Stub, Totally Stubby, and Not So Stubby Areas

External LSAs are flooded through the OSPF backbone as well as through all regular areas. Let’s test this using TraderMary’s network of Figure 6-10. A static route for 192.168.3.0 is defined (pointing to null0) on Chicago and redistributed into OSPF. Router Chicago then advertises an external LSA with a link state ID of 192.168.3.0:

hostname Chicago
!
router ospf 10
 redistribute static metric 100 metric-type 1 subnets
 network 172.16.0.0 0.0.255.255 area 0
!
ip route 192.168.3.0 255.255.255.0 Null0

The LSA is flooded to all routers in the network. Let’s check Paris as an instance:

Paris#sh ip ospf database external

       OSPF Router with ID (192.168.1.5) (Process ID 10)


           AS External Link States

  Routing Bit Set on this LSA
  LS age: 158
  Options: (No TOS-capability)
  LS Type: AS External Link
  Link State ID: 192.168.3.0 (External Network Number )
  Advertising Router: 192.168.1.3
  LS Seq Number: 80000001
  Checksum: 0x8F67
  Length: 36
  Network Mask: /24
    Metric Type: 1 (Comparable directly to link state metric)
    TOS: 0 
    Metric: 100 
    Forward Address: 0.0.0.0
    External Route Tag: 0

The route to 192.168.3.0 also appears in the routing table:

Paris#sh ip route
...
Gateway of last resort is not set
...
O E1 192.168.3.0/24 [110/302] via 10.0.1.2, 00:02:08, Serial1
...

Flooding external LSAs throughout an OSPF network may be a waste of resources. Stub areas block the flooding of external LSAs, as we will see in the next section.

Stub Areas

Referring to Figure 6-1, the router ...

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