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 is defined (pointing to null0) on Chicago and redistributed into OSPF. Router Chicago then advertises an external LSA with a link state ID of

hostname Chicago
router ospf 10
 redistribute static metric 100 metric-type 1 subnets
 network area 0
ip route 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 ( (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: (External Network Number )
  Advertising Router:
  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:
    External Route Tag: 0

The route to also appears in the routing table:

Paris#sh ip route
Gateway of last resort is not set
O E1 [110/302] via, 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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