IGP Redistribution Question - Task2.4 Lab 2

Hello all,

I have some concerns with respect to IGP redistribution. Kindly help explain this because it's somehow confusing me.

From my own understanding, one has to consider carefully the administrative distance before redistributing between routing protocols.

I actually have some concerns with Task 2.4, Lab 2, CCIE R&S Lab Workbook Volume 2 Version 5 (Please refer to the workbook, or find attached).

The topology has EIGRP, OSPF and RIP routing protocols, and we are required to redistribute between them.

My own understanding is this:

-When a router learns a route from both OSPF (AD 110) and EIGRP External (Redistributed, AD 170), the router will still prefer the OSPF route due to lower AD.

-When a router learns a route from EIGRP Internal (AD 90) and OSPF External (AD 110), it will prefer the EIGRP route due to lower AD.

- For RIP (AD 120) and OSPF External (AD 110), the router prefers OSPF even if the RIP route is shorter - this is suboptimal routing, I think?

- For RIP (AD 120) and EIGRP Internal (AD 90), the router prefers OSPF even if the RIP route is shorter - this also is suboptimal routing, I think?

Now please refer to the LAB 2 topology. R2, R3 and R4 all have connections to both EIGRP and OSPF. R1 connects both RIP and OSPF domains and redistributes between both protocols. With my own understanding the redistribution should work normally without any filtering, since there is no conflict with the AD, especially since EIGRP External AD is 170.

But in the solution manual, the RIP routes were configured to have AD 110, so that they should be reachable via OSPF. Ordinarily since they were redistributed by R1 into OSPF, I thought they would have AD 110 and there would not have been any routing loops or suboptimal routing. So I dont understand the reason for the router ospf sub-command

"distance 110 EXTERNAL_VIA_OSPF" 

Also, please kindly explain why we have to increase the AD of OSPF external routes to 171 with the router ospf sub-command

"distance ospf external 171"

I am actually confused about the two commands and the Access Lists and I really wish to understand.

Please help me out.

Thank you.




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