 150.1.1.1  
 Routing entry for 150.1.1.1/32  
  Known via "ospf 1", distance 110, metric 2, type intra area  
  Last update from 13.13.13.1 on GigabitEthernet1.13, 00:04:55 ago  
  Routing Descriptor Blocks:  
  * 13.13.13.1, from 150.1.1.1, 00:04:55 ago, via GigabitEthernet1.13  
    Route metric is 2, traffic share count is 1  

 R3# sh ip os nei   
 Neighbor ID   Pri  State      Dead Time  Address     Interface  
 150.1.2.2     0  FULL/ -      -    23.23.23.2   OSPF_VL1  
 150.1.1.1     1  FULL/DR     00:00:03  13.13.13.1   GigabitEthernet1.13  
 150.1.2.2     1  FULL/DR     00:00:03  23.23.23.2   GigabitEthernet1.23  
 150.1.4.4     1  FULL/DR     00:00:03  34.34.34.4   GigabitEthernet1.34  

 R3#sh ip os int brief   
 Interface  PID  Area      IP Address/Mask  Cost State Nbrs F/C  
 VL1     1   0        23.23.23.3/24   1   P2P  1/1  
 Gi1.13    1   1        13.13.13.3/24   1   BDR  1/1  
 Gi1.23    1   1        23.23.23.3/24   1   BDR  1/1  
 Gi1.34    1   2        34.34.34.3/24   1   BDR  1/1  

Post comments on how you can get R3 to choose (Gi1.23) R2 -> R1 to reach R1's loopback 150.1.1.1/32.

Many more articles to come so ....

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