OSPF Default Route Cost

In OSPF design, STUB and NSSA areas can have multiple Area Border Routers (ABRs) to achieve redundancy or load-balancing in traffic. By default if a router in a STUB or a NSSA area is dual homed to two ABRs with equal links, and that ABR is injecting a default route into that area, the STUB router will load-balance the traffic to the ABRs. The problem occurs when those ABRs doesn’t have equal links connected to the backbone area. Even though traffic from the stub router will be load balanced to the ABRs, the traffic from the ABR with the lower bandwidth link will drop traffic going to the backbone if it exceeds the its maximum link bandwidth.

Let’s take a look at this simple topology

Area 1 has two ABRs, R2 and R10, Both are injecting a default route in Area 1. Since R7 has two Gig links to both ABRs, it will load balance traffic on those two links, which get us to an interesting concept here.

The default route is a generated route, meaning that it is originated from the ABR itself, and by default it will have a metric of 1. Meaning, even though R2 have a 155 Mb link connected to Area 0 and R10 has a 1000 Mb link to Area 0, R7 will still have the default route metric with the cost of 2 ( 1 for the link to the ABR + 1 for the generated default route cost)

Now let’s see how that looks from the prospective of R7 in normal conditions.

R7# show ip route

Gateway of last resort is 10.7.10.10 to network 0.0.0.0

     7.0.0.0/32 is subnetted, 1 subnets

C       7.7.7.7 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 2 subnets

C       10.7.10.0 is directly connected, FastEthernet0/0

C       10.2.7.0 is directly connected, FastEthernet0/1

O*IA 0.0.0.0/0 [110/2] via 10.7.10.10, 00:30:57, FastEthernet0/0

               [110/2] via 10.2.7.2, 00:27:16, FastEthernet0/1

As the routing table of R7 shows, it will load balance traffic equally to both ABRs.

Let’s see the OSPF Database from R7 Also

R7#show ip ospf database summary

            OSPF Router with ID (7.7.7.7) (Process ID 1)

                Summary Net Link States (Area 1)

  Routing Bit Set on this LSA

  LS age: 1820

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

  LS Type: Summary Links(Network)

  Link State ID: 0.0.0.0 (summary Network Number)

  Advertising Router: 2.2.2.2

  LS Seq Number: 80000003

  Checksum: 0x71C2

  Length: 28

  Network Mask: /0

        TOS: 0  Metric: 1

  Routing Bit Set on this LSA

  LS age: 1461

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

  LS Type: Summary Links(Network)

  Link State ID: 0.0.0.0 (summary Network Number)

  Advertising Router: 10.10.10.10

  LS Seq Number: 80000002

  Checksum: 0x8292

  Length: 28

  Network Mask: /0

        TOS: 0  Metric: 1

As mentioned earlier, this will cause R2 to drop traffic exceeding 155 Mb even though its share of traffic can reach 500 Mb.

Now that can be fixed either by increasing the cost of link towards R2, but since that I might have multiple routers in that area, and the default route is propagated to all routers in the stub area, we can use the sub area command default-cost.

The default-cost commands sets the initial metric of the default route injected in the area, it has a range from 0 to 16777215. In our case here, we can either set default-cost of R2 to any number higher than 1 or we can lower the default cost of R7 to 0

Let’s higher the default cost of R2

R2(config)#router ospf 1

R2(config-router)#area 1 default-cost 1000

Showing the router table of R7, only the default route from R10 in installed in the routing table

R7#show ip route

Gateway of last resort is 10.7.10.10 to network 0.0.0.0

     7.0.0.0/32 is subnetted, 1 subnets

C       7.7.7.7 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 2 subnets

C       10.7.10.0 is directly connected, FastEthernet0/0

C       10.2.7.0 is directly connected, FastEthernet0/1

O*IA 0.0.0.0/0 [110/2] via 10.7.10.10, 00:02:15, FastEthernet0/0

Also checking the database of R7

R7#show ip ospf database summary
            OSPF Router with ID (7.7.7.7)(ProcessID1)
                Summary Net Link States (Area 1)

  Routing Bit Set on this LSA
  LS age: 303
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 0.0.0.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000006
  Checksum: 0x95B0
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 1000
   
  Routing Bit Set on this LSA
  LS age: 1063
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 0.0.0.0 (summary Network Number)
  Advertising Router: 10.10.10.10
  LS Seq Number: 80000003
  Checksum: 0x8093
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 1

The route is still in the OSPF database, in case the link to R10 fails, R7 will install the other route to R2.

One thing to remember here is that for an ABR to advertise a generated default route into an area, it must have at least a single interface in area 0

If the link between R10 and backbone area fails, R7 will still prefer R10 as its gateway if R10 has it’s loopback 0 advertised in Area 0, to overcome this, you’ll have to advertise R10 Loopback 0 in area 1 so that it will not have any interfaces in area 0, but that’s a whole other debate.