Cisco OSPF Route Summary Configuration Example

Table of Contents

Summary

This is an example of configuring OSPF inter-area route summary on a Cisco router. Notice the default cost of the default summary route.

Related articl

For more information of configuring route summary, please see the following article.

N-Study

OSPF Route Summary and Configuration | N-Study

https://www.n-study.com/en/ospf-detail/ospf-route-summary/

In OSPF, route summary can be performed with ABR and ASBR.This section explains the mechanism of route summary with ABR/ASBR in OSPF and the configuration commands on Cisco routers.

Network diagram

Configuration Conditions

Enable OSPF based on the network diagram.
On R2, make sure that the OSPF route looks like the following.

R2#show ip route ospf
     10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
O       10.3.1.0/24 [110/20] via 192.168.23.3, 00:00:13, Ethernet0/1
O       10.3.3.0/27 [110/40] via 192.168.23.3, 00:00:13, Ethernet0/1
O       10.3.2.0/25 [110/30] via 192.168.23.3, 00:00:13, Ethernet0/1

On R1, make sure that the OSPF route looks like the following.

R1#show ip route ospf
     10.0.0.0/16 is subnetted, 1 subnets
O IA    10.3.0.0 [110/30] via 192.168.12.2, 00:00:41, Ethernet0/0
O IA 192.168.23.0/24 [110/20] via 192.168.12.2, 00:04:44, Ethernet0/0

Initial Configuration

hostname
IP address

Configuration and Verification

Step1:Enabling OSPF

Enable OSPF on R1/R2/R3 based on the network diagram. On R3, the Loopback interface configures the networks 10.3.1.0/24, 10.3.2.0/25, and 10.3.3.0/27. On R3, ip ospf network point-to-point is required on the Loopback interface.

router ospf 1
 router-id 1.1.1.1
 log-adjacency-changes
 network 192.168.12.0 0.0.0.255 area 0

router ospf 1
 router-id 2.2.2.2
 log-adjacency-changes
 network 192.168.12.0 0.0.0.255 area 0
 network 192.168.23.0 0.0.0.255 area 1

interface Loopback0
 ip ospf network point-to-point
!
interface Loopback1
 ip ospf network point-to-point
!
interface Loopback2
 ip ospf network point-to-point
!
router ospf 1
 router-id 3.3.3.3
 log-adjacency-changes
 network 10.3.0.0 0.0.255.255 area 1
 network 192.168.23.0 0.0.0.255 area 1

Step2:Verifying OSPF

Verify that the OSPF configuration in Step 1 is working correctly. Verify the OSPF neighbor and OSPF route on R2.

R2#show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/BDR        00:00:34    192.168.12.1    Ethernet0/0
3.3.3.3           1   FULL/BDR        00:00:35    192.168.23.3    Ethernet0/1
R2#show ip route ospf 
     10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
O       10.3.1.0/24 [110/11] via 192.168.23.3, 00:01:39, Ethernet0/1
O       10.3.3.0/27 [110/11] via 192.168.23.3, 00:01:39, Ethernet0/1
O       10.3.2.0/25 [110/11] via 192.168.23.3, 00:01:39, Ethernet0/1

We can see that R2 has established a neighbor with R1 and R3 and has correctly received the route beginning with “10.3” .

Step3:Changing OSPF cost

To make the OSPF route appear to be a configuration condition on R2, change the cost of each Loopback interface on R3. the cost of E0/1 on R2 is 10. To make the OSPF route look like the cost of the configuration condition, set the OSPF cost as follows

Loopback0 10.3.1.0/24 cost 10
Loopback1 10.3.2.0/25 cost 20
Loopback2 10.3.3.0/27 cost 30

interface Loopback0
 ip ospf cost 10
!
interface Loopback1
 ip ospf cost 20
!
interface Loopback2
 ip ospf cost 30

Step4:Verifying OSPF costs

Verify the OSPF routes on R2.

R2#show ip route ospf 
     10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
O       10.3.1.0/24 [110/20] via 192.168.23.3, 00:00:57, Ethernet0/1
O       10.3.3.0/27 [110/40] via 192.168.23.3, 00:00:57, Ethernet0/1
O       10.3.2.0/25 [110/30] via 192.168.23.3, 00:00:57, Ethernet0/1

You can see that the cost of the OSPF route is displayed in the same format as the configuration conditions.

Step5:Configuring Route Summary

In order for OSPF routes to appear as configuration conditions in R1’s routing table, summarize 10.3.1.0/24, 10.3.2.0/25, and 10.3.3.0/27 into 10.3.0.0/16. Since route summary in OSPF is done by ABR, generate summary route in R2.

router ospf 1
 area 1 range 10.3.0.0 255.255.0.0

Step6:Verifying Route Summary

Verify the route summary configuration. On R2, which generates summary route, use the show ip opsf command. Also, there is an LSA type 3 in the LSDB of area 0 that represents the summary route.

R2#show ip ospf 
 Routing Process "ospf 1" with ID 2.2.2.2
-- omitted --
 Number of areas in this router is 2. 2 normal 0 stub 0 nssa
 Number of areas transit capable is 0
 External flood list length 0
    Area BACKBONE(0)
        Number of interfaces in this area is 1
        Area has no authentication
        SPF algorithm last executed 00:13:20.256 ago
        SPF algorithm executed 6 times
        Area ranges are
        Number of LSA 5. Checksum Sum 0x0308BA
        Number of opaque link LSA 0. Checksum Sum 0x000000
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0
    Area 1
        Number of interfaces in this area is 1
        Area has no authentication
        SPF algorithm last executed 00:13:20.268 ago
        SPF algorithm executed 12 times
        Area ranges are
           10.3.0.0/16 Active(20) Advertise 
        Number of LSA 4. Checksum Sum 0x022C3D
        Number of opaque link LSA 0. Checksum Sum 0x000000
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0
R2#show ip ospf database summary 

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Summary Net Link States (Area 0)

  LS age: 894
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 10.3.0.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0x6FA4
  Length: 28
  Network Mask: /16
        TOS: 0  Metric: 20 

  LS age: 524
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 192.168.23.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000002
  Checksum: 0xFFA9
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 10 


                Summary Net Link States (Area 1)

  LS age: 527
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 192.168.12.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000002
  Checksum: 0x793B
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 10

By default, the summary route metric is the minimum metric for detailed routes. The minimum metric for a detailed route is 20 for 10.3.1.0/24, so the metric for an summary route is 20.

And in the routing table of R1, we can see the OSPF routes as well as the configuration conditions.

R1#show ip route ospf 
     10.0.0.0/16 is subnetted, 1 subnets
O IA    10.3.0.0 [110/30] via 192.168.12.2, 00:17:08, Ethernet0/0
O IA 192.168.23.0/24 [110/20] via 192.168.12.2, 00:43:58, Ethernet0/0