Problem: OSPF Neighbor Is Not Advertising External Routes

Whenever there is a redistribution in OSPF, it generates an external LSA (Type 5) that is flooded throughout the OSPF network. External LSAs are not leaked into stub, totally stubby, and NSSA areas.

The most common possible causes of this problem are as follows :

• The area is configured as a stub or NSSA.

• The NSSA ABR is not translating Type 7 into Type 5 LSA.

OSPF Neighbor Is Not Advertising External Routes ‚ Cause: Area Is Configured as a Stub Area or NSSA

In OSPF, Type 5 LSAs are not allowed in a stub or NSSA area. When entering the redistribute command on a router that is completely in a stub or NSSA area, a warning message is displayed. This redistribute command in the configuration is incapable of importing any external LSAs into a stub or NSSA area.

Figure 9-56 shows the flowchart to follow to solve this problem.

Figure 9-56. Problem-Resolution Flowchart

Debugs and Verification

Example 9-147 shows the configuration error when trying to redistribute into OSPF from another routing protocol on a router in a stub area.

Example 9-147 Errors Caused by Redistributing into OSPF on a Stub Area Router

 R1(config)#  router ospf 1  R1(config-router)#  redistribute rip subnets   Warning: Router is currently an ASBR while having only one area which is a stub area  

Example 9-148 shows the configuration on R1. Even though RIP is being redistributed, R1 will not generate Type 5 LSAs for RIP subnets because R1 is completely in a stub area. For more information on Type 5 LSAs, refer to Chapter 8.

Example 9-148 Redistributing RIP into OSPF While an OSPF Area Is Defined as a Stub

 R1#  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   area 2 stub  

Example 9-149 shows that no external LSAs are generated by R1 because R1 is completely in a stub area. R1 will ignore the redistribution command.

Example 9-149 No External LSAs Are Generated by R1

 R1#  show ip ospf database external 132.108.3.0  OSPF Router with ID (131.108.2.1) (Process ID 1) R1# 

Solution

This problem can be solved in two ways. One solution is to make area 2 a normal area by taking out the area 2 stub command on all the routers in area 2. This is not a good solution because sometimes, a stub area does not require external LSAs.

The second solution is a preferred solution that involves changing the entire area as an NSSA. An NSSA permits redistribution of external routes. However, the NSSA will not generate Type 5 LSAs. It will generate Type 7 LSAs, which will be converted to Type 5 LSAs by the NSSA ABR. Chapter 8 explains NSSAs in more detail.

Example 9-150 shows the configuration that converts a stub area into an NSSA. The area id nssa command must be issued on all routers in the NSSA.

Example 9-150 Converting a Stub Area to an NSSA to Permit Redistribution of External Routes

 R1#  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   area 2 nssa  

Example 9-151 shows that R1 is generating Type 7 LSAs for two RIP routes into the NSSA area. The NSSA ABR converts these Type 7 LSAs further into Type 5 LSAs, which then can be flooded to the rest of the OSPF domain.

Example 9-151 Showing OSPF Generating Type 7 LSAs for RIP Routes

 R1#  show ip ospf database nssa-external 132.108.3.0  OSPF Router with ID (131.108.2.1) (Process ID 1)  Type-7 AS External Link States (Area 2)  LS age: 1161   Options: (No TOS-capability, Type 7/5 translation, DC)   LS Type: AS External Link  Link State ID: 132.108.3.0 (External Network Number )  Advertising Router: 131.108.2.1   LS Seq Number: 80000001   Checksum: 0x550   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 1         Forward Address: 0.0.0.0         External Route Tag: 1 R1# R1#  show ip ospf database nssa-external 132.108.4.0  OSPF Router with ID (131.108.2.1) (Process ID 1)  Type-7 AS External Link States (Area 2)  LS age: 1161   Options: (No TOS-capability, Type 7/5 translation, DC)   LS Type: AS External Link  Link State ID: 132.108.4.0 (External Network Number )  Advertising Router: 131.108.2.1   LS Seq Number: 80000001   Checksum: 0x550   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 1         Forward Address: 0.0.0.0         External Route Tag: 1 R1# 

OSPF Neighbor Is Not Advertising External Routes ‚ Cause: NSSA ABR Not Translating Type 7 LSAs into Type 5 LSAs

NSSA RFC 1587 states that before NSSA ABR converts from Type 7 into Type 5, all NSSA ABRs must examine all NSSA ABRs for a particular NSSA. The ABR with the highest router ID must do the conversion of Type 7 into Type 5. If Type 7 is not translated into Type 5 by the NSSA ABR, no routers outside of NSSA become aware of the external route redistribution that is happening within the NSSA. This defeats the entire purpose of NSSA.

Figure 9-57 shows an OSPF network experiencing this problem. Router 2 is an NSSA ABR, but it's not converting Type 7 LSAs into Type 5 LSAs.

Figure 9-57. OSPF Network in Which OSPF Neighbor R2 Is Not Advertising External Routes into Area 0

Figure 9-58 shows the flowchart to follow to solve this problem.

Figure 9-58. Problem-Resolution Flowchart

Debugs and Verification

Example 9-152 shows the output of show ip ospf database nssa-external on route 132.108.4.0, indicating that R2 is generating Type 7 LSAs.

Example 9-152 show ip ospf database nssa-external Command Output Indicates That Type 7 Is Being Generated

 R2#  show ip ospf database nssa-external 132.108.4.0  OSPF Router with ID (131.108.1.2) (Process ID 1)  Type-7 AS External Link States (Area 2)  LS age: 1161   Options: (No TOS-capability, Type 7/5 translation, DC)   LS Type: AS External Link  Link State ID: 132.108.4.0 (External Network Number )  Advertising Router: 131.108.1.2   LS Seq Number: 80000001   Checksum: 0x550   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 1         Forward Address: 0.0.0.0         External Route Tag: 1 

Example 9-153 shows the output of the show ip ospf database external command for 132.108.4.0, which indicates that R2 is not converting Type 7 LSAs into Type 5 LSAs.

Example 9-153 show ip ospf database external Command Output Indicates That Conversion of Type 7 to Type 5 Is Not Occurring

 R2#  show ip ospf database external 132.108.4.0  OSPF Router with ID (131.1081.2) (Process ID 1) R2# 

Example 9-154 shows that R2 is an NSSA ABR, so it is supposed to do the conversion.

Example 9-154 Verifying That R2 Is an NSSA ABR, Subject to Type 7-to-Type 5 LSA Conversion

 R2#  show ip ospf  Routing Process "ospf 1" with ID  131.108.1.2  Supports only single TOS(TOS0) routes  It is an area border router  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 3. Checksum Sum 0x14DAA  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 4. 3 normal 0 stub 1 nssa     Area BACKBONE(0)         Number of interfaces in this area is 2         Area has no authentication         SPF algorithm executed 60 times         Area ranges are         Number of LSA 16. Checksum Sum 0x9360D         Number of DCbitless LSA 7         Number of indication LSA 0         Number of DoNotAge LSA 0     Area 2         Number of interfaces in this area is 1  It is a NSSA area   Perform type-7/type-5 LSA translation  Area has no authentication         SPF algorithm executed 54 times         Area ranges are         Number of LSA 11. Checksum Sum 0x7A449         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 

Example 9-155 shows another router in the NSSA area, R1, which also claims to be an ABR. This router is a fake ABR because it is not connected to area 0.

Example 9-155 show ip ospf Command Output Indicates That R1 Claims to Be an ABR

 R1#  show ip ospf   Routing Process "ospf 1" with ID 131.108.2.1  Supports only single TOS(TOS0) routes  It is an area border router  Summary Link update interval is 00:30:00 and the update due in 00:29:48  External Link update interval is 00:30:00 and the update due in 00:19:43  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 2. 1 normal 0 stub 1 nssa  Area BACKBONE(0) (Inactive)  Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 2 times         Area ranges are         Link State Update Interval is 00:30:00 and due in 00:29:47         Link State Age Interval is 00:20:00 and due in 00:19:47         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0     Area 2         Number of interfaces in this area is 1         It is a NSSA area         Area has no authentication         SPF algorithm executed 65 times         Area ranges are         Link State Update Interval is 00:30:00 and due in 00:16:27         Link State Age Interval is 00:20:00 and due in 00:14:39         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 

Example 9-156 shows that R1 is doing all the conversion instead of R2.

Example 9-156 R1 Performs Type 7-to-Type 5 LSA Conversion Instead of R2

 R1#  show ip ospf database external 132.108.3.0  OSPF Router with ID (  131.108.2.1  ) (Process ID 1)  Type-5 AS External Link States  LS age: 1161   Options: (No TOS-capability, DC)   LS Type: AS External Link  Link State ID: 132.108.3.0 (External Network Number )  Advertising Router: 131.108.2.1   LS Seq Number: 80000001   Checksum: 0x550   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 1         Forward Address: 0.0.0.0         External Route Tag: 1 R1# R1#  show ip ospf database external 132.108.4.0  OSPF Router with ID (131.108.2.1) (Process ID 1)  Type-5 AS External Link States  LS age: 1161   Options: (No TOS-capability, DC)   LS Type: AS External Link  Link State ID: 132.108.4.0 (External Network Number )  Advertising Router: 131.108.2.1   LS Seq Number: 80000001   Checksum: 0x550   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 1         Forward Address: 0.0.0.0         External Route Tag: 1 R1# 

Example 9-157 shows the OSPF configuration on R1. A network statement on R1 includes one interface of R1 in area 0. This network statement is causing problems because none of the inter-faces on R1 belongs to the backbone.

Example 9-157 R1's Configuration Shows One Interface in Area 0

 R1#  router ospf 1   network 131.108.1.0 0.0.0.255 area 2   network 131.108.2.0 0.0.0.255 area 2    network 131.108.5.0 0.0.0.255 area 0    area 2 nssa  

Solution

In this case, R1 has a router ID (RID) of 131.108.2.1 and R2 has a router ID of 131.108.1.2, as shown in the output of Examples 9-154 and 9-155. Because R1 has a higher RID than R2, R1 does the conversion and the Type 5 LSA goes into a black hole.

You can resolve this problem using multiple solutions:

• Remove the wrong network statement.

• Change the router ID of R1 so that it is lower than R2's.

• Change the router ID of R2 so that it is higher than R1's.

Example 9-158 shows the easiest solution, which is to remove the incorrect network statement.

Example 9-158 Correcting the network Statement on R1 so That R2 Performs the Type 7-to-Type 5 LSA Conversion

 R1#  router ospf 1   network 131.108.1.0 0.0.0.255 area 2   network 131.108.2.0 0.0.0.255 area 2    no network 131.108.5.0 0.0.0.255 area 0   area 2 nssa 

The other solution is to alter the router ID of R1 or R2 so that either R1's is lower than R2's or R2's is higher than R1's. To change the router ID, configure the loopback interface of R1 with a lower IP address than R2's loopback, and then restart the OSPF process. Restarting OSPF will cause network downtime for few seconds. The faster you enter the OSPF configu-ration back on the router, the less downtime there will be. In Cisco IOS Software Release 12.0 and later, a command has been added to configure the router ID manually and then restart OSPF. Example 9-159 shows how to change the router ID with Cisco IOS Software Release 12.0.

Example 9-159 Changing the Router ID of R1 to Be Lower Than R2's

 R2(config)#  router ospf 1  R2(config-router)#  router-id 131.108.0.1  R2(config-router)#  end  R2#clear ip ospf process 

Example 9-160 shows that after fixing the configuration, R2 starts converting Type 7 LSAs into Type 5 LSAs properly. The output of show ip ospf database external on R2 shows that it is generating Type 5 LSAs and that R1 is receiving Type 5 LSAs.

Example 9-160 Confirming That the LSA Type Conversion Problem Has Been Resolved

 R2#  show ip ospf database external 132.108.3.0  OSPF Router with ID (131.108.1.2) (Process ID 1) +  Type-5 AS External Link States  LS age: 1161   Options: (No TOS-capability, DC)   LS Type: AS External Link  Link State ID: 132.108.3.0 (External Network Number )  Advertising Router: 131.108.1.2   LS Seq Number: 80000001   Checksum: 0x550   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 1         Forward Address: 0.0.0.0         External Route Tag: 1 R1# R1#  show ip ospf database external 132.108.4.0  OSPF Router with ID (131.108.1.2) (Process ID 1)  Type-5 AS External Link States  LS age: 1161   Options: (No TOS-capability, DC)   LS Type: AS External Link  Link State ID: 132.108.4.0 (External Network Number )  Advertising Router: 131.108.1.2   LS Seq Number: 80000001   Checksum: 0x550   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 1         Forward Address: 0.0.0.0         External Route Tag: 1 R1#