@sales2161 You are correct, just fixed this. Thanks for letting us know!
Rene
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After reading the tutorial, I’m a bit confused about the configuration for Totally Stub Area and Totally NSSA.
How do we determine which router to run “area X stub no-summary” or “area X nssa no-summary”
I can see that no-summary has been run on the middle of router but what if we have many routers.
Hello Po
First of all, all four stub area types (Stub, Totally stub, NSSA, and totally NSSA) are characterized by the fact that they only have a single ABR. Remember, an ABR is an OSPF router that has two or more areas connected to it. This essentially means that there is only one entry and exit point from that particular area. This also means that area 0 can never be a stub network, because it is the backbone network to which all stubs connect. Remember that all ABRs must have at least one interface in area 0 for OSPF to function correctly. For a refresher, take a look at the following lesson:
Since all stub areas have only a single ABR, this means that these commands must be implemented on the ABRs that connect to the stub network in question, and must be applied to the area connected to that ABR which is a stub.
Looking at this topology from the lesson:
If we want to configure area 1 as a totally stub area, the command must be implemented in the ABR connected to that totally stub area. R2 is the ABR connected to that totally stub area, and by definition there is only one ABR in area 1 (otherwise it would not be a stub). Secondly, the command must be implemented on Area 1. So in R2, you would implement:
area 1 stub no-summary
Note that this command must be implemented only on the ABR. R3 is not an ABR, but must be configured as part of a normal stub area, this is why the command area 1 stub is configured there.
I hope this has been helpful!
Laz
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Thank you very much for your detailed explanation!
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in the above diagram , both of the ABR will generate the default route …Right…As we have configured the stub to area 1. Now the thing is in Area-1 all router will use as ECMP for external routes…SO the conclusion is we have to redistribute as E1 for one ABR and E2 for another abr so that we can the use one path as primary and another one is backup…Right…bcz ECMp is not the best solution for production…!!
Hello Narad
In this topology, yes both the ABRs will generate the default route, and the stub router will have an equal cost to all networks in Area 0. Thus, both routes will be placed in the routing table, and thus traffic will be load-balanced across both ABRs.
Now how that load balancing will take place depends on various factors. One way to load balance is indeed equal-cost multi-path (ECMP) routing. This allows the packets from a single flow to traverse multiple paths. This can cause problems in packet reordering and MTU differences in the various paths, and this can disrupt the operation of some protocols, especially TCP, or VoIP applications.
For this reason, Cisco has employed CEF, which, regardless of the routing protocol being used, is a routing mechanism that aids in the methods by which such traffic is transmitted. Take a look at this post for more information about load balancing and CEF:
Actually, equal cost routing, with the appropriate precautions, is actually beneficial and desired in production networks.
I hope this has been helpful!
Laz
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