OSPF Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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OSPF Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Configure OSPF scale and topology controls

Updated: June 26, 2026

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Set up multiple OSPF areas on a primary interface and summarize routes to reduce LSA count and optimize network resource usage.

When subnetworks are configured on different interfaces, OSPF can summarize these into a single LSA, reducing LSA numbers and conserving network resources. Interarea route summarization applies only to internal routes within your autonomous system and must be performed on the ABR. Summarization does not affect external routes injected via redistribution. Multi-area adjacency is supported where only two OSPF speakers are attached; for broadcast networks, configure the interface as OSPF point-to-point.

Enforce secure OSPF routing protocol operations by implementing authentication at the process, area, and interface levels to prevent unauthorized routing updates.

Before you begin

Ensure the interface used for multi-area adjacency has only two OSPF speakers attached.

For native broadcast networks, configure the interface as point-to-point using the network point-to-point command.

Procedure

  1. Configure the primary loopback interface IPv4 address.

    Example:

    Router(config)# interface Loopback0
Router(config-if)# ipv4 address 192.0.2.2/32
Router(config-if)# commit

  2. Associate the loopback interface with the first OSPF area.

    Example:

    Router(config)# router ospf 100
Router(config-ospf)# router-id 10.10.10.1
Router(config-ospf)# area 1
Router(config-ospf-ar)# interface Loopback0
Router(config-ospf-ar-if)# commit

  3. Reuse the same loopback interface in another OSPF area using the multi-area-interface command.

    Example:

    Router(config)# router ospf 100
Router(config-ospf)# router-id 10.10.10.1
Router(config-ospf)# area 0
Router(config-ospf-ar)# multi-area-interface Loopback0
Router(config-ospf-ar-mif)# commit

  4. Verify the configuration using show running-config.

    Example:

    
Router# show running-config
!
interface Loopback0
ipv4 address 192.0.2.2/32
!
router ospf 100
router-id 10.10.10.1
area 0
multi-area-interface Loopback0
area 1
interface Loopback0
!

  5. Check how the ABR identifies Loopback0 in area 1 using show ospf routes.

    Example:

    
Router# show ospf routes 192.0.2.2/32 backup-path detail
...
Route type: Intra-area
10.10.10.1, directly connected, via Loopback0
LSA: 1/10.10.10.1/10.10.10.1, Area: 1

    You can verify configuration changes at the RI LSA level.

  6. After enabling Loopback0 for area 1, verify that the summary LSA is not advertised by the ABR in area 0 with show ospf database summary.

    Example:

    
Router# show ospf database summary 10.10.10.1
...
Summary Net Link States (Area 0)
Link State ID: 10.10.10.1
Advertising Router: 10.0.0.1

  7. Verify the RI LSA for area 0 with show ospf database opaque-area. Confirm the multi-area loopback interface is enabled.

    Example:

    
Router# show ospf database opaque-area ext-prefix 192.0.2.2/32
...
Type-10 Opaque Link Area Link States (Area 0)
Route-type: 1
Prefix: 10.10.10.1/32
SID sub-TLV: SID Index: 4
SID sub-TLV: SID Index: 2004

The OSPF configuration is complete and verified. The router reflects the intended operational state with summarized LSAs and correct area interface assignments.