Jul
26

Welcome back to our series on OSPF areas. Click here for Part 1 of the series. It is time to focus on normal areas and stub areas in this post. Recall our topology:

blogospf

We have gone to R1 and created a prefix (11.11.11.0/24) using a loopback interface. We run RIP version 2 on this interface and redistribute this into OSPF Area 0. What should this create on R3 in Area 11 (a normal OSPF area)? That’s right  – a Type 5 LSA for an External prefix. Let us examine the OSPF database on R3 now and the accompanying IP routing table:

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

		Router Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         1216        0x80000002 0x00023C 1
3.3.3.3         3.3.3.3         1215        0x80000002 0x00C075 1

		Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.3     3.3.3.3         1215        0x80000001 0x003577

		Summary Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      2.2.2.2         1281        0x80000001 0x0048C4
172.16.10.0     2.2.2.2         1241        0x80000001 0x00C79B

		Summary ASB Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         449         0x80000001 0x0075B0

		Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
11.11.11.0      1.1.1.1         456         0x80000001 0x0075AB 0
R3#

R3#show ip route

Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.10.0 [110/21] via 192.168.1.2, 00:24:41, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/20] via 192.168.1.2, 00:24:41, FastEthernet0/0
     11.0.0.0/24 is subnetted, 1 subnets
O E2    11.11.11.0 [110/20000] via 192.168.1.2, 00:11:53, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
R3#

Sure enough, there is the Type 5 prefix in the normal area. And we cannot forget about the LSA Type 4 (Summary ASB Link State). This informs the OSPF domain of the location of the Autonomous System Boundary Router (ASBR). I am sure you have been noticing how some of the LSAs in the database do not translate directly into routing table entries. For example, the LSA Type 4. This is reminiscent of the EIGRP topology table. That protocol sure tries to act link state as well!

OK, well let us see what happens when we convert Area 11 into a STUB AREA. Remember, this is a simple configuration. All we need to do is go to ALL of the routers in the stub area (there can be many), and issue the router configuration command area 11 stub. Now that we have done that, let us examine the databases on R3.

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

		Router Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         7           0x80000005 0x001A23 1
3.3.3.3         3.3.3.3         6           0x80000005 0x00D85C 1

		Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.3     3.3.3.3         6           0x80000004 0x004D5E

		Summary Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         33          0x80000001 0x0075C0
10.10.10.0      2.2.2.2         33          0x80000003 0x0062AA
172.16.10.0     2.2.2.2         33          0x80000003 0x00E181
R3#

R3#show ip route

Gateway of last resort is 192.168.1.2 to network 0.0.0.0

     172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.10.0 [110/21] via 192.168.1.2, 00:01:23, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/20] via 192.168.1.2, 00:01:23, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
O*IA 0.0.0.0/0 [110/11] via 192.168.1.2, 00:01:23, FastEthernet0/0
R3#

Wow, things really changed here. Notice the Stub Area effect worked just as advertised in our Core Knowledge studies. The Type 4 and 5 LSAs were removed from the OSPF database! They were replaced with a “special” LSA Type 3. It is special because it is an automatically generated default route by the Area Border Router (ABR).

Join us in the next part of this blog series where we examine the next OSPF area type, the Totally Stubby Area.


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14 Responses to “OSPF Areas, Part 2, Normal and Stub Areas”

 
  1. Nish says:

    THanks….this clarifies a lot of things…

  2. Hi Nadeem!

    Notice the LSA Type 4 does not advertise a network. It advertises the router that is the ASBR.

    It is used internally by the OSPF Shortest Path First algorithm in order to have routers calculate optimal paths to external prefixes (Type 5 LSAs).

  3. Nadeem Rafi says:

    great explanation…… quite helpful. Please can you explain little more about Type 4 and how they behave?

  4. Nadeem Rafi says:

    Thanks for clearing it…. you are doing great job
    I request to do same with other technologies also.

  5. Nish says:

    Hi,

    One request, please in the final part of this blog series, kindly prepare an OSPF network with all the different area types: NSSA, Stub, Totally Stubby…and then list their routing tables and OSPF databases….so that we can fully understand the propagation of the different LSA’s…..

    This will act as a good summary for this blog series…and will help us verify and confirm all that we wold have learnt so far…I hope I am not asking for too much ….

    I really like your approach so far of breaking down this blog series on the basis of area types…

  6. Harry Markham says:

    There’s only one AS in play here, yeah ? So why is a Type 5 “AS External Link” Type 5 LSA being produced ?

    I have just passed my BSCI exam…and everything I have read on OSPF tells me that Type 5 LSA’s are generated by ASBR’s to advertise external routes…..i.e. routes that are redistribute from one AS into another….or from another AS.

  7. Hi Harry!

    Great point – I will edit the article to clarify – the loopback we create is running RIP and we redistribute that into Area 0.

  8. Please fix the “static default route” wording on the 0.0.0.0/0 Type-3 LSA advertised into the stub area. It’s not a “static” route ;)

  9. Done Ivan – great catch – thanks.

  10. Faisal says:

    Hi,

    Just need a clarification about LSA Type-5 from what Harry has stated.

    Type-5 LSA’s are generated even by redistributing connected routes it is not mandatory to run an External IGP and redistribute it into OSPF to generate LSA Type-5 or AS External Link.

    In other words in the same above topology you need not run rip and just a simple redistribute connected subnets would make R1 announce itself as ASBR and generate type-5 LSA for the loopback 0.

    Please correct me if I’m not correct
    Faisal

  11. Excellent job Faisal. You can also use the redistribute connected approach in order to create the Type 5 LSA.

    I did not have to redistribute a RIP route in my above example.

  12. Marlon says:

    Hey, let’s if I understand:
    When you wanted to make Area 11 as ‘stub’, you go to R3 (or every router in the area you wish to make as stub) and enter ‘area 11 stub’ command. I understand that you described that step here:”…. All we need to do is go to ALL of the routers in the stub area (there can be many), and issue the router configuration command area 11 stub”.

    Then when you wanted to make the Area 11 as a totally stubby area (part 3), you did that on R2 (Area 0). Not on R3 Area 11. Is that right? Intesting… you configured ‘stub’ on R3 Area 11, but totally stub was controlled on R2, Area 0.

  13. Nice post. I used to be checking continuously this weblog and I’m impressed!
    Very helpful info specially the final part :) I deal with such
    info a lot. I used to be looking for this certain info for a very long time.
    Thanks and best of luck.

 

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