OSPFv3 LSA Types: Type 2007 (Type-7) – Part 6

In this last article of the series, we will explore how OSPFv3 NSSA areas inject routes external to the OSPF domain. In OSPFv2, this function is performed by NSSA-External (Type 7) LSAs. In OSPFv3, this is a function of, very creatively named, Type-7 (Type 2007) LSAs.Our faithful test network is still running and is configured according to the below diagram.

In this case, we’ll focus on router R6 and the Loopback6, which is redistributed. Below is a quick reminder of that configuration.

R6:

interface Loopback6
 ipv6 address 2001:DB8:6::6/64
!
ipv6 router ospf 1
 redistribute connected route-map CON-to-OSPF
!
route-map CON-to-OSPF permit 10
 match interface Loopback6
!

Since R6 is the router in area 26 and area 26 is NSSA, this configuration should generate a Type-7 (Type 2007) LSA. Let’s inspect that.

R6:

R6#show ipv6 ospf database nssa-external self-originate

            OSPFv3 Router with ID (6.6.6.6) (Process ID 1)

                Type-7 AS External Link States (Area 26)

  LS age: 1864
  LS Type: AS External Link
  Link State ID: 0
  Advertising Router: 6.6.6.6
  LS Seq Number: 80000010
  Checksum: 0xC4F9
  Length: 36
  Prefix Address: 2001:DB8:6::
  Prefix Length: 64, Options: P
  Metric Type: 2 (Larger than any link state path)
  Metric: 20

Indeed, the LSA is there. One thing we should immediately notice is the absence of the Forwarding Address (FA), which is installed by default in all NSSA-External LSAs in OSPFv2. The reason for its absence is that OSPFv3 requires the next-hop for route to be link-local address, which won’t make any sense on a router that’s not directly connected. On the face of it, this makes things only simpler. As far as CCIE exam goes – yes, since we don’t have to worry about FA being reachable through OSPF, or about FA suppression on the ABRs. However, in real life, it makes things less scalable.

The presence of the FA allowed optimization of the forwarding using multiple paths in inter-area operation. In OSPFv3, we can use only the path through the ABR which converted Type 7 to Type 5, since we don’t have FA to calculate the next-hop. I have to admit, this limitation came as a surprise to me when I was writing this blog. Let’s not dwell on that.

Since Type-7 LSA has area flooding scope, router is other area need to learn how to forward the traffic for those prefixes. Just like in OSPFv2, ABR with the highest OSPF Router-ID will convert Type-7 LSA into AS-External and advertise it to area 0. In our case, this is a function performed by R2. Let’s take a look.

R2:

R2#show ipv6 ospf database nssa-external 2001:db8:6::/64

            OSPFv3 Router with ID (2.2.2.2) (Process ID 1)

                Type-7 AS External Link States (Area 26)

  Routing Bit Set on this LSA
  LS age: 240
  LS Type: AS External Link
  Link State ID: 0
  Advertising Router: 6.6.6.6
  LS Seq Number: 80000011
  Checksum: 0xC2FA
  Length: 36
  Prefix Address: 2001:DB8:6::
  Prefix Length: 64, Options: P
  Metric Type: 2 (Larger than any link state path)
  Metric: 20

We can see the Type-7 LSA present in the database on R2. We can also observe it’s identical to the one originated by R6. Let’s now see the converted AS-External (Type 4005) LSA.

R2:

R2#show ipv6 ospf database external 2001:db8:6::/64

            OSPFv3 Router with ID (2.2.2.2) (Process ID 1)

                Type-5 AS External Link States

  LS age: 1428
  LS Type: AS External Link
  Link State ID: 1
  Advertising Router: 2.2.2.2
  LS Seq Number: 8000000E
  Checksum: 0x3188
  Length: 36
  Prefix Address: 2001:DB8:6::
  Prefix Length: 64, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 20

This LSA should be exactly the same, as the one on R4 (since AS-External has domain flooding scope).

R4:

R4#show ipv6 ospf database external adv-router 2.2.2.2

            OSPFv3 Router with ID (4.4.4.4) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 1488
  LS Type: AS External Link
  Link State ID: 1
  Advertising Router: 2.2.2.2
  LS Seq Number: 8000000E
  Checksum: 0x3188
  Length: 36
  Prefix Address: 2001:DB8:6::
  Prefix Length: 64, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 20

This is indeed the case!

I hope you enjoyed this OSPFv3 series and that you have learned something new.

Happy studies!

–
Marko Milivojevic – CCIE #18427
Senior Technical Instructor – IPexpert
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OSPFv3 LSA Types: Type 2007 (Type-7) - Part 6, 3.0 out of 5 based on 4 ratings

Tags: CCIE, CCIE Routing & Switching, CCIE Security, CCIE Service Provider, IPv6, OSPFv3, OSPFv3 LSA