OSPFv3 Forwarding Address under NSSA vs Regular External

I am sharing something I researched recently at work, and still do not have the final answer, however it is interesting so I decided to write about it, some of the development group asked us why and if it is really important to use the Forwarding address in OSPFv3 implementation.

I have used the above model (and another that may be I will share later).

External NSSA

External without NSSA

I am advertising from R6 and R5 net 2001:11:145:56:: On the External database of R1 there is only a single selected although under R3 both options (from R5 and R6) are there, so it looks like it is being filtered. R1#sh ipv ospf database external             OSPFv3 Router with ID (150.1.1.1) (Process ID 1)                 Type-5 AS External Link States   …   Routing Bit Set on this LSA   LS age: 143   LS Type: AS External Link   Link State ID: 1   Advertising Router: 150.3.3.3   LS Seq Number: 80000002   Checksum: 0xEC3F   Length: 56   Prefix Address: 2001:11:145:56::   Prefix Length: 64, Options: None   Metric Type: 2 (Larger than any link state path)   Metric: 20 Forward Address: 2001:11:143:36::2   External Route Tag: 600 … R3 is the ABR and as such it translate the LSA-7 to 5 To reach Advertising router we must use link local R1#sh ipv ospf database link             OSPFv3 Router with ID (150.1.1.1) (Process ID 1)                 Link (Type-8) Link States (Area 0)   LS age: 1175   Options: (V6-Bit, E-Bit, R-bit, DC-Bit)   LS Type: Link-LSA (Interface: FastEthernet1/1)   Link State ID: 4 (Interface ID)   Advertising Router: 150.1.1.1   LS Seq Number: 80000001   Checksum: 0x8CFC   Length: 56   Router Priority: 1   Link Local Address: FE80::C800:19FF:FE9C:1D   Number of Prefixes: 1   Prefix Address: 2001:11:141:13::   Prefix Length: 64, Options: None   LS age: 1131   Options: (V6-Bit, E-Bit, R-bit, DC-Bit)   LS Type: Link-LSA (Interface: FastEthernet1/1)   Link State ID: 5 (Interface ID) Advertising Router: 150.3.3.3   LS Seq Number: 80000001   Checksum: 0xCC98   Length: 56   Router Priority: 1 Link Local Address: FE80::C802:19FF:FE9C:38   Number of Prefixes: 1   Prefix Address: 2001:11:141:13::   Prefix Length: 64, Options: None R1# R1#show ipv route 2001:11:145:56:: Routing entry for 2001:11:145:56::/64   Known via "ospf 1", distance 110, metric 20   Tag 600, type extern 2   Route count is 1/1, share count 0   Routing paths: FE80::C802:19FF:FE9C:38, FastEthernet1/1       Last updated 00:16:00 ago R1# The FWD address used is the selected default gateway to reach the External prefix as it can have 2 paths (one from R5 and the other from R6) However it looks like the FWD address is ignored as R3 install both paths and use both of them R1#traceroute 2001:11:145:56::1 Type escape sequence to abort. Tracing the route to 2001:11:145:56::1   1  *     2001:11:141:13::2 8 msec 4 msec   2 2001:11:143:36::2 16 msec     2001:11:143:35::2 16 msec     2001:11:143:36::2 12 msec R1#traceroute 2001:11:145:56::2 Type escape sequence to abort. Tracing the route to 2001:11:145:56::2   1 2001:11:141:13::2 4 msec 4 msec 8 msec   2 2001:11:143:36::2 20 msec     2001:11:143:35::2 8 msec     2001:11:143:36::2 16 msec R1#

On that case we both LSA to the External route R1#show ipv os database external             OSPFv3 Router with ID (150.1.1.1) (Process ID 1)                 Type-5 AS External Link States   …   Routing Bit Set on this LSA   LS age: 54   LS Type: AS External Link   Link State ID: 7   Advertising Router: 150.5.5.5   LS Seq Number: 80000001   Checksum: 0x442   Length: 40   Prefix Address: 2001:11:145:56::   Prefix Length: 64, Options: None   Metric Type: 2 (Larger than any link state path)   Metric: 20   External Route Tag: 500 … Routing Bit Set on this LSA   LS age: 28   LS Type: AS External Link   Link State ID: 4   Advertising Router: 150.6.6.6   LS Seq Number: 80000001   Checksum: 0xAC35   Length: 40   Prefix Address: 2001:11:145:56::   Prefix Length: 64, Options: None   Metric Type: 2 (Larger than any link state path)   Metric: 20   External Route Tag: 600 To reach them R3 the ABR is advertising an Inter area router (similar to LSA 4) R1#show ipv os database inter-area router             OSPFv3 Router with ID (150.1.1.1) (Process ID 1)                 Inter Area Router Link States (Area 0)   Routing Bit Set on this LSA   LS age: 143   Options: (V6-Bit, E-Bit, R-bit, DC-Bit)   LS Type: Inter Area Router Links   Link State ID: 2516911365   Advertising Router: 150.3.3.3   LS Seq Number: 80000001   Checksum: 0x45D5   Length: 32   Metric: 1   Destination Router ID: 150.5.5.5   Routing Bit Set on this LSA   LS age: 116   Options: (V6-Bit, E-Bit, R-bit, DC-Bit)   LS Type: Inter Area Router Links   Link State ID: 2516977158   Advertising Router: 150.3.3.3   LS Seq Number: 80000001   Checksum: 0x4BC9   Length: 32   Metric: 1   Destination Router ID: 150.6.6.6 R1# And again we need link local to reach ABR R1#show ipv os database link             OSPFv3 Router with ID (150.1.1.1) (Process ID 1)                 Link (Type-8) Link States (Area 0)   LS age: 1959   Options: (V6-Bit, E-Bit, R-bit, DC-Bit)   LS Type: Link-LSA (Interface: FastEthernet1/1)   Link State ID: 4 (Interface ID)   Advertising Router: 150.1.1.1   LS Seq Number: 80000001   Checksum: 0x8CFC   Length: 56   Router Priority: 1   Link Local Address: FE80::C800:19FF:FE9C:1D   Number of Prefixes: 1   Prefix Address: 2001:11:141:13::   Prefix Length: 64, Options: None   LS age: 34   Options: (V6-Bit, E-Bit, R-bit, DC-Bit)   LS Type: Link-LSA (Interface: FastEthernet1/1)   Link State ID: 5 (Interface ID)   Advertising Router: 150.3.3.3   LS Seq Number: 80000002   Checksum: 0xCA99   Length: 56   Router Priority: 1 Link Local Address: FE80::C802:19FF:FE9C:38   Number of Prefixes: 1   Prefix Address: 2001:11:141:13::   Prefix Length: 64, Options: None R1# · Forwarding is correctly. R1#traceroute 2001:11:145:56::1 Type escape sequence to abort. Tracing the route to 2001:11:145:56::1   1 2001:11:141:13::2 4 msec 8 msec 4 msec   2 2001:11:143:35::2 8 msec     2001:11:143:36::2 32 msec     2001:11:143:35::2 12 msec R1#traceroute 2001:11:145:56::2 Type escape sequence to abort. Tracing the route to 2001:11:145:56::2   1 2001:11:141:13::2 4 msec 4 msec 8 msec   2 2001:11:143:35::2 8 msec     2001:11:143:36::2 20 msec     2001:11:143:35::2 16 msec R1#

I have to say that the following implementation was based on Cisco IOS 15.2, older versions, do not use the Forwarding address at all although from the results above technically I do not see how it is used here as well forwarding wise.

As for the RFC’s the reference is https://tools.ietf.org/html/rfc5340

• The forwarding address is present in the AS-external-LSA if and only if the AS-external-LSA's bit F is set.
• case the F bit is marked:
• It MUST NOT be set to the IPv6 Unspecified Address (0:0:0:0:0:0:0:0) or an IPv6 Link-Local Address (Prefix FE80/10)
• forwarding address MUST advertise a global IPv6 address
• Section A.4.8.  NSSA-LSAs

"The selection should proceed the same as OSPFv2 NSSA support [NSSA]with additional checking to ensure IPv6 link-local address are not selected."

• that is referencing to NSSA RFC 3031 where it clearly indicate under section 2.3  Type-7 LSAs

          "6. Those Type-7 LSAs that are to be translated into Type-5 LSAs must have their forwarding address set."

What do you think?