OSPF Troubleshooting Lab - It's All About LSA Magic

Yesterday I got this OSPF Troubleshooting Challenge sent by one CCIE candidate friend on email. So I thought to share it with you guys.


With that being said , here is the topology {Which I redraw to look better of course :-) } & quick requirements.

Note: Click Images To Enlarge

Requirements :
---------------------


On R3 we have redistribute the Loopback interfaces into OSPF process using "redistribute connected subnets" command. Over R2 we have configured "Summary x.x.x.x x.x.x.x" command under OSPF process to summarize these loopbacks of R3. But R1 is still seeing the all more specific routes for R3's loopbacks. In short it's not getting the summary route from R2. 


So how we gonna fix it ?


Ummm.... Interesting ? .... May Be :-)


Lets hop on to all three routers and check the basic configuration already done for us.

So based on this we can see that it's just a normal misconfiguration. Because to summarize the External Routes we need to put this command on R3 since it's the one who is ASBR. Or in short - Only ASBR can summarize the external prefixes not ABR. It looks simple and meaningful though but I have seen many people with this misconception that since as most of books we read says in OSPF the summarization can only be done at Boundary Routers so they assume that external routes can also be summarized by ABR. Which we can see here is that not the case. Or may be they should have mentioned this behavior explicitly :-)

So by that same token if we configure "Summary x.x.x.x x.x.x.x" command on R3 instead of R2 in that case it will work just fine.

But that will be two simple... how about if we are not allowed to put this command on R3 ?

Ummm.... that will actually make life more complicated. Lets check the database on R2 and see how it looks like with command "sh ip ospf database"

 So R2 sees those prefixes as LSA Type - 5 and we are Adv router (Originator) is 33.33.33.33 which is Router ID (RID) of R3 which I setup manually.

Ummm.... so only LSA 5 Originator can summarize the prefixes by using "Summary" command. 

Is there any way we can take charge of these prefixes and convert them to LSA Type 5 with Originator set being Ourself ?

How about NSSA ? :-) 

Because if convert Area 1 into NSSA in that case we will see those external prefixes being advertised by R3 as Type 7 LSA. Since R2 is the ABR it will convert LSA 7 into LSA 5 before advertising into Area 0. So in that sense we are now originator of LSA - 5 and very well can use summary command.

Lets see this on command line by configuring AREA 1 as NSSA on R2 & R3:

Now lets take a look at database on R2 again:

Lets see if this worked for us just fine by taking a look at R1's Routing Table.

So far so good. :-) Time for Celebration

But what if we are not even allowed to make Area 1 as NSSA ? ..... horrible ?


See if you can make it work with this restriction before I present the solution which came from a great Network Engineer and friend Michael Miller.


Take it as challenge and see if you can make this work with this last restriction by not configuring Area 1 as NSSA. 


I'll post the solution though some time next week.


And Of course.... Merry Christmas Everyone

HTH...

Deepak Arora

CCIE - 0::0/0 Null0