Hello Carlo,
Per RFC 3101, Section 2.5, Step 6:
Preference is defined as follows:
(a) Intra-area and inter-area paths are always preferred over
AS external paths.
[OSPF]
(b) Type 1 external paths are always preferred over type 2
external paths. When all paths are type 2 external paths,
the paths with the smallest advertised type 2 metric are
always preferred.
[OSPF]
(c) If the new AS external path is still indistinguishable
from the current paths in N's routing table entry, and
RFC1583Compatibility is set to "disabled", select the
preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section 16.4.1.
Here intra-NSSA paths are equivalent to the intra-area
paths of non-backbone regular OSPF areas.
[NSSA]
(d) If the new AS external path is still indistinguishable
from the current paths in N's routing table entry, select
the preferred path based on a least cost comparison. Type
1 external paths are compared by looking at the sum of the
distance to the ASBR/forwarding addresses and the
advertised type 1 metric (X+Y). Type 2 external paths
advertising equal type 2 metrics are compared by looking
at the distance to the ASBR/forwarding addresses.
~[OSPF]
(e) If the current LSA is functionally the same as an
installed LSA (i.e., same destination, cost and non-zero
forwarding address) then apply the following priorities in
deciding which LSA is preferred:
1. A Type-7 LSA with the P-bit set.
2. A Type-5 LSA.
3. The LSA with the higher router ID.
Hence, if a router knows both LSA-7 and LSA-5 for the same route and they both provide the same total metric of the same type, the information from LSA-7 is preferred.
Note that the RFC 3101 is an upgraded version of OSPF NSSA functionality. The NSSA was first specified in (now obsolete) RFC 1587 which states a different preference (Section 3.5 Step 5):
When a type-5 LSA and a type-7 LSA are found to have the
same type and an equal distance, the following priorities
apply (listed from highest to lowest) for breaking the tie.
a. Any type 5 LSA.
b. A type-7 LSA with the P-bit set and the forwarding
address non-zero.
c. Any other type-7 LSA.
If the new path is shorter, it replaces the present paths
in the routing table entry. If the new path is the same
cost, it is added to the routing table entry's list of
paths.
Hence, different RFC versions define different NSSA behavior. In Cisco IOS, the updated RFC 3101 behavior was implemented in 15.0SY, 15.1S and 15.2M onwards. You may therefore end up with differing route selections based on different IOS versions.
Best regards,
Peter
