If the forwarding address is not known through an intra-area or interarea
route, OSPF will not install the route in the routing table.
Figure
shows a network
with the following specifications:
- R3 is an ASBR and is redistributing RIP routes into OSPF.
- R4 is running RIP with R3.
- R4 is learning 200.200.200.0/24 through RIP.
- R2 is running OSPF with R3.
- R2 is the ABR.
The network is experiencing a problem of external routes not getting
installed in the routing table. Figure
shows
the output of show ip route for 200.200.200.0/24 on router R1. This network
resides in a RIP domain. Because RIP is being redistributed into OSPF on router
R3, all OSPF routers should see this external OSPF route. However, router R1 is
not seeing it in its routing table.
Debugs and
Verification
Figure
shows the
external LSA for 200.200.200.0/24 on R1. The output of the show ip
ospf database external command shows that the external LSA exists in
the OSPF database, but the route is still not being installed in the routing
table. Note the forwarding address involved in this external LSA.
Figure
shows that the
route to the forwarding address of 131.108.0.4, is known through an OSPF
external route. The ABR is summarizing 131.108.0.0/24 with the area range
command, so the more specific intra-area routes are summarized into one route.
This range
summarizes all routes under the 131.108.0.0/26 range.
Figure
shows that the
ASBR is doing the redistribution from RIP into OSPF. It also shows that the
connected networks in the range of 131.108.0.0/26 are being redistributed into
OSPF because RIP owns those connected routes. This configuration redistributes
131.108.0.4/26, which is a connected interface on router R3. This subnet covers
the forwarding address that appeared in Figure
.
Router R1
is seeing the forwarding address learned through OSPF external because R3 has
redistribute connected under router ospf. This leaks a more
specific route for the connected interfaces of R3. This also includes the
forwarding address and subnet, which is 131.108.0.4/26. Also, the intra-area
route for this subnet is suppressed by R2 because R2 is summarizing
131.108.0.0/16. Because the more specific route is always preferred, R1 prefers
the more specific external route of 131.108.0.4/26 over the less specific
summarized route of 131.108.0.0/16.
Solution
The problem can
be solved in one of two ways:
- Do not summarize at the ABR.
- Filter the connected subnet from being redistributed into OSPF at the ASBR.
To implement the first solution, remove the area
range command on the ABR.

To
implement the second solution, add a filter to control the redistributed routes
on the ASBR. A route-map is added to the redistribute rip
command, as shown in Figure
to prevent the
route 131.108.0.0/26 from getting redistributed into OSPF, permitting only the
200.200.200.0/24 route.
Figures
and
show the changes
in the R1 routing table for the external 200.200.200.0/24 route and that the
forwarding address is now known through OSPF interarea instead of OSPF
external.