OSPF ABR Type 3 LSA Filtering on Cisco IOS

Lessons Discussion
61

I dont see an Area 0 in this topology, how did you get this to work without a backbone area to send the Type 3 LSA’s into?

Thanks.

62

Hello Chris

If you’ll notice, the L0 interface on R4 with an IP address of 4.4.4.4/32 is in Area 0. This is not shown in the diagram with a colored ellipse, but is shown within the configuration of R4.

The Area 0 of an OSPF network can simply be applied to a single interface, even to a loopback interface, fulfilling the backbone area requirement of OSPF.

I hope this has been helpful!

Laz

1 Like
63

Lazaros,

Thanks so much! I did not know this! if I may get some help on something though. I have noticed that if I am summarizing routes on an ABR outbound into area 0 using the area <area #> range command, I am not able to filter those same routes outbound using the area < area # > filter-list command. I have to remove the summarization and then it works. Does the summary take precedence? or am I just not defining the subnet correctly in my ACL or prefix list?

Thanks!

65

Hello Chris

I labbed this one up, but I was unable to reproduce your results. I found that the area X filter-list command always took precedence over any summarization I would include. In my lab, I used the lesson’s topology, and added a summarization for 3.3.0.0/16 from area 3 using the following command:

area 3 range 3.3.0.0 255.255.0.0

I found that the outbound filter list was still blocking the redistribution of the 3.3.3.3/32 address of the loopback on R3. I even tried to configure a summarization that was exactly the same as the prefix being advertised, namely 3.3.3.3/32 but I got the same result. The prefix was being filtered.

Can you share the configuration you are applying with us so we can take a closer look? Also, I used IOSv 15.9(3)M2. What IOS are you using? It may also have to do with how each software implements these features.

I hope this has been helpful!

Laz

1 Like
66

Hello awesome teachers,

I love your brilliantly articulated lessons !

Just a quick question, I thought Cisco requires OSPF to have all areas to be connected to the backbone area.

So, it was a must for each ABR to be connected to “Area 0”, using either a physical link or a virtual link. Therefore , it was weird, the example in your lesson, had shown R4 to be the central “ABR” , R4 was missing a backbone area. Any exemptions ?

What was the basis for Cisco, to actually allow R4 to be an “ABR” ? Much Appreciated !

67

Hello Jenny

Actually, you will find that R4 does have a backbone area. It is the area to which the loopback L0 is connected. So the rule was not violated here. In the diagram, Area 0 is not explicitly shown with a colored oval, so this may have been misleading. If you take a look at the OSPF configuration of R4, you will find the following command:

R4(config-router)#network 4.4.4.4 0.0.0.0 area 0

which fulfills the requirement of a backbone area. The backbone area can also be applied to a loopback interface as well as a physical interface and virtual link.

All other areas are directly connected to this area 0 via the R4 ABR.

I hope this has been helpful!

Laz

1 Like
68

Awesome Laz ! you had resolved one of my doubts: that loopback by itself , could actually help us , enabled this central router R4 to be our “ABR”. We break no rules here ! :grinning:

But, another suspicious doubt of mine: Shouldn’t the Backbone area ( ie the transit area ) not be a stub area at the same time ?

Aha ! !! Thank you Laz, from this good example of yours … I can comprehend and appreciate better now:

ie, using loopback to form “Area 0”, was indeed, a valid strategy, where there was no ASBR used anywhere in the OSPF domain.

Must say, I love our Kooool universal flexible loopbacks. Many thanks Laz !!

1 Like
69

Hello Jenny

Glad to hear that it was helpful! Now as for your question:

Remember that:

  1. All inter-area traffic must pass through area 0 because only the backbone area can generate the summary routing topology information that is used by the other areas.
  2. Area 0 is never a stub area, even if it is configured on a loopback.
  3. In this scenario, traffic from non-backbone areas will pass through area 0 (the loopback interface) before going to other non-backbone areas.

I hope this has been helpful!

Laz

1 Like
70

OMG !! That was super detailed. Your answer had cleared my doubt !!
Until today, I did not know, area 0 is never a stub area, even if it was configured on a loopback !! you are really clear-cut !! Thank you so much Laz !!

1 Like
71

Hi,

Is the:

area range not-advertise

not also a valid way to filter Type-3 LSAs?

Thanks,

Sam

72

Hello Samir

Yes, you’re correct. The command area range not-advertise is indeed a valid way to filter Type-3 LSAs in OSPF. By using this command, the area border router (ABR) will not advertise the specified range of addresses to other areas, effectively filtering out the Type-3 LSAs for that range.

However, note that this command only prevents the specific range of IP addresses from being advertised, it does not stop the ABR from advertising other Type-3 LSAs.

For more info on this command, take a look at this Cisco command reference:

I hope this has been helpful!

Laz