OSPFv3 LSA Types: Type 4005 and 2004 (AS-External and Inter-Area-Router) – Part 5

By Marko Milivojevic on July 21st, 2011

So far in this series, we’ve covered all LSAs used by OSPFv3 to propagate routing information internal to the OSPF domain. In this article, we’ll take a look at how OSPFv3 propagates information about external (redistributed) routes. Primary for this operation is Type 4005 (AS-External) LSA, however it sometimes needs to enlist help from Type 2004 (Inter-Area-Router) LSA.The test network is identical to our previous example and can be seen on the diagram below.

OSPFv3 Test Network

Router R4 is configured with Loopback4 interface, which is redistributed into the OSPFv3 domain. This is just a quick reminder of that configuration.

R4:

interface Loopback4
 ipv6 address 2001:DB8:4::4/64
!
ipv6 router ospf 1
 redistribute connected route-map CON-to-OSPF
!
route-map CON-to-OSPF permit 10
 match interface Loopback4
!

This very configuration will generate AS-External LSA on R4. AS-External has the value of 4005 and that means the flooding scope is the domain. In plain English, this means that we should be seeing exact same LSA on R4 and say R1. Let’s take a look.

R4:

R4#show ipv6 ospf database external adv-router 4.4.4.4

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

                Type-5 AS External Link States

  LS age: 961
  LS Type: AS External Link
  Link State ID: 0
  Advertising Router: 4.4.4.4
  LS Seq Number: 8000000E
  Checksum: 0xDED5
  Length: 36
  Prefix Address: 2001:DB8:4::
  Prefix Length: 64, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 20

R1:

R1#show ipv6 ospf database external adv-router 4.4.4.4

            OSPFv3 Router with ID (1.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 1021
  LS Type: AS External Link
  Link State ID: 0
  Advertising Router: 4.4.4.4
  LS Seq Number: 8000000E
  Checksum: 0xDED5
  Length: 36
  Prefix Address: 2001:DB8:4::
  Prefix Length: 64, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 20

Obviously, flooding scope works as expected. This information by itself should be more than enough for R5 to reach this prefix. R5 has the database in the same area as R4 and knows how to reach 4.4.4.4. However, R1 is in entirely different area and it doesn’t share the same database as R4 and R5. For R1, entity 4.4.4.4 is unknown, unless it has a helping hand. Just like with OSPFv, where External (Type 5) LSA is assisted in inter-area operation by ASBR-Summary (Type 4) LSA, in OSPFv4, Inter-Area-Router (Type 2004) LSA is used for the same purpose.

Inter-Area-Router LSA is originated by the ABR in the area where ASBR is present. In our case, this is R5. Let’s confirm.

R5:

R5#show ipv6 ospf database inter-area router 4.4.4.4

            OSPFv3 Router with ID (5.5.5.5) (Process ID 1)

                Inter Area Router Link States (Area 0)

  LS age: 1641
  Options: (V6-Bit, E-Bit, R-bit, DC-Bit)
  LS Type: Inter Area Router Links
  Link State ID: 67372036
  Advertising Router: 5.5.5.5
  LS Seq Number: 8000000E
  Checksum: 0xB6CE
  Length: 32
  Metric: 64
  Destination Router ID: 4.4.4.4

It looks like all the information we need is right here. We see that R5 originates LSA informing routers in area 0 that 4.4.4.4 is reachable through it with metric 64.

Area 26 is configured as NSSA area, which prohibits Type 4005 and Type 2004 LSAs. Let’s just quickly confirm this is the case.

R6:

R6#show ipv6 ospf database

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

                Router Link States (Area 26)

ADV Router      Age         Seq#        Fragment ID  Link count  Bits
2.2.2.2         696         0x80000016  0            1           B E
6.6.6.6         558         0x80000017  0            1           E

                Inter Area Prefix Link States (Area 26)

ADV Router      Age         Seq#        Prefix
2.2.2.2         696         0x8000000C  2001:DB8::2/128
2.2.2.2         696         0x8000000C  2001:DB8:25::/64
2.2.2.2         696         0x8000000C  2001:DB8:12::/64
2.2.2.2         696         0x8000000C  2001:DB8::5/128
2.2.2.2         696         0x8000000C  2001:DB8:45::/64
2.2.2.2         696         0x8000000C  2001:DB8::4/128
2.2.2.2         1674        0x80000007  2001:DB8::1/128

                Type-7 AS External Link States (Area 26)

ADV Router      Age         Seq#        Prefix
2.2.2.2         696         0x8000000C  ::/0
6.6.6.6         1565        0x8000000E  2001:DB8:6::/64

                Link (Type-8) Link States (Area 26)

ADV Router      Age         Seq#        Link ID    Interface
2.2.2.2         698         0x80000010  6          Se0/1/0
6.6.6.6         1323        0x8000000F  6          Se0/1/0

                Intra Area Prefix Link States (Area 26)

ADV Router      Age         Seq#        Link ID    Ref-lstype  Ref-LSID
2.2.2.2         698         0x8000000F  0          0x2001      0
6.6.6.6         1324        0x80000012  0          0x2001      0

As we expected, there is no trace of them. NSSA areas are loads of fun though. We’ll look into them next time.

Happy studies!


Marko Milivojevic – CCIE #18427
Senior Technical Instructor – IPexpert
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OSPFv3 LSA Types: Type 4005 and 2004 (AS-External and Inter-Area-Router) - Part 5, 5.0 out of 5 based on 2 ratings
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    One Response to “OSPFv3 LSA Types: Type 4005 and 2004 (AS-External and Inter-Area-Router) – Part 5”

    1. Imtiyaz says:

      What is the use of “Destination Router ID: 4.4.4.4″ as gained from the inter-area-route LSA. For OSPFv2, I understand it can be used to have a route to the area on the otherside of the ABR but in case of OSPFv3, how does it help ?

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