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OSPFCCNA 200-301

OSPF Summary Routes Not Being Generated at ABR

Presenting Symptom

Remote networks reachable via OSPF are not being summarized at the ABR; instead, individual /24 routes appear in the routing table of the backbone area.

Network Context

The network consists of an enterprise campus with a three-tier OSPF design: Area 0 (backbone) and Area 1 (non-backbone). An ABR (Cisco 4321 router, IOS 16.9) connects Area 1 to Area 0. Area 1 has multiple /24 subnets (e.g., 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24) that should be summarized as 10.1.0.0/16 into Area 0. The ABR has been configured with the 'area 1 range' command, but the summary route is not being generated.

Diagnostic Steps

1

Check OSPF neighbor state on ABR

show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.2         1   FULL/DR         00:00:35    10.0.12.2       GigabitEthernet0/0
10.0.0.3         1   FULL/DR         00:00:32    10.0.13.2       GigabitEthernet0/1

Verify that OSPF neighbors are in FULL state. If neighbors are not FULL, summarization won't work due to lack of adjacency. If FULL, proceed to check routing and configuration.

2

Examine OSPF routes on ABR

show ip route ospf
O    10.1.1.0/24 [110/2] via 10.0.12.2, 00:10:15, GigabitEthernet0/0
O    10.1.2.0/24 [110/2] via 10.0.12.2, 00:10:15, GigabitEthernet0/0
O    10.1.3.0/24 [110/2] via 10.0.12.2, 00:10:15, GigabitEthernet0/0
     10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

The ABR sees individual /24 routes from Area 1. If summarization were working, you would see a single O IA 10.1.0.0/16 route. The absence of the summary indicates the 'area 1 range' command is not effective.

3

Verify OSPF configuration on ABR

show running-config | section router ospf
router ospf 1
 network 10.0.0.0 0.255.255.255 area 0
 network 10.1.0.0 0.0.255.255 area 1
 area 1 range 10.1.0.0 255.255.0.0

Check that the 'area 1 range' command is present. If missing, that's the root cause. If present, note that the range command must be applied under router ospf, not under an interface. Also ensure the area number matches.

4

Check OSPF database for summary LSA

show ip ospf database summary
            OSPF Router with ID (10.0.0.1) (Process ID 1)
                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.0.0        10.0.0.1        100         0x80000001 0x00A1B2

If the summary LSA (Link ID 10.1.0.0) appears in Area 0's database, the ABR is generating it. If not, the ABR is not creating the summary. If the LSA exists but routes are still not in the routing table, check for route filtering or administrative distance issues.

Root Cause

The 'area 1 range' command was configured under the wrong OSPF process or was missing entirely. In this scenario, the command was accidentally omitted during initial configuration. Without the range command, the ABR does not generate a Type 3 summary LSA for Area 1 subnets, so individual routes are advertised into Area 0.

Resolution

1. Enter global configuration mode. 2. Enter router ospf configuration mode: 'router ospf 1' 3. Add the range command: 'area 1 range 10.1.0.0 255.255.0.0' 4. Exit and save configuration: 'end', 'write memory' Explanation: The 'area 1 range' command instructs the ABR to summarize routes from Area 1 into a single Type 3 LSA for Area 0. The network mask 255.255.0.0 defines the summary prefix.

Verification

1. Run 'show ip route ospf' on the ABR and a router in Area 0. Expected output: 'O IA 10.1.0.0/16 [110/2] via <ABR-IP>' instead of multiple /24 routes. 2. Run 'show ip ospf database summary' on the ABR. Expected output: a summary LSA with Link ID 10.1.0.0 and ADV Router set to the ABR's router ID. 3. Run 'show ip ospf' to confirm the range is active: 'Area 1 range 10.1.0.0/16 Advertise'.

Prevention

1. Always verify OSPF summarization configuration using 'show ip ospf' or 'show running-config | section router ospf' after initial setup. 2. Use a configuration template or automation to ensure consistent application of range commands on all ABRs. 3. Implement a change management process that includes verification steps for OSPF summarization after any routing changes.

CCNA Exam Relevance

On the CCNA 200-301 exam, OSPF summarization is tested in troubleshooting scenarios, often as a multiple-choice question or a drag-and-drop where you must identify missing or incorrect 'area range' commands. The exam expects you to know that the 'area range' command is configured under router ospf and that it generates a Type 3 summary LSA. A common distractor is confusing 'area range' with 'summary-address' (used for redistribution).

Exam Tips

1.

Remember that 'area range' is used for inter-area summarization (ABR), while 'summary-address' is for external route summarization (ASBR).

2.

In troubleshooting, if you see individual routes instead of a summary, check the ABR's OSPF configuration for the correct 'area range' command.

3.

The 'show ip ospf database summary' command is key to verifying that the ABR is generating the summary LSA.

Commands Used in This Scenario

show ip ospf

Displays general information about OSPF routing process, including router ID, areas, and LSDB statistics, used to verify OSPF configuration and operational status.

show ip route

Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.

show running-config

Displays the current active configuration in DRAM, showing all non-default settings.

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