Refer to the exhibits. The output of the trace route from R5 shows a loop in the network.
Which configuration prevents this loop?
A.
B.
C.
D.
Exam 300-410 All QuestionsBrowse all questions from this exam
Question 5
Correct Answer:
The configuration needed to prevent the routing loop involves the proper tagging and filtering of routes between the two different routing protocols, EIGRP and OSPF. This is essential to ensure routes are not constantly reintroduced into the other protocol, creating a loop. The configuration must include tagging the routes when they are redistributed from EIGRP to OSPF and then filtering them out when they are redistributed back from OSPF to EIGRP. The provided configuration in Option A follows these steps correctly: R3 redistributes EIGRP into OSPF with a tag using a route-map, and R4 redistributes from OSPF to EIGRP while filtering out these tagged routes, thus preventing the loop. Therefore, the correct configuration to prevent the loop is shown in Option A.
Discussion
Deadliftn
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.
Koume
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
net_eng10021
I see the same thing as Koume has described above. The network is not tagged in the ospf domain.
HungarianDish
"A" I have redone this lab. Introduced the loop, then applied solution "A". It did actually prevented the loop. Before applying "A": R5#trac 10.1.1.1 Type escape sequence to abort. Tracing the route to 10.1.1.1 VRF info: (vrf in name/id, vrf out name/id) 1 10.1.35.3 2 msec 1 msec 2 msec 2 10.1.23.2 2 msec 2 msec 2 msec 3 10.1.24.4 2 msec 2 msec 2 msec 4 10.1.45.5 1 msec 2 msec 2 msec 5 10.1.35.3 3 msec 2 msec 2 msec 6 10.1.23.2 3 msec 2 msec 3 msec 7 10.1.24.4 3 msec 3 msec 3 msec 8 10.1.45.5 2 msec 2 msec 2 msec 9 10.1.35.3 4 msec 3 msec 3 msec 10 10.1.23.2 3 msec 4 msec 4 msec <output ommited> After applying "A": R5#trac 10.1.1.1 Type escape sequence to abort. Tracing the route to 10.1.1.1 VRF info: (vrf in name/id, vrf out name/id) 1 10.1.35.3 2 msec 2 msec 1 msec 2 10.1.23.2 2 msec 2 msec 2 msec 3 10.1.12.1 2 msec * 2 msec R5#
HungarianDish
Prefix is tagged: R4#sh ip route 10.1.1.1 Routing entry for 10.1.1.0/24 Known via "ospf 1", distance 110, metric 20 Tag 1, type extern 2, forward metric 2 Redistributing via eigrp 1 R4#sh run | sec router eigrp router eigrp 1 network 10.1.24.0 0.0.0.255 redistribute ospf 1 metric 1000000 1 255 1 1500 route-map FILTER-TAG R4# R4#sh run | sec route-map redistribute ospf 1 metric 1000000 1 255 1 1500 route-map FILTER-TAG route-map FILTER-TAG deny 10 match tag 1 route-map FILTER-TAG permit 20
HungarianDish
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)
HungarianDish
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.
anonymous1966
***(A) is correct Blue ---> Yelow (SET a tag) Blue <--- Yelow (BLOCK tagged updates back and permit not tagged) (B) wrong direction back Yelow --> Blue (SET a tag) Yelow --> Blue (BLOCK tagged updates and permit not tagged) (C) a permit statement is missing Blue ---> Yelow (SET a tag) Blue <--- Yelow (BLOCK tagged updates back and DO NOT permit not tagged) (D) a deny statement is used wrongly Blue ---> Yelow (DO NOT SET a tag) Blue <--- Yelow (BLOCK tagged updates back and DO NOT permit not tagged)
[Removed]
beautiful explanation.
Mohammad963
I'll go with A, 100% .
KZM
Option A is the solution.
net_eng10021
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.
Chiaretta
The right answer is A.
inteldarvid
100%% option "A"
SeMo0o0o0
A is correct we must permit tag 1 on R3 and dney it on R4
Dumpsvibe_com_exams
'b' is correct
edson91
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.
MasoudGhorbani
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.
LanreDipeolu
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.
XBfoundX
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.
XBfoundX
ok I have read half of it please ignore my comment LanreDipeolu
HungarianDish
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.
AinsB
Answer is B. R1 is advertising the connected 10.1.1.0 as an external network AD 170. OSPF advertises it as 110 so by default R4 will take the path through R5->R3 to get to 10.1.1.0. R2 IS advertising it at 170 to R4 even though it is a shorter path. If we block advertisement from R5 for this network then the better path of R4 -> R2 will be chosen.
Dacusai
In answer C and D is missing the permit 20 on the route map mining that no other routes will be added to the routing table and one of them has a permit so it still has the loop.
Dacusai
According the configuration in R3 you redistribute EIGRP into OSPF and answer B say other wise, so A is the correct one.