What is the sequence of the OSPF external route selection?

[GozoHUN01]

Dear friends,

I have a question about the OSPF external route selection.

Which is the correct sequence of the OSPF route selection?

External Type 1 (E1)

External Type 2 (E2)

NSSA Type 1 (N1)

NSSA Type 2 (N2) 

or

External Type 1 (E1)

NSSA Type 1 (N1)

External Type 2 (E2)

NSSA Type 2 (N2) 

?

I read some article about it, but some of them say the first is the right and some say the second.

[paul driver]

Hello

My understanding is still as follows for certain IOS's:

0

0IA

E1

E2

N1

N2

[Samer R. Saleem]

Hi,

you are correct

Since Cisco IOS release 15.1(2)S, Cisco uses the path selection order from RFC 3101 which obsoletes RFC 1587. What this means is that it prefers N1 routes before E1 and N2 over E2 routes. In other words, the preferred path list is O > O IA > N1 > E1 > N2 > E2

If this was helpful please mark it.

thanks

[paul driver · Accepted solution]

Hello

cheers @Samer R. Saleem  -I wasn't aware what version it change at, Just looked it up , its standard for all IOS-XE and as you stated 15.1(2)s for IOS  which seems to suggest a type7 metric 1 or 2 will succeed  type 5 metric 1 or 2

1587
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.

3101
The LSA type of two AS-external-LSAs plays no role in
         determining path preference except when the LSAs are
         functionally the same (i.e., same destination, cost and non-
         zero forwarding address)


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.

[GozoHUN01]

Ok, thanks your reply!

If I'm right, these two standards contain two different statements.

Does RFC1587 assert the correct order is E1, E2, N1, N2? And does the RFC 3101 assert it is  E1, N1, E2, N2?

[Samer R. Saleem]

Yes,

both are correct but I would follow the newer IOS version path selection in future which uses RFC 3101

but if you have older IOS then RFC 1587 will be in use....