OSPF ABR Not Advertising Summary Routes to Backbone
Presenting Symptom
Remote networks behind an OSPF ABR are not reachable from the backbone area, even though the ABR has full routing information for those networks.
Network Context
The network consists of an enterprise campus with a backbone area 0 and a non-backbone area 1 connected via a Cisco router acting as an ABR. The ABR is a Cisco 4321 running IOS 15.9. Area 1 contains several loopback networks (e.g., 10.1.1.0/24, 10.1.2.0/24) that are redistributed into OSPF. The ABR is configured to summarize these networks into a single summary route 10.1.0.0/16. However, routers in area 0 do not see the summary route, causing reachability issues.
Diagnostic Steps
Check OSPF neighbor relationships on the ABR
show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface 10.0.0.2 1 FULL/DR 00:00:34 192.168.1.2 GigabitEthernet0/0 10.0.0.3 1 FULL/BDR 00:00:38 192.168.1.3 GigabitEthernet0/0
Ensure the ABR has FULL adjacency with backbone routers. If neighbors are not FULL, OSPF is not working properly and summary routes won't be advertised.
Verify the ABR's OSPF database for summary routes
show ip ospf database summary OSPF Router with ID (10.0.0.1) (Process ID 1)
Summary Net Link States (Area 0)
LS Age: 360
Options: (No TOS-capability, DC)
LS Type: Summary Links(Network)
Link State ID: 10.1.0.0 (summary Network Number)
Advertising Router: 10.0.0.1
LS Seq Number: 80000001
Checksum: 0x1234
Length: 28
Network Mask: /16
TOS: 0 Metric: 10
If the summary route is missing, the ABR is not generating it.Look for the summary route in area 0's database. If it's absent, the ABR is not advertising it. If present, check if it's being filtered or if there's a metric issue.
Check the ABR's routing table for the summary route
show ip route ospf | include 10.1.0.0O IA 10.1.0.0/16 [110/11] via 192.168.1.1, 00:00:12, GigabitEthernet0/0
If the summary route appears as an O IA route, the ABR has it in its routing table. If not, the summary is not being generated or is being filtered.
Examine the ABR's OSPF configuration for the area range command
show running-config | section router ospfrouter ospf 1 network 192.168.1.0 0.0.0.255 area 0 network 10.0.0.0 0.0.0.255 area 1 area 1 range 10.1.0.0 255.255.0.0
Verify that the 'area 1 range' command is present. If missing, the summary is not configured. If present, check for typos or incorrect area number.
Root Cause
The ABR is missing the 'area 1 range 10.1.0.0 255.255.0.0' command under the OSPF process, which is required to advertise a summary route for area 1 into area 0. Without this command, the ABR advertises individual /24 routes instead of the summary, or if those routes are not in the OSPF database, nothing is advertised.
Resolution
Verification
After applying the configuration, verify the summary route appears: show ip ospf database summary Expected output should include the summary LSA for 10.1.0.0/16 with advertising router 10.0.0.1. Also verify reachability from a backbone router: ping 10.1.1.1 source 192.168.1.1 Should succeed.
Prevention
1. Always configure area range commands on ABRs when designing multi-area OSPF to reduce LSA flooding and routing table size. 2. Use network documentation to ensure summary addresses correctly cover all subnets in the area. 3. Verify OSPF database on ABRs after configuration changes to confirm summary LSAs are generated.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests understanding of OSPF multi-area operation and the role of ABRs. Questions may present a troubleshooting scenario where remote networks are unreachable and require identifying missing 'area range' configuration. The exam expects candidates to know that Type 3 summary LSAs are generated by ABRs using the 'area range' command.
Exam Tips
Remember that 'area range' is configured on the ABR, not on other routers.
The summary route appears as an O IA (inter-area) route in other areas.
If the summary route is missing, check the ABR's OSPF database for Type 3 LSAs.
Commands Used in This Scenario
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