OSPF Summary Address Cost

By default, OSPF route summarization  generate  the summary route metric based on lowest metric of the more specific routes present in its routing table.

Let’s check this topology

R3 is an Area Border Router that receives two route from R1 and R2, Assuming that we manually adjust the OSPF cost of the link towards R1 to 2000 and the link to R2 to 3000.

Let’s check R3 routing table

R3#show ip route

     100.0.0.0/24 is subnetted, 2 subnets

O       100.0.1.0 [110/2001] via 10.1.3.1, 00:00:03, FastEthernet0/0

O       100.0.2.0 [110/3001] via 10.2.3.2, 00:00:03, FastEthernet0/1

     3.0.0.0/32 is subnetted, 1 subnets

C       3.3.3.3 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 3 subnets

C       10.1.3.0 is directly connected, FastEthernet0/0

C       10.2.3.0 is directly connected, FastEthernet0/1

C       10.3.4.0 is directly connected, FastEthernet1/0

Now let’s enable summarization of the 100.0.0.0/8 route from area 5

R3(config)#router ospf 1

R3(config-router)#area 5 range 100.0.0.0 255.0.0.0

Let’s check R3 routing table again

R3#show ip route

     100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks

O       100.0.0.0/8 is a summary, 00:10:01, Null0

O       100.0.1.0/24 [110/2001] via 10.1.3.1, 00:00:06, FastEthernet0/0

O       100.0.2.0/24 [110/3001] via 10.2.3.2, 00:00:23, FastEthernet0/1

     3.0.0.0/32 is subnetted, 1 subnets

C       3.3.3.3 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 3 subnets

C       10.1.3.0 is directly connected, FastEthernet0/0

C       10.2.3.0 is directly connected, FastEthernet0/1

C       10.3.4.0 is directly connected, FastEthernet1/0

The summary route has been generated and its next hop is Null0 since R3 is the originator of the summary route.

Checking the OSPF Database for self-originated summary LSAs

R3#show ip ospf database summary 100.0.0.0 self-originate

            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Summary Net Link States (Area 0)

  LS age: 245

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(Network)

  Link State ID: 100.0.0.0 (summary Network Number)

  Advertising Router: 3.3.3.3

  LS Seq Number: 80000001

  Checksum: 0x876C

  Length: 28

  Network Mask: /8

        TOS: 0  Metric: 2001

It’s seems that The R3 Generated the summary route with the lowest metric after comparing between the 100.0.1.1/24 and 100.0.2.2/24  metrics.

Now let’s check what R4 is receiving

R4#show ip route

O IA 100.0.0.0/8 [110/2002] via 10.3.4.3, 00:00:05, FastEthernet0/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/2] via 10.3.4.3, 00:40:39, FastEthernet0/0

     4.0.0.0/32 is subnetted, 1 subnets

C       4.4.4.4 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 4 subnets

O IA    10.1.3.0 [110/2001] via 10.3.4.3, 00:40:39, FastEthernet0/0

C       10.4.5.0 is directly connected, FastEthernet0/1

O IA    10.2.3.0 [110/3001] via 10.3.4.3, 00:39:51, FastEthernet0/0

C       10.3.4.0 is directly connected, FastEthernet0/0

Indeed R4 is receiving the summary route with a metric of 2002

A point that needs to be clarified, this behavior was based on RFC1583, in a later implementation of RFC2328 the behavior was changed to the highest metric instead of the lowest metric. You can refer to that in RFC2328 section 3.5

“Area border routers then summarize the area

 contents (for distribution to the backbone) by advertising a

 single route for each address range.  The cost of the route is

 the maximum cost to any of the networks falling in the specified

 range.”

You can disable OSPF compatibility to the old RFC1583 and make the normal behavior as stated in RFC2328

R3(config)#router ospf 1

R3(config-router)#no compatible rfc1583

Now let’s check R3 self-originated summary routes again

R3#show ip ospf database summary 100.0.0.0 self-originate

            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Summary Net Link States (Area 0)

  LS age: 8

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(Network)

  Link State ID: 100.0.0.0 (summary Network Number)

  Advertising Router: 3.3.3.3

  LS Seq Number: 80000004

  Checksum: 0xB450

  Length: 28

  Network Mask: /8

        TOS: 0  Metric: 3001

And R4 for a final check

R4#show ip route

Gateway of last resort is not set

O IA 100.0.0.0/8 [110/3002] via 10.3.4.3, 00:02:16, FastEthernet0/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/2] via 10.3.4.3, 00:15:31, FastEthernet0/0

     4.0.0.0/32 is subnetted, 1 subnets

C       4.4.4.4 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 4 subnets

O IA    10.1.3.0 [110/2001] via 10.3.4.3, 00:12:36, FastEthernet0/0

C       10.4.5.0 is directly connected, FastEthernet0/1

O IA    10.2.3.0 [110/3001] via 10.3.4.3, 00:12:46, FastEthernet0/0

C       10.3.4.0 is directly connected, FastEthernet0/0

Hopefully that clears up how OSPF ABR summarization works