The two most common problems associated with OSPF and redistribution are:
- OSPF is not installing external routes in the routing table.
- ASBR is not advertising redistributed routes.
OSPF is Not Installing External Routes in the Routing
Table
When OSPF redistributes any routes whether connected, static, or
from a different routing protocol, it generates a Type 5 LSA for those external
routes. These Type 5 routes are flooded into every OSPF router, with the
exception of those in stub and NSSA areas. Sometimes, the problem is that the
external routes are in the OSPF database but are not being installed in the
routing table.
The most common causes of this problem are:
- The forwarding address is not known through the intra-area or interarea
route.
- The ABR is not generating Type 4 Summary LSAs.
Both of these problems are discussed in the Troubleshooting OSPF
section.
ASBR is Not Advertising Redistributed
Routes
Whenever a route is known to be connected or static, or when any
other routing protocol is redistributed into OSPF, an external LSA is generated
for that route. If an OSPF router is not advertising the external route even
after the redistribution, this indicates a problem on a router that is doing
the redistribution. Mostly, the problem stems from configuration mistakes.
The most common causes of this problem are:
- The subnets keyword is missing from the ASBR
configuration.
-
distribute-list out command is blocking the routes.
Distribute list issues are discussed in the section on Common IGP
Routing Protocol Issues, Causes and Solutions. The following is an example of a
problem caused by the missing subnets keyword on the ASBR
configuration.
When any protocol is redistributed into OSPF, if the
networks that are being redistributed are subnets, the
subnets keyword must be used under the OSPF configuration.
If the subnets keyword is not added, OSPF will ignore all the subnetted routes
when generating the external LSA.
The situation could arise when
connected or static routes are being redistributed into or out of OSPF. In that
case, the same rule applies: The subnets keyword must be entered to
redistribute subnetted routes.
Figure
shows a network
setup that is redistributing into OSPF.
Debugs and
Verification
Figure
shows
the output of show ip ospf database external for
132.108.3.0. The output shows no LSA information, which means that R1 is not
even originating the external LSA for 132.108.3.0. Figure
shows the OSPF
configuration of R1, and that the redistribute rip command
under OSPF is missing the subnets keyword.
Solution
The solution to this problem is to add the
subnets keyword to the subnets command
under OSPF.
After this
option has been added, OSPF will redistribute all of the routes that are
subnetted; for example 131.108.3.0 with the /24 mask. Figure
shows that R1 is
now generating the external LSA for 132.108.3.0/24 and 132.108.4.0/24.