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Question 71

AS 111 wanted to use AS 200 as the preferred path for 172.20.5.0/24 and AS 100 as the backup. After the configuration, AS 100 is not used for any other routes.

Which configuration resolves the issue?

route-map SETLP permit 10 match ip address prefix-list PLIST1 set local-preference 99 route-map SETLP permit 20

router bgp 111 no neighbor 192.168.10.1 route-map SETLP in neighbor 192.168.20.2 route-map SETLP in

route-map SETLP permit 10 match ip address prefix-list PLIST1 set local-preference 110 route-map SETLP permit 20

router bgp 111 no neighbor 192.168.10.1 route-map SETLP in neighbor 192.168.10.1 route-map SETLP out

Correct Answer: A

There is an implicit deny all at the end of any route-map which means all other traffic that does not match 172.20.5.0/24 would be dropped. Therefore, we need to add a permit sequence at the end of the route-map to allow other traffic. The default value of Local Preference is 100 and higher values are preferred. Therefore, we have to set the local preference of AS100 lower than that of AS200. By setting the local preference of 99 for AS100, the route preference for 172.20.5.0/24 will prioritize AS200 over AS100, ensuring the desired path. Additionally, adding sequence 20 will allow other routes to take the path through AS100 without being dropped by the implicit deny.

Discussion

JingleJangusOption: A

A works because the default local pref is 100. Making the local pref 99 will decrement the quality of the route enough to install the route from AS200 into the RIB. The reason traffic wasnt using AS100 for any other routes is because of the logic of the route map. Seq 10 matches on a specific range of ip's. Therefore traffic that doesnt match that range will move onto the next seq #, but in our case there is none so all other traffic hits the implicit deny at the end of the route map. Adding a seq 20 to match on all traffic (not having a match statement) will allow all other NLRI thru from AS100 to AS111.

SeMo0o0o0Option: A

A is correct

AinsBOption: C

correct answer is C, higher local preference is preferred

AinsB

I am correcting my answer to "A" for the reason that we need to route to go to AS200 so if we drop the local preference to 99 for AS100 the route will prefer AS200 "BUT" note that there is an implicit deny at the end of the prefix list/access list that we need to override so that other routes an take this path. This is the trick of these exam questions "good review"

Noproblem22Option: A

A is correct