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?
A.
route-map SETLP permit 10 match ip address prefix-list PLIST1 set local-preference 99 route-map SETLP permit 20
B.
router bgp 111 no neighbor 192.168.10.1 route-map SETLP in neighbor 192.168.20.2 route-map SETLP in
C.
route-map SETLP permit 10 match ip address prefix-list PLIST1 set local-preference 110 route-map SETLP permit 20
D.
router bgp 111 no neighbor 192.168.10.1 route-map SETLP in neighbor 192.168.10.1 route-map SETLP out
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.
Option C adjusts the local preference for routes matching prefix-list PLIST1 (which should contain the prefix 172.20.5.0/24) and sets the local-preference to 110, which will make AS 200 the preferred path. The permit 20 in the second route-map statement ensures that all other routes are allowed without changing their local preference.
setting the local preference to a value under the default of 100, will make as 200 the preferred egress. However, what was actually needed for this scenario was the second sequence in the routemap; putting the permit in front of the implicit deny.
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"
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