4.12 - Design Issue in the task?

Hi everyone. 

 

I have 2 issues with task 4.12.

 

1. Does the task 4.11 set the problem correctly that task 4.12 is supposed to solve? I think that if you'll perform all redistributions stated in 4.11, it's not necessarily that you'll get a loop between R5, R1 and R4 for original RIP networks. I think it only depends on the order of operations. For example if you do the 4.11 task line by line where redistribution on R5 is requested in the second place (before redistribution on R1), then you'll not get any loops. The reason in my opinion is that if R5 manages to redistribute RIP into OSPF first (before R1 redisrtributes from EIGRP), then R1 will receive those routes via OSPF and will install them in his routing table. Once this happens, R1 will never install these routes from EIGRP because of worse AD of external EIGRP (170) vs OSPF (110). Therefore R1 will not redistribute them into OSPF himself. Therefore R5 will keep all his original RIP routes and everything will work fine wothout any loops. It's only if R1 redistributes them first and R5 learns them through OSPF, pointing to R1, you'll get a loop as R1 will be pointing to R4 for them, R4 to R5 and R5 back to R1. This can happen after reload of the routers and if R1 will load and perform redistribution first (before R5 performs redistribution). Basically, they cancel each other redistribution. Please correct me if I am wrong here and share your thoughts.

2. Secondly,  in the event that we do have loops (second option as described above) I don't understand why do we need to prevent redistribution of original RIP routes back into RIP on R5 (all tagging and filtering based on those tags as SG suggests). Wouldn't just setting AD for OSPF external to 121 on R5 do the trick? If all OSPF external routes are given AD of 121, R5 wouldn't install those routes into his routing table because he has them with better AD of 120 from RIP and therefore shouldn't redistribute them back into RIP? What am I missing here? Please help to understand.

 

Thanks in advance!

 

 

 

 

Comments

  • Hi,

    Your understanding is correct on both the points.

    You have R5 & R1 which are the mutual redistribution point for OSPF & RIP. I think you should not give the preference on which router would redistribute first. Assuming that the R1 could also perform redistribution with preference, we should use loop prevention mechanism here for avoiding the possibilities of loop. So, we usually shouldn't think of it from one way perspective since anything could happen once the routers get rebooted. :) or re-converged.

    For the second topic, you are asked to use route tagging in the question. So, you have to use route-tag rather than manipulating AD values. According to your view for the loop prevention, its correct & works as well. Many other labs have the same solution as you explained here for your second doubt. But, you should follow anything that you are explicitly told to do. ;)

    Good luck!

  • Hi, Hari!

     

    Thank you extremely much for your valuable input. I hope that this thread will help others who are confused when trying to understand the problems of these loops. Regarding the second point I only pointed that because SG does state to use AD as well. This is why I couldn't understand the point of tagging on top of that. I thought it was excessive.

     

    Thank you again for helping to sort that out.

     

    Regards,

    Timur

     

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