In the topology above, routers R1 and R2 are the ABRs
interfacing with backbone area 0
Area 1 in this topology has been configured as NSSA this
time. My
earlier topology referenced a Totally NSSA area.
As mentioned earlier, NSSA configuration in OSPF allows
redistribution to occur within the NSSA area.
These redistributed prefixes are Type 7 LSAs and are converted to Type 5
LSAs as they are injected in area 0 by one of the ABRs interfacing area 0
Router with higher
router ID is elected to do this translation where there are multiple ABRs
interfacing backbone area 0
Like stub area,
Type-5 external LSAs are blocked from entering the NSSA area.
Compared to Totally NSSA, area configured as NSSA will not filter Type 3 LSAs and will only filter Type 4/Type 5 LSAs.
Compared to Totally NSSA, area configured as NSSA will not filter Type 3 LSAs and will only filter Type 4/Type 5 LSAs.
As opposed to Totally NSSA; in the case of NSSA, the default
route is not auto injected.
And this needs to be injected manually using command area
<area_id> nssa default-information-originate
===================================
Area specific configuration on routers R1, R2, R3 and R4 is
as below
Configuration on Router R1
Conf t
Router ospf 1
Area 1 nssa
default-information-originate
Configuration on Router R2
Conf t
Router ospf 1
Area 1 nssa
default-information-originate metric 100
Configuration on Routers R3 and R4
Conf t
Router ospf 1
Area 1 nssa
For demonstration purpose, note in the above configuration
of Router R2, we have increased the metric of default route to 100
This will force
traffic from routers R3 and R4 to follow the default route of Router R1 since route
from R1 has lower metric.
We are just trying to
cover this use-case whereby you can force the traffic through one of the
upstream routers in case there is a better WAN link there.
Below output shows default route on router R4
 |
| Default Route on Router R4 |
Below output shows clearly that metric through Router R1 is lower as compared to Router R2
=======================================
If you want the traffic moving from R3/R4 towards backbone
area 0 to be load balanced across R1/R2, then there is no need to increase the
metric of default route on R1/R2
In that case, OSPF topology information for default route on Router R4 will be as below.
In that case, OSPF topology information for default route on Router R4 will be as below.
