4.10 - Loop formed due to 7.1

Folks,

For 7.1, I needed to configured SW1 and SW2 to support IPv6. In doing so, I caused a loop to form between R4 and R5 for all the prefixes directly redistributed into EIGRP on SW1, SW2 and R6. In order to solve this, I had to break the loop manually by shutting down one of the frame-relay interfaces on R4. Can this method be a valid fix to re-align the lab?

I find when digging into the problem, R5 received notification via EIGRP that SW1 and SW2 (and inevitably R6) were dropping their adjacencies because of the reboot. Therefore, since R4 loops the control plane back to R5 via OSPF, R5 decided to pick up the route with the higher distance (AD 200) and point itself to R4 for these prefixes. What is interesting though, is once it converges again, R5 doesn't natually point back to the correct location (even in the presence of lower AD routes.). Why is this?

Since this lab specifically asked us to only use distance to solve the original looping problem, we cannot add more resiliency to the loop and therefore we are vulnerable to this behavior if the topology reconverges.

Thanks

Mike

Comments

  • I had to break the loop manually by shutting down one of the frame-relay interfaces on R4. Can this method be a valid fix to re-align the lab?

    Mike,

    You can't take an action to prevent the loop using such an option. A routing protocol may install the route with higher AD where the specific route does exist. If you have less specific network with lower AD vaue, the more specific route with the higher AD is preferred. 

    Please check for the same & get back to us.

    Hope this helps!

  • Hari,

    I'm afraid I didn't understand your answer. My topology followed in line with the solutions guide yet I still experienced this problem. The only way to fix it without adding configuration was to introduce a break and re-enable.

    Mike

  • Mike,

    You can't shut any of the link down in order to solve the loop issue. Any route with more specific prefix is preferred regardless of the AD value. So, I was just trying to drag your attention on it. Sorry if you still didn't understand my point.

    Good luck!

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