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:
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.
About Anthony Sequeira, #15626:
Anthony Sequeira brings decades of teaching, technical writing, and consulting experience to INE. Anthony began his career as an author and lecturer within the IT community, featuring best-selling titles for Microsoft and Cisco Press. Best known as one of the training voices for the revolutionary e-learning company called KnowledgeNet, Anthony now teaches online and in-classroom exclusively for INE. When not helping his students master Cisco networking, Anthony can be found at the poker tables, or flying the Florida skies in a Cessna.
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13 Responses to “OSPF Areas, Part 2, Normal and Stub Areas”
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great explanation…… quite helpful. Please can you explain little more about Type 4 and how they behave?
THanks Anthony….this clarifies a lot of things…
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).
Thanks for clearing it…. you are doing great job
I request to do same with other technologies also.
Anthony,
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…
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.
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.
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
Done Ivan – great catch – thanks.
Anthony,
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
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.
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.
Marlon:
Yes – but do not forget – R2 is in Area 0 and Area 11. The no-summary keyword is done on the ABR. By definition, the ABR participates in multiple areas.