4.13. Redistribution Loop Prevention

Hi,

The task says the following :

• Ensure that the RIP routes redistributed into EIGRP on R3 do not get
passed back into EIGRP from OSPF on R2 or R4.

How would this happen taking into account that in a previous section the external ospf distance has been set to 171 ?

I think that with this distance configured the following happens. For example, let's take a RIP route coming from BB3 :

- route gets redistributed by R3 into Eigrp 10 domain => AD 170

- route gets to R2, which redistributes it into OSPF

- route gets to R4, by OSPF transport with an AD of 171

- routes doesn't get installed in routing table through OSPF as the AD is higher that the AD of the already installed route (EIGRP 170)

The same would happen with the routes that get redistributed into OSPF by R4 and get ultimately to R2.

 

So my question is, why would the tagging solution be presented in SG as I understand that distance solution from previous task already has taken care of this issue.

 

Please help me on this one.

Thanks,
Ciprian.

 

Comments

  • So my question is, why would the tagging solution be presented in SG as I understand that distance solution from previous task already has taken care of this issue.

    Please help me on this one.

    Thanks,

    Hi Ciprian,

    Your understanding in the first topic is correct! 

    After analysing your query, I found the reason of using tag option which is necessary to prevent loop between OSPF & EIGRP domain. Since the OSPF external administrative distance has already been changed to 171, All the routes redistributed from OSPF into EIGRP via R4 get redistributed back into OSPF domain via R2. This is because of the modified AD values for OSPF external routes. In this situation, EIGRP would have 170 for external routes that are coming from OSPF, when the external routes get to R2 via EIGRP, AD values are compared between OSPF & EIGRP External and then EIGRP would break the tie 170<171, which cause the routes to be redistributed back into OSPF. [H]

    Hope this helps! 

    Good luck!

  • Hi Hari,

    Thanks for your answer.

    Ok, i understand that the routes will continously loop in the EIGRP domain if not filtered by using tagging.

    so, this makes sense if mutually redistributing on both r4 and r2, right ?

    inthe SG only on r2 , if I remember correctly, is being done both ways, so tagging route filtering makes sense, otherways it wouldn't. Don't you agree?

    but as the task says to mutually redistribute on both r2 and r4, this filtering makes sense.

    please provide your input if it's different thaan mine.

    many thanks, Ciprian.

  • so, this makes sense if mutually redistributing on both r4 and r2, right ?

    inthe SG only on r2 , if I remember correctly, is being done both ways, so tagging route filtering makes sense, otherways it wouldn't. Don't you agree?

    but as the task says to mutually redistribute on both r2 and r4, this filtering makes sense.

    Hi Ciprian,

    If you look into the task 4.11, you are asked to perform mutual redistribution between EIGRP & OSPF on R2 & R4. So, route tagging is necessary as part of the solution to prevent the possible loop between these routing domains. In the solution guide, mutual redistribution has been performed, please check & confirm.

    Good luck!

  • Hi Hari,

    Following has been copy-pasted from the WB and solution guide (legacy version, not the latest version which has just been released) :

    WB :

    4.11. IGP Redistribution
    • Perform mutual redistribution between OSPF and EIGRP on R2 and R4.
    • These routes should be redistributed in such a way that the cost does not
    change as the routes are advertised throughout the OSPF domain.
    2 Points
    10 Minutes

    SG :

    Task 4.11

    R2:
    router eigrp 10
    redistribute ospf 1 metric 1 1 1 1 1
    !
    router ospf 1
    redistribute eigrp 10 subnets
    distance ospf external 171
    R4:
    router eigrp 10
    redistribute ospf 1 metric 1 1 1 1 1
    !
    router ospf 1
    distance ospf external 171    >>>>>> no EIGRP redistributed into OSPF

     

    Based on the info above, I would conclude the following :

    - 4.11 configuration from SG is not complete, it should include eigrp redistribution into OSPF on R4 also

            * with this config ("incomplete"), I consider that route tagging filtering is not necessary

    - if configuration includes also R4 mutual redistribution, then route filtering makes sense, otherwise routes will continously loop in the EIGRP doman.

     

    Do you confirm the above ?

    Thanks,

    Ciprian.

     

     

  • router ospf 1

    distance ospf external 171    >>>>>> no EIGRP redistributed into OSPF

    Do you confirm the above ?

    Yes, you are right. In the solution, EIGRP has not been redistributed into OSPF. It could be missing one since this particular configuration has been corrected in the task 4.13. If the question asks that you need to perform mutual redistribution on R2 & R4, R4 should also have two way redistribution configured. So, it can be considered as missing one & we can't always rely on the answers. :)

    Thanks!

    Good luck!

     

  • Thanks for confirming Hari, have a great day !

    Ciprian.

     

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