OSPFv3 LSA Types: Type 2003 (Inter-Area-Prefix) – Part 4

Past two articles in this blog series covered information exchange inside an OSPFv3 area using Router, Network, Link and Intra-Area-Prefix LSAs. In the third article of this series, we will explore how routes are carried between areas in OSPFv3. In OSPFv2, this is the responsibility of Type 3 (Summary) LSA. In OSPFv3, inter-area routes are carried in Type 2003 (Inter-Area-Prefix) LSAs.The examples will continue with the familiar network, shown on the diagram below. In our network, we have two routers that act as Are Border Routers (ABR). Those routers are R2, between areas 0 and 26 and R5, between areas 0 and 45. In the first step, we’ll focus on R5 and Type 2003 LSA generated by it for the Loopback0 of R4. We can use R1 to take a look into this information. R1:

R1#show ipv6 ospf database inter-area prefix adv-router 5.5.5.5

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

                Inter Area Prefix Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 856
  LS Type: Inter Area Prefix Links
  Link State ID: 0
  Advertising Router: 5.5.5.5
  LS Seq Number: 8000000D
  Checksum: 0xB7AB
  Length: 44
  Metric: 64
  Prefix Address: 2001:DB8::4
  Prefix Length: 128, Options: None

  Routing Bit Set on this LSA
  LS age: 856
  LS Type: Inter Area Prefix Links
  Link State ID: 1
  Advertising Router: 5.5.5.5
  LS Seq Number: 8000000D
  Checksum: 0xC7A1
  Length: 36
  Metric: 64
  Prefix Address: 2001:DB8:45::
  Prefix Length: 64, Options: None

Just by looking at the information shown, we can see that Inter-Area-Prefix LSA is no different from OSPFv2 Summary LSA. We have routes from another area advertised by the ABR, together with the cost to reach them, from the ABR. Total cost to the announced prefix is the sum of the announced cost to the prefix by the ABR and our own cost to reach ABR. In this case, cost to reach Loopback0 on R4 from R1 should be 64 (announced cost from R5) + 1 (cost of FastEthernet to R2) + 64 (cost of Serial link between R2 and R5) = 129. Lets’ take a look. R1:

R1#show ipv6 route 2001:db8::4 | begin ^OI
OI  2001:DB8::4/128 [110/129]
     via FE80::218:19FF:FEF3:42B0, FastEthernet0/0

Those with sharp eyes will notice a small discrepancy in comparison to OSPFv2. In OSPFv2, Loopback interfaces have the default cost of 1. In OSPFv3, the default cost of Loopback interfaces in OSPFv3 is 0. Another thing we should look into is how the information will be propagated to R6, which is in yet another area. The very value of the LSA (2003) tells us that the flooding scope is area. That means that R2 needs to originate new LSA to provide reachability information to R6. Let’s take a quick look. R2:

R2#show ipv6 ospf database inter-area prefix 2001:db8::4/128

            OSPFv3 Router with ID (2.2.2.2) (Process ID 1)
  Routing Bit Set on this LSA
  LS age: 1953
  LS Type: Inter Area Prefix Links
  Link State ID: 0
  Advertising Router: 5.5.5.5
  LS Seq Number: 8000000D
  Checksum: 0xB7AB
  Length: 44
  Metric: 64
  Prefix Address: 2001:DB8::4
  Prefix Length: 128, Options: None

  LS age: 712
  LS Type: Inter Area Prefix Links
  Link State ID: 5
  Advertising Router: 2.2.2.2
  LS Seq Number: 8000000B
  Checksum: 0x65C6
  Length: 44
  Metric: 128
  Prefix Address: 2001:DB8::4
  Prefix Length: 128, Options: None

We can see that R2 has two Type 2003 LSAs for 2001:db8::4/128 in the database. One identical to the LSA we observed in R1′s database and originated by R5. This is obviously in area 0. Other Inter-Area-Prefix for the Loopback0 of R4 is originated by R2. We can see that the cost on that one is 128, which should be R2′s cost to reach this route. Is it? R2:

R2#show ipv6 route 2001:db8::4
Routing entry for 2001:DB8::4/128
  Known via "ospf 1", distance 110, metric 128, type inter area
  Route count is 1/1, share count 0
  Routing paths:
    FE80::20A:B8FF:FE19:C8F0, Serial0/2/0
      Last updated 05:47:06 ago

We can see this is true indeed. Using the logic from before, we can calculate the end cost on R6 to be 128 + 64 = 192. Let’s confirm. R6:

R6#show ipv6 route 2001:db8::4
Routing entry for 2001:DB8::4/128
  Known via "ospf 1", distance 110, metric 192, type inter area
  Route count is 1/1, share count 0
  Routing paths:
    FE80::2, Serial0/1/0
      Last updated 05:48:03 ago

Next time, we’ll explore how the information from external sources (redistribution) is carried through the OSPF domain. Happy studies!

Marko Milivojevic – CCIE #18427
Senior Technical Instructor – IPexpert
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Tags: CCIE R&S, CCIE Routing & Switching, CCIE Security, IPv6, OSPFv3, OSPFv3 LSA