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
hostname Chicago ! router ospf 10
redistribute static metric 100 metric-type 1 subnetsnetwork 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
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.
Referring to Figure 6-1, the router ...