Redistribution Questions
Question 1
Question 2
Explanation
Use the external keyword along with the redistribute command to redistribute OSPF external routes.
In order to use an prefix-list in a “match” statement, we have to use the command “match ip address prefix-list …”. The syntax of a “match” statement is as follows:
match ip address {access-list-number [access-list-number… | access-list-name…] | access-list-name [access-list-number…| access-list-name] | prefix-list prefix-list-name [prefix-list-name…]}
Reference: https://www.cisco.com/c/en/us/td/docs/ios/iproute_pi/command/reference/iri_book/iri_pi1.html
Question 3
Explanation
The first statement of the above route-map (route-map BGP-TO-OSPF deny 10) will prevent network 172.16.1.0/24 from being redistributed into OSPF.
Question 4
Question 5
Explanation
It seems there is a loop because of mutual redistributions among RIP, OSPF and EIGRP domains. So we should filter out the prefix 10.1.1.0/24 when redistributed from OSPF to EIGRP (the second redistribution point) to prevent routing loop.
Question 6
Explanation
Only the subnet that matches prefix-list OSPF-TAG-PRF-1 will be redistributed into OSPF (as indicated by “route-map OSPF-TAG-1 permit 10”). This subnet must match the prefix-list OSPF-TAG-PRF-1 so it must be 10.2.0.0/18 to 10.2.0.0/24. Only the subnet 10.2.2.0/24 matches this requirement.
Note: The prefix-list “OSPF-TAG-PRF” denies a route (deny 10.1.0.0/16 le 24) is used in a route-map deny clause (route-map OSPF-TAG-1 deny 5) so according to Cisco:
“If you use an ACL in a route-map permit or deny clause, and the ACL denies a route, then the route-map clause match is not found and the next route-map clause is evaluated.”
Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/49111-route-map-bestp.html
Therefore in this case the first route-map clause will be ignored so we don’t need to care about it.
Question 7
Explanation
If you configure the redistribution of OSPF into BGP without keywords, only OSPF intra-area and inter-area routes are redistributed into BGP, by default.
You can redistribute both internal and external (type-1 & type-2) OSPF routes via this command: “Router(config-router)#redistribute ospf 1 match internal external 1 external 2”
Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5242-bgp-ospf-redis.html
Question 8
Explanation
In this topology, we are doing mutual redistribution at multiple points (between OSPF and EIGRP on R3 & R4), which is a very common cause of network problems, especially routing loops so you should use route-map to prevent redistributed routes from redistributing again into the original domain.
In this question, route-map is also used for this purpose. In particular, the route-map “SET-TAG” is used to prevent any routes that have been redistributed into OSPF from redistributed again into EIGRP domain by tagging these routes with tag 1:
| R3 route-map SET-TAG permit 10 set tag 1 |
These routes are prevented from redistributed again by route-map FILTER_TAG by denying any routes with tag 1 set:
| R4 route-map FILTER-TAG deny 10 match tag 1 |
Hi
In question 6, the prefix list is denying 10.1.0.0/16 le 24 whereas the route-map is also denying.
We should only have one deny statement to really deny the subnets but two denies like this example will permit the routes
@Anonymous: We have just updated the explanation for Q.6 to answer your question:
Note: The prefix-list “OSPF-TAG-PRF” denies a route (deny 10.1.0.0/16 le 24) is used in a route-map deny clause (route-map OSPF-TAG-1 deny 5) so according to Cisco:
“If you use an ACL in a route-map permit or deny clause, and the ACL denies a route, then the route-map clause match is not found and the next route-map clause is evaluated.”
Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/49111-route-map-bestp.html
Therefore in this case the first route-map clause will be ignored so we don’t need to care about it.
Question 8 – strange, answer b, we redistribute ospf to eigrp, but isn’t it the other way around?
i suppose we have to redistribute eigrp to ospf firs on R3, because 10.1.1.1 route folows from R3 to R4 via R5 with already set tag and when it redistributed from ospf to eigrp on R5 it is matched and denyed…
and also redistribute ospf 1 subnets – incorrect statement
subnets should be after eigrp
What is difference between OSPF external type 1 and 2?
@Nim
for redistribution of external routes in ospf u can use metric-type 2 (default value p.e. “redistribute connected subnet”) or set the redistribution to metric-type 1 (p.e. “redistribute connected subnet metric-type 1”). The difference is that in metric-type 2 the metric-value for the route will always stay the same (p.e. 20) between the different OSPF-router. With type 1 the metric increases with every OSPF-router by adding the own cost value to the neighbor that propagated the route. By comparison between Routes to same subnet with type 1 vs type 2 propagated the type 1 route will be installed only in the RIB, whatever the metric value was.
p.e.:
O E1 1.1.1.1 [110/40] via 10.10.10.1 Ethernet0/0
vs
O E2 1.1.1.1 [110/20] via 10.10.10.5 Ethernet0/1
“Type” [“AD”/”Metric-value”]
-> First route will be installed in the RIB
Is question 8 really B? That is confusing.
Q8 answer cannot be B , it is C
@Anonymous
Q8 the answer is B or C ?
please may you explain
@networktut
@networktut
about Q7 i think it is B , because they said by default which means by general so not for this example
please need an explain
@networktut. About Q8 It is indeed A, the tag and filter should be in different protocols. In B the tag and filter are both in eigrp process.
@Networktut. Sorry I meant D on Q8
R3
router ospf 1
redistribute eigrp 1 subnets route-map SET-TAG
!
route-map SET-TAG permit 10
set tag 1
—————————————————————————————-
R4
router eigrp 1
redistribute ospf 1 metric 2000000 1 255 1 1500 route-map FILTER-TAG
!
route-map FILTER-TAG permit 10
match tag 1
@Heisenberg: Thanks your your detection but answer D is not correct either because of the command “route-map FILTER-TAG permit 10”. It should be:
route-map FILTER-TAG deny 10
match tag 1
route-map FILTER-TAG permit 20
We rechecked this question and the answers were changed and answer A is correct. We have just updated this question so please check again.
@networktut Thanks for following up
@networktut. About Q5 ,is the answer A?
please explain in more detail.
Hi! Why is question 8 in “Big Composite Quiz 60 Questions” the answer is option B and here is option A? Please help me!!
@Any: The answer choices in Composite Quizzes are shuffled so in fact they are the same.
@networktut, there is a problem in the quiz when you take a test for question 8. When we read tha question with the answer is Option A ( R3 – Ospf . R4- EIGRP ) , but when you take the quiz only for the Redistribution part for this question it is giving correct answer option B ( R3- EIRGP , R4-EIGRP ) which I belive is not correct. Please fix it
@bobo: Thanks for your detection, we have just fixed it!
I am confused with Q5. Why is the answer A? It seems to me that the loop’s cause is the route being advertised from R1-> R2->R4->R3->R2. When R2 received route from R1 with AD 120 and from R3 with AD 110 it chooses the route from R3 and installs in in its RIB thus ending the redistribution of 10.1.1.0 since the route must be from RIP and be in the RIB.
It seems to me as answers B or C would break the loop while still allowing for connectivity.
Please correct me if my logic is flawed.
Yup,
Your logic is not flawed but the question is. There would actually be no loop. RIP 120, OSPF 110, External EIGRP (any route redistributed into EIGRP) 170. Therefore at the R3 redistribution point the OSPF route (from R2) would be preferred over the EIGRP route from R4. R4 would have two valid paths being R2 or R3, but this would not effect the forwarding decision of either R2 or R3.
My best guess is the person who wrote the question to test your knowledge does not understand how it works:).
Upon further review, if we assume that the arrows are accurate and redistribute points are RIP to EIGRP and EIGRP to OSPF and that answer “a” is a typo this would be correct. “a” should read “Filter the prefix 10.1.1.0/24 when redistributed from EIGRP to OSPF”
Upon further review, if we assume that the arrows are accurate and redistribute points are RIP to EIGRP and EIGRP to OSPF and that answer “a” is a typo this would be correct. “a” should read “Filter the prefix 10.1.1.0/24 when redistributed from EIGRP to OSPF”
@NETWORKTUT.
Can you please change the explanation for #8. It should be EIGRP route redistributed into OSPF at R3 are tagged 1 and are being denied at the redistribution of OSPF to EIGRP at R4.
At least that is my understanding of what is going on for Answer A. The current explanation is the reverse.
@supalonely: Yes, thanks for your detection, our explanation was reversed. We have just fixed it!
I got this explanation for my answer:
Note: Please type the corresponding numbers of each item on the left to the blank below to match the options on the right. For example: 5321 (which means 53 for first group, 21 for second group from top to bottom).
Please type your answer here: 5213 (correct answer: 5231)
In my book, my answer is correct, but your system has marked in wrong. Please fix this.
Same test, with 5 (including the above mentioned) questions wrong, I got 59.18% score. Could it be that the scoring is not 100% accurate?
@Arusha: Coud you please send us the URL where you saw this problem so that we can have a closer check?
@ me again …..you are exactly correct. I even labbed the scenario and that is the case.
Q6
What does it mean ?
set tag 40
!
set tag 80
@Gabriel8338: For example:
route-map OSPF-TAG-1 deny 5
match ip address prefix-list OSPF-TAG-PRF
set tag 40
So if the packets match OSPF-TAG-PRF prefix then they will be attached a tag with the value of 40.