Inter Area traffic

[GeorgeMonier_1982_2]

Hello All ,

                 I was having the below network

                         ---((MPLS))----R2(PE)--(Area 0 )---R1----(Area120)----XR2---(Area 0)---------(PE)XR1-----------((MPLS))  ,

               XR2 was having a loopback in Area 20 , and normally it was injecting it as LSA 3 . R1 was get this LSA3 from 2 sides , from XR2 directly over the AREA 120 and from its PE over area 0 .  The point is R1 always uses route send from the PE no matter how bad the metric was tot the PE ?!! . What is more supprising that if PE stopped redstibuting this routes R1 never uses the route from XR2 . Only when I shut down the inteface to the PE R1 starts using the route sent by XR2 .

                      Does this mean that if you have a link to the BB you must used for inter-area traffic ? and if you did not get this route from the BB do not ever use any LSA from any other non-BB area ?

                      Am I correct ?

George Kamel

[Rolf Fischer]

Hi George,

if  the same prefix is learned by different paths (inra-area, inter-area, external), the metrics and even the administrative distance are subordinated in the decision process.

The order of path selection preference as defined in RFC 1583 (still default in IOS) is:

1. intra-area
2. inter-area
3. external type 1
4. external type 2

This has been changed in RFC 2328 (default in NX-OS):

Intra-area paths using non-backbone areas are always the most preferred.
The other paths, intra-area backbone paths and inter-area paths, are of equal preference.

You can change the default behavior with the no compatible rfc... command under the OSPF process.

Mixing both types should be avoided since this could lead to routing loops.

Hope that helps

Rolf

[GeorgeMonier_1982_2]

Hello Rolf ,

                  Your are answer is focused on Intra area traffic , but my question is about Inter-area routes .

                  Question is if you are part of the BB area , is it allowed to use LSA  3 from non-backbone areas  Yes or NO ?. 

[Rolf Fischer]

After re-reading your original posting I've to ask if Area 20 for the loopback was a typo and you meant 120.

If not, remenber that a non-backbone area always has to be connected to the backbone-area and traffic is also forwarded though the backbone.

Question is if you are part of the BB area , is it allowed to use LSA  3 from non-backbone areas  Yes or NO ?.

Of course it is.

An ABR will re-inject OSPF routing-table entries from the non-backbone area into the backbone as Type-3 and vice versa.

Since this are distance-vector mechanisms, you can summarize and filter them if you want.

[EDIT] : I think I misunderstood your question, sorry. Please see the latest posting, probably this is what you wanted to know.

[GeorgeMonier_1982_2]

No it is not a typo , and if the answer "Yes it is allowed " so why R1 refused to use the LSA 3 of XR2 as long as it is connected to the BackBone ?

[Rolf Fischer]

OK, then ignore my first posting.

So the answer is

• the IP of the loopback belongs to Area 20
• the link between R1 and XR2 is in Area 120
• communication between Area 20 and 120 has to traverse Area 0, so the link R1-XR2 is out of question
• XR2 is ABR for Area 20 and generates a Type-3 LSA for the loopback's prefix in Area 0

There's a great document writen by Petr Lapukhov with all the details of inter-area loop-prevention:

http://blog.ine.com/wp-content/uploads/2011/01/Loop-Prevention-in-OSPF.pdf

Perhaps the most important paragraph regarding your question:

ABR expects summary LSAs from Area 0 only. This means there should be at least one adjacency in FULL state built over Area 0 interface. In case if ABR has such adjacency, it will ignore summary-LSAs received over non-backbone areas. These LSAs will be installed in the database, but not used for SPF calculations.