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The ASBR in an OSPF NSSA Area Fails to Perform the Conversion from the Type 7 LSA to the Type 5 LSA

Publication Date:  2012-07-27 Views:  70 Downloads:  1
Issue Description

  Version information: ME60 (V100R005C02B01B), S8505 (version 3.10)
Networking description: The ME60 is connected to a downstream S8505 and several downstream NE20s. The upstream device of the ME60 is NE80E-2. The ME60, NE80E-2, and other upstream devices are in OSPF backbone area 0, whereas the S8505 and the NE20s operate in different NSSA areas.
Fault symptom: Some users of the S8505 (functioning as the gateway) cannot access the Internet. 

Alarm Information

Handling Process
 Delete Area 0 from the OSPF process on the S8505, and the problem can be solved. 

Root Cause
 Assume that the IP addresses of the users who cannot access the Internet belong to network segment A.
1. Check the OSPF configurations on the S8505, and confirm that the direct route A has been redistributed into the OSPF process.
2. Check the routing table on the ME60 and it is found that the ME60 can learn the route to network segment A and the next hop is the S8505.
3. Check the routing tables on NE80E-2 and the upstream device of NE80E-2. It is found that there are no entries recording the route to network segment A in these routing tables.
4. Check the LSA on the ME60 that contains information about network segment A. It is found that the NP in the type 7 LSA is not set. The ASBR in the NSSA area, however, does not convert such an LSA. Therefore, the upstream devices of the ME60 do not have the LSA containing information about network segment A and as a result cannot give a correct calculation of routes.
5. Check the OSPF configurations on the S8505. It is found that there is Area 0, and some interface in the Down state exists in this area. Therefore, the S8505 claims to be an ASBR. As defined in the OSPF protocol, the ASBR in an NSSA area does not set the NP in the LSAs generated by the ASBR itself to prevent the LSA conversion on the other ASBRs in the same area so that route loops can be averted. The cause of the problem is found. 

 This problem occurred after the cutover of services. The S8505 originally ran in OSPF backbone area 0. After the cutover, the S8505 was incorporated into an NSSA area. As a result, the interface originally running in OSPF backbone area 0 was shut down but the corresponding network configurations on the interface were not deleted. Thus, the problem occurred. It is suggested to delete useless configurations when services are cut over to avert such problems.