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Redistribution Questions

March 23rd, 2020 Go to comments

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
Comments (7) Comments
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  1. Judgestone
    April 14th, 2022

    Why is the answer to Q5 not – Set the administrative distance 100 under the process on R2

  2. Anonymous
    August 10th, 2022

    @Judgestone – I thought the same because in one of the LAB, changing AD was solution for selecting an optimal path.

  3. turdburglar
    September 14th, 2022

    It’s excessive to change the AD of the entire routing process unless absolutely necessary. Just tag and block the traffic. Way simpler, then you don’t have to remember that your AD is a non-standard value.

  4. dontask
    September 18th, 2022

    I think Q5 is “Set the administrative distance 100 under the process on R2”. Answer B.

    The prefix 10.1.1.0/24 will be redistributed from RIP to EIGRP, and then continuing from EIGRP redistributesd to OSPF. So, at OSPF routing domain, the prefix 10.1.1.0/24 has 110 AD value.

    When PC2,PC3 or PC4 try to get PC1, the traffic will go to R2. and at that, R2 will decide to route to R3 (because of lower AD which is 110). then R3 decide send traffic to R4 and a loop will begin, which is R2->R3->R4->R2

    @networktut Please check!

  5. Bank
    June 27th, 2023

    Hi sir.

    I need update diagram of Re-distribution LAB Exam (Including Question would be good).
    Pls send to me at Email : prisonerpriest(at)hotmail.com

    Thx boss.

  6. DB_Cooper
    May 15th, 2024

    @dontask you are correct. I tested this in a lab and confirmed that once setting “distance 100” under RIP configuration mode in R2, the RIP route became more favorable and pings from PCs 2, 3, and 4 to PC1 were successful.

    @networktut, can you confirm and possibly update the answer to B?

  7. Merlin
    May 17th, 2024

    Please check this question Q5 as people has tested this in the lab and doubt the answer to be “Filter the prefix 10.1.1.0/24 when redistributed from OSPF to EIGRP” as the correct answer.

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