notes: OSPF Troubleshooting Redistribution problems in OSPF

This section describes problems related to redistribution in OSPF. When a router in OSPF does the redistribution, it becomes an ASBR. The routes that are redistributed into OSPF could be directly connected routes, static routes, or dynamically learned routes from another routing protocol or another OSPF process.

The following are problems that can happen during redistribution:

• ASBR is not advertising redistributed routes.
  

  Whenever a route is known to be connected or static, or when any other routing protocol is redistributed into OSPF, an external LSA is generated for that route. If an OSPF router is not advertising the external route even after the redistribution, this indicates a problem on a router that is doing the redistribution. Mostly, the problem stems from configuration mistakes.

  
  

  • The subnets keyword is missing from the ASBR configuration.
    When any protocol is redistributed into OSPF, if the networks that are being redistributed are subnets, you must define the subnets keyword under OSPF configuration. If the subnets keyword is not added, OSPF will ignore all the subnetted routes when generating the external LSA.
    The situation could arise when connected or static routes are being redistributed into or out of OSPF. In that case, the same rule applies: The subnets keyword must be entered to redistribute subnetted routes.
    show ip ospf database external x.x.x.x
    missing the subnet keyword
    router ospf 1 
    redistribute rip  
    network 131.108.1.0 0.0.0.255 area 0  
    network 131.108.2.0 0.0.0.255 area 0  
    !  
    router rip network 132.108.0.0
    
    
     Solution: add the subnet keyword.  

             router ospf 1  
             redistribute rip subnets  
             network 131.108.1.0 0.0.0.255 area 0 
             network 131.108.2.0 0.0.0.255 area 0
           
             Verification:
              show ip ospf database external x.x.x.x
  
  

  • distribute-list out is blocking the routes.

             When distribute-list out is configured on an ASBR, the access list associated with the distri-bute list is examined and Type 5 external LSAs are originated for networks that are explicitly permitted in the distribute list. All other networks are denied, and no Type 5 external LSAs are generated for those networks.

  
  

            The first logical step is to look at the configuration.

            router ospf 1  

            redistribute rip subnets 

            network 131.108.1.0 0.0.0.255 area 2  

            network 131.108.2.0 0.0.0.255 area 2 

            distribute-list 1 out  

            !

            access-list 1 permit 132.108.4.0 0.0.0.255

            

            Solution:
  

            To solve this problem, you must either remove the distribute list so that R1 generates Type 5 external LSAs for all the RIP routes or modifies the distribute list so that it contains all the necessary networks for which Type 5 LSAs are required. This problem also can happen when using route maps instead of distribute lists. In any case, be sure to permit the desired network in the access list.

           

            Verification:

             show ip ospf database external x.x.x.x

  
  
• OSPF is not installing external routes in the routing table.
  - discussed in previous section.