Answer is A but the available answers are all written wrong either way. Whoever wrote this is crazy. But, the CLOSEST possible answer would be A. Whoever writes questions for the Cisco exams are absolutely ignorant in how they write questions OR they are being deliberate in trying to fool the test takers, which is sad.
After a thoght analysis is not B, Il explain Why, R3 are redistributing OSPF into EIGRP and setting the tag 1, but notice that the tag 1 is being announced on EIGRP process, when R4 redistribute OSPF into EIGRP with the route map it will not match anything because that tag is no been announce by ospf process.
So on R4 the R1 network will be redistributed back and being announced to R2, as the reported distance reset by redistribution then when packet arrives to R2 the R4 router will be prefered.
In conclusion B is not Correct, the most closest is A
Before applying "A" - 10.1.1.0/24 is learned from OSPF:
R4#sh ip eigrp 1 top 10.1.1.0/24 | sec External
Composite metric is (2816/0), route is External
External data:
AS number of route is 1
External protocol is OSPF, external metric is 20
Administrator tag is 1 (0x00000001)
After applying "A" - tagged ospf routes are filtered, 10.1.1.0/24 is learned from redistribute connected via eigrp:
R4#sh ip eigrp 1 top 10.1.1.0/24 | sec External
Composite metric is (131072/130816), route is External
External data:
AS number of route is 0
External protocol is Connected, external metric is 0
Administrator tag is 0 (0x00000000)
Before applying solution A, R2 sees two redistributed routes in eigrp, one from redistribute connected, and another from redistribute ospf. R2 trusts ospf more, and sends traffic to R4. Loop is created.
After careful examination of the contested answers. A is the appropiriate answer because one cannot redistribute into eigrp without specifying the metrics. So B cannot be the answr.
A is the correct answer.
The reason for the loop is that R2 is forwarding the packets destined to 10.1.1.1 to R4, instead of R1. This is because in the redistribute OSPF statement, BW metric has a higher value and delay has a value of 1. So, R2 chooses R4 over R1 for 10.1.1.0/24 subnet causing a loop.
Now, R5 learns 10.1.1.0/24 from R3 and advertises the same route to R4, that R4 redistributes back in EIGRP. If R3 sets a tag of 1 while redistributing EIGRP in OSPF, and R4 denies all the OSPF routes with tag 1 while redistributing, it will not advertise 10.1.1.0/24 back into EIGRP. Hence, the loop will be broken.
We are redistributing an OSPF subnet to EIGRP, so you need to go to EIGRP and issue the OSPF redistribution inside EIGRP.
Answer is A, no debate is needed.
Just look how the configuration is being applied and save your time.
Answer is A. To stop routing loops when mixing EIGRP and OSPF, we use route tagging and filtering. when a router sends routes from one type of routing (like EIGRP) into another (like OSPF), it adds a special tag to those routes. This tag is like a note that says, 'Hey, I came from EIGRP!' Then, when another router is moving routes back from OSPF into EIGRP, it looks for that tag. If it sees the tag, it knows not to send those routes back into EIGRP again. This way, we avoid having routes go in circles, causing loops. if router R3 is moving routes from EIGRP to OSPF, it tags them. Then, R4, which is moving routes the other way, blocks any routes with that tag from going back into EIGRP. So, R3 tags the EIGRP routes with a '1' when sending them to OSPF, and R4 makes sure not to let any routes with a '1' tag back into EIGRP.
I like A here. The problem with B is that the 10.1.1.0/24 subnet is not getting tagged on the eigrp to ospf redistribution at R3. Hence, R4, can't block it from on the ospf to eigrp redistribution at R4.
B is the correct answer from the fact that R4 advertised the important route of 10.1.24.4, which other options did not. Also technically set tag1 in R3 and denied it in R4.
no is not, in eigrp you need to specify the K values for redistribution otherwise the routes will be redistributed with infinite metric which means they are not valuable routes.
EIGRP needs the K values because it does not have an active algorithm like ospf but is just a formula using by default bandwith and delay.
For me also "A" seems to be the closest, because it is applying the tag on the correct combination of protocol & router. I labbed this scenario in CML, but I was unable to reproduce a loop with this configuration.
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