IP Routing

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 ...