Refer to the exhibit. Currently the two eBGP links between AS100 and AS200 have an average inbound load of 65% and 20% respectively. After further investigation, traffic to 10.10.1.16/28 accounts for 45%, and traffic to 10.10.1.32/28 and to 10.10.1.48/28 each account for 20% of the inbound load. The BGP attributes are currently set at their default values in both autonomous systems.
If you want to influence how AS200 sends traffic to AS100, which eBGP configurations would you configure in AS100 to influence AS200 to use the eBGP links more evenly? (Choose two.)
Select 2 response(s).
A.
neighbor 192.168.30.2 route-map as_50 out
B.
neighbor 192.168.20.2 route-map as_50 out
C.
route-map as_50 permit 10
match ip address 50
set metric 150
access-list 50 permit 10.10.1.16 0.0.0.240
D.
route-map as_50 permit 10
match ip address 50
set metric 150
access-list 50 permit 10.10.1.32 0.0.0.240
Answer: B C
Explanation
Note: The wildcard masks in the access-list are not correct, they should be 0.0.0.15 instead of 0.0.0.240.
First let’s recall about MED. If you want to influence external neighbors about the path it sends traffic then MED, also called the metric, should be used. A lower MED value is preferred over a higher value. The default MED value is 0.
Also, an important point I wish to notice in this question is: there is a “deny all” statement at the end of each access-list. In other words, all the routes that do not match the access-list used by the route-map will be filtered out. For example in answer C, only network 10.10.1.16/28 is matched by the access-list and all other networks will be dropped (not advertised to EBGP).
Therefore in answer C, only traffic to 10.10.1.16/28 will be go through routerA (45%). All other traffic must go through routerB (40%) -> links are used more evenly -> C is correct.
There are some debates saying D is correct but in fact it is not. If answer D is applied for router A, the MED for 10.10.1.32 set to 150 would make traffic for this route go through router B (because the MED default value is 0, which is lower than 150). The access-list will also filter out network 10.10.1.16 -> router E can not send traffic for network 10.10.1.16 to router A. Now all traffic must go through router B (85%).
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This doesn’t make sense. if you apply a metric (MED) of 150 to 10.1.1.16/28 on router A using that route map outward to 192.168.20.2 this will make the line utilization more saturated on the link between Routers B and F because what you are actually doing is telling AS 200 “My Med is 150 from A (higher) than my metric from B 0 (Lower)” this will have the ISP in the example use 192.168.30.0/24 as the path to reach 10.10.1.16/28.
The answer given by you cannot be correct looking at this logically the right answers are (A) and (D) this will tell the ISP “hey I have a metric of 150 from Router A, the metric from router B of 0 is better” thus utilization will become 45% and 40% respective.
A and D are the correct answer logically.
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