LAB: OSPF Default Routing

- Configure R6 with a static default route pointing to BB1.
- Configure R6 to originate an external type-2 default route with a metric of 60 into OSPF as long as the static default route is installed in the routing table.
- Do not use a route-map to accomplish this.
- Configure R4 to originate an external type-1 default route with a metric of 40 regardless of whether it has a default route already installed in the routing table.

On a non-stub areas default routing is achieve by using default-information originate command.  OSPF first checks to see if a default route is installed in the routing table. if the default route exists either via static or learned via other routing protocols, the OSPF default route is originated.  but if the default route is not found, no origination will occur.  This design is desirable whereby you have 2 or more exit points out of the OSPF domain to ISP network.  Individual exit point will only originate a default traffic if they themselves have a default exit point to upstream network.

the default-information originate always argument skips over the checking for a default route already being installed in the table.

Rack1R1#show ip ospf database external 0.0.0.0

            OSPF Router with ID (150.1.1.1) (Process ID 1)

command sets

Rack1R6(config)#router ospf 1                
Rack1R6(config-router)#default-information originate metric 60

Rack1R4(config)#router ospf 1
Rack1R4(config-router)#default-information originate always metric 40 metric-type 1

verification:

R4 is originating a E1 default route with metric of 40  and R6 with default E2 metric of 60. same route logic is applied to these default routes. E1 is proffered over E2. and if both are E2 then forward metrics are compared.

Rack1R1#show ip ospf database external 0.0.0.0

            OSPF Router with ID (150.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 2 (DoNotAge)
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: 150.1.4.4
  LS Seq Number: 80000001
  Checksum: 0xB7B4
  Length: 36
  Network Mask: /0
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0
        Metric: 40
        Forward Address: 0.0.0.0
        External Route Tag: 1

  Routing Bit Set on this LSA
  LS age: 173
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: 150.1.6.6
  LS Seq Number: 80000001
  Checksum: 0xE9E9
  Length: 36
  Network Mask: /0
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 60
        Forward Address: 0.0.0.0
        External Route Tag: 1

Rack1R1#show ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "ospf 1", distance 110, metric 41, candidate default path
  Tag 1, type extern 1
  Last update from 155.1.146.4 on FastEthernet0/0, 00:00:13 ago
  Routing Descriptor Blocks:
  * 155.1.146.4, from 150.1.4.4, 00:00:13 ago, via FastEthernet0/0
      Route metric is 41, traffic share count is 1
      Route tag 1