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Sometimes, OSPF uses a default route for destinations that are unknown to OSPF. Most of the time, these destinations are networks external to OSPF. Instead of importing all the external routes into OSPF, just one default route is needed that will be used for all unknown external destinations. In the absence of such a default route, all the traffic destined for any unknown address will be dropped by OSPF. The most common possible causes for an OSPF router not to advertise the default route are as follows :
OSPF Neighbor Not Advertising Default Routes ‚ Cause: Missing default-information originate CommandsOSPF does not originate the default route unless the OSPF default-information originate command is present in the OSPF configuration. This command originates the default route on the router on which it is configured. There is no other way in OSPF to generate the default route. Figure 9-59 shows a network setup that produces this problem. Figure 9-59. Network Setup That Produces This Problem
Figure 9-60 shows the flowchart to follow to solve this problem. Figure 9-60. Problem-Resolution Flowchart
Debugs and VerificationExample 9-161 shows that R1 is receiving a default route through RIP. This is important because if R1, which is the originator of a default route, does not have the default route, it can't generate a default route in OSPF. Example 9-161 R1 Receives a Default Route Through RIPR1# show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "rip", distance 120, metric 1, candidate default path Redistributing via rip Last update from 132.108.0.2 on Serial0, 00:00:16 ago Routing Descriptor Blocks: 132.108.0.2, from 132.108.0.2, 00:00:16 ago, via Serial0 Route metric is 1, traffic share count is 1 Example 9-162 shows the configuration of R1 illustrating that R1 is redistributing all the RIP routes into OSPF. As mentioned in the beginning of this problem, the only way to originate a default in OSPF is through the default-information originate command. So, redistribution of a default route will not cause OSPF to generate the default route. Example 9-162 R1 Redistributes All RIP Routes into OSPFR1# 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 ! router rip network 132.108.0.0 ! Example 9-163 shows that 0.0.0.0 is not in R1's database but that other RIP routes are in the database. This indicates that the default route that was a part of RIP did not get redistributed into OSPF. Example 9-163 R1's OSPF Database Is Missing Information About the 0.0.0.0 RouteR1#show ip ospf database external 0.0.0.0 OSPF Router with ID (131.108.2.1) (Process ID 1) R1# 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 Example 9-164 shows the configuration of R1, which shows that the default-information originate command is missing from the configuration. Example 9-164 R1's Configuration Is Missing the default-information originate CommandR1# 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 SolutionIn this example, even though R1 is receiving a default route from RIP, it's not getting redistributed into the OSPF database. To solve this problem, configure the default-information originate command under router ospf on R1. Example 9-165 shows the configuration change on R1 that fixes this problem. Example 9-165 Configuring R1 to Include the default-information originate CommandR1# 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 default-information originate Example 9-166 shows that R1 is originating the default route in the database after fixing the configuration on R1. Example 9-166 The New Configuration of R1 Fixes the Default Route ProblemR1# show ip ospf database external 0.0.0.0 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: 0.0.0.0 (External Network Number ) Advertising Router: 131.108.2.1 LS Seq Number: 80000001 Checksum: 0x550 Length: 36 Network Mask: /0 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 1 Forward Address: 0.0.0.0 External Route Tag: 1 Example 9-167 shows that R2 starts receiving this default route from R1 after changing the configuration on R1. Example 9-167 Verifying That R2 Receives the Default Route from R1R2# show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "ospf 1", distance 110, metric 1, candidate default path Tag 1, type extern 2, forward metric 128 Redistributing via ospf 1 Last update from 131.108.1.2 on Serial0, 00:54:59 ago Routing Descriptor Blocks: * 131.108.1.2, from 131.108.2.1, 00:54:59 ago, via Serial0 Route metric is 1, traffic share count is 1 OSPF Neighbor Not Advertising Default Routes ‚ Cause: Default Route Missing from the Neighbor's Routing TableOSPF will not originate the default route unless the command default-information originate is configured under router OSPF. This command has one more restriction: The router must have a default route in its routing table to originate a default route. Figure 9-61 shows the flowchart to follow to solve this problem. Figure 9-61. Problem-Resolution Flowchart
Debugs and VerificationExample 9-168 shows that R1 is not generating the default route in OSPF database because no default route is present in the routing table. Refer to Figure 9-56 for the network setup. Example 9-168 R1 Is Not Generating the Default RouteR1# show ip route 0.0.0.0 % Network not in table R1# R1#show ip ospf database external 0.0.0.0 OSPF Router with ID (131.108.2.1) (Process ID 1) R1# Example 9-169 shows the configuration on R1 to confirm that the command default-information originate is also configured to rule out the cause addressed in the previous section. Example 9-169 Confirming That R1's Configuration Includes the default-information-originate CommandR1# 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 default-information originate SolutionThis problem can be solved in two ways. One is to make sure that the default route is present in the routing table. It could be derived from any other routing protocol, including static or default route information. Example 9-170 shows that after the default route is back in R1's routing table, it starts generating a default external LSA. Example 9-170 Default Route in R1 Is Back So That It Generates the External LSA for 0.0.0.0R1# show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "rip", distance 120, metric 1, candidate default path Redistributing via rip Last update from 132.108.1.1 on Serial1, 00:00:16 ago Routing Descriptor Blocks: 132.108.1.1, from 132.108.1.1, 00:00:16 ago, via Serial0 Route metric is 1, traffic share count is 1 R1# show ip ospf database external 0.0.0.0 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: 0.0.0.0 (External Network Number ) Advertising Router: 131.108.2.1 LS Seq Number: 80000001 Checksum: 0x550 Length: 36 Network Mask: /0 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 1 Forward Address: 0.0.0.0 External Route Tag: 1 The second method is to use the always keyword with default-information originate. The always keyword instructs the router to originate the default whether or not there is a default present in the routing table. Example 9-171 shows the configuration, which originates a default route in the OSPF database. Example 9-171 Configuring the always Keyword to Force Origination of the Default RouteR1# 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 default-information originate always Example 9-172 shows that the default originated in the OSPF database of R1. Example 9-172 Verifying That R1 Originates the Default RouteR1# show ip ospf database external 0.0.0.0 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: 0.0.0.0 (External Network Number ) Advertising Router: 131.108.2.1 LS Seq Number: 80000001 Checksum: 0x550 Length: 36 Network Mask: /0 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 1 Forward Address: 0.0.0.0 External Route Tag: 1 Example 9-173 shows that R2 starts receiving this default route from R1 after changing the configuration on R1. Example 9-173 R2 Is Receiving the Default from R1R2# show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "ospf 1", distance 110, metric 1, candidate default path Tag 1, type extern 2, forward metric 128 Redistributing via ospf 1 Last update from 131.108.1.2 on Serial0, 00:54:59 ago Routing Descriptor Blocks: * 131.108.1.2, from 131.108.2.1, 00:54:59 ago, via Serial0 Route metric is 1, traffic share count is 1 OSPF Neighbor Not Advertising Default Routes ‚ Cause: Neighbor Trying to Inject a Default into a Stub AreaWhen an area is defined as a stub, no external LSAs are allowed in it. This includes the Type 5 default external LSAs created by the default-information originate command on a router. An ABR automatically generates a Type 3 summary LSA default in a stub area; however, if a default-information originate command is configured on the ABR or non-ABR, it is not advertised as the default route because this command generates Type 5 LSAs, which are not allowed in a stub area. In Figure 9-62, R1 is a router that is completely in area 2, which is defined as a stub area. Figure 9-62. Network Setup Used to Produce This Problem
Figure 9-63 shows the flowchart to follow to solve this problem. Figure 9-63. Problem-Resolution Flowchart
Debugs and VerificationExample 9-174 shows the warning message that R1 gets when it tries to originate a default route with the default-information originate command. R1 has all interfaces in area 2, which is defined as a stub, so default-information originate is not possible for R1 because it will make R1 an ASBR. Example 9-174 Warning Message Generated When Attempting to Originate a Default Route with the default-information originate CommandR1(config)# router ospf 1 R1(config-router)# default-information originate Warning: Router is currently an ASBR while having only one area which is a stub area Example 9-175 shows the configuration on R1, which reveals that R1 has the default-information originate command configured. However, R1 will not generate default Type 5 LSAs into area 2 because it's a stub area. Also, no information on 0.0.0.0 in the OSPF database of R1 can be found. Example 9-175 R1's Configuration Indicates That It's Defined as a Stub AreaR1# 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 default-information originate area 2 stub R1#show ip ospf database external 0.0.0.0 OSPF Router with ID (131.108.2.1) (Process ID 1) R1# SolutionIn this example, all the interfaces of R1 are included in area 2, which is a stub area. When the default-information originate command is configured, it simply ignores the area with a warning. Normally, when an area is defined as a stub, a Type 3 summary default is generated by the ABR. This problem can be solved in three ways:
The first solution is discussed later in this section. The second solution is not a good one because area 2 will become a normal area and will contain unnecessary Type 5 LSAs. In this case, however, it is an acceptable solution. Example 9-176 shows the configuration changes necessary to achieve this. Example 9-176 Configuring R1 as a Normal Area and Originating a Default RouteR1# 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 default-information originate no area 2 stub Example 9-177 shows that R2 is receiving a default from R1 after the configuration change. Example 9-177 Verifying That Reconfiguring R1 as a Normal Area Resolves the ProblemR2# show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "ospf 1", distance 110, metric 1, candidate default path Tag 1, type extern 2, forward metric 128 Redistributing via ospf 1 Last update from 131.108.1.1 on Serial0, 00:54:59 ago Routing Descriptor Blocks: * 131.108.1.1, from 131.108.2.1, 00:54:59 ago, via Serial0 Route metric is 1, traffic share count is 1 The third solution is simple to implement. The requirement is to override the summary default route that is coming from the ABR in case the area is defined as a stub. If that summary default is desirable, there is no need for any change in the configuration. Example 9-178 shows that R1 has a static default route configured. To override the summary default on all the routers in an area, the static default route must be configured on all the routers in an area. This makes this solution less desirable. Example 9-178 R1 Has a Static Default Route ConfiguredR1(config)# ip route 0.0.0.0 0.0.0.0 131.108.2.2 R1# show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "static", distance 1, metric 0, candidate default path Routing Descriptor Blocks: * 131.108.2.2 Route metric is 0, traffic share count is 1 OSPF Neighbor Not Advertising Default Routes ‚ Cause: NSSA ABR/ASBR Not Originating Type 7 DefaultWhen an NSSA is defined, by default, the NSSA ABR does not originate any default route. This is not the case in stub areas or totally stubby areas. When an area is defined as a stub, the ABR originates the Type 3 default route in the form of a summary LSA. This is because a stub area cannot have any Type 5 LSAs, so the default route must not be a Type 5 LSA. This is also true in totally stubby areas. When an NSSA area is defined with the no-summary option, the NSSA ABR automatically generates a Type 3 summary default. This creates a total NSSA. In Figure 9-64, R1 is an NSSA ASBR that is trying to originate a default route into area 2, which is an NSSA. Figure 9-64. Network Setup Used to Produce This Problem
Figure 9-65 shows the flowchart to follow to solve this problem. Figure 9-65. Problem-Resolution Flowchart
Debugs and VerificationExample 9-179 shows the configuration of R1. The configuration shows that R1 is trying to originate a default. Example 9-179 R1 Is Misconfigured to Originate the Default RouteR1# 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 default-information originate area 2 nssa Example 9-180 shows that R1 is not originating a default route. Example 9-180 R1 Is Not Originating a Default RouteR1# show ip ospf database external 0.0.0.0 OSPF Router with ID (131.108.2.1) (Process ID 1) R1# R1# show ip ospf database nssa-external 0.0.0.0 OSPF Router with ID (131.108.2.1) (Process ID 1) R1# SolutionType the highlighted command in Example 9-181 to originate a default route in an NSSA. This command works on either NSSA ABRs or NSSA ASBRs. Example 9-181 Originating a Default Route in an NSSAR1# 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 area 2 nssa default-information originate In the past, this command worked only on an NSSA ABR, but Cisco IOS Software Release 12.0.11 and 12.1.2 provide support for this to work on NSSA ASBRs as well. Example 9-182 shows that after configuring this command, R2 receives a default from R1. Example 9-182 R1 Successfully Originates a Default RouteR2# show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "ospf 1", distance 120, metric 1, candidate default path, type NSSA extern 2, forward metric 64 Redistributing via ospf 1 Last update from 131.108.1.1 on Serial0, 00:00:03 ago Routing Descriptor Blocks: * 131.108.1.1, from 131.108.2.1, 00:00:03 ago, via Serial0 Route metric is 1, traffic share count is 1 |
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