One of the functions of a Type 4 summary LSA is to announce the reachability
of an ASBR to other areas. The Type 4 LSA is not required if the ASBR exists in
the same area.
The ASBR does not generate the Type 4 summary LSA if it
is not connected to area 0. To generate a summary LSA of Type 3 or Type 4, a
router must have a connection to area 0. As a result, the external routes will
not be installed in the network.
Figure
shows a network
in which R3 redistributes RIP routes into OSPF. Figure
shows
that R1 is not installing the external route 200.200.200.0/24 into the routing
table.
Debugs and Verification
The output of show ip
ospf database external command in Figure
shows that the
route exists in the external OSPF database of router R1. However, the output of
show ip ospf database asbr-summary command in Figure
shows that there
is no Type 4 LSA for this route.
The next logical step is to go on the
ABR and see if it is indeed an ABR. If it is an ABR it will generate a summary
LSA of Type 3 or Type 4. If it is not an ABR, it will not generate that
summary. The output of the show ip ospf command in Figure
shows that
router R2, which is between two areas and may look like an ABR, is not
identifying itself as an ABR. If R2 were an ABR, the output would display that
is an "Area Border Router."
 |
NOTE:
All areas in an OSPF autonomous system must be physically or virtually
connected to the backbone area (area 0). For a router to be an ABR, one of its
interfaces must be part of area 0.
|
Solution
In this example, router R2 does not generate the
Type 4 summary LSA because it is not connected to area 0. To generate a summary
LSA of Type 3 or Type 4, a router must have a connection into area 0.
To
solve this problem, router R2 must be connected to area 0, either physically or
virtually by creating a virtual link, as shown in Figure
. By configuring
a virtual link on R2, the router is now virtually connected to area 0;
therefore, it now considers itself an ABR. After connecting R2 to area 0, the
output of show ip ospf shows that it is now an ABR.

After the
configuration change on R2, as shown in Figure
, R1 is now
generating a Type 4 summary LSA into area 3. Because the Type 4 LSA is now
being received, R1 installs the external route 200.200.200.0/24 into its
routing table.
