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 |
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 …
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#
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# |
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
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#
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?
