Exam 350-501 topic 1 question 72 discussion

For those who say A is correct... explain how the administrator of 64502 can modify the outgoing traffic of the other AS(64501) using Local pref... Local pref is for traffic leaving your network to prefer one path over the other... and since the issue is with traffic coming into his network not out of his network the only way is to modify MED or a more specific prefix. The only one that will surely get the job done is C because ROUTING IS ALWAYS BASED ON THE MOST SPECIFIC ROUTE REGARDLESS OF AAAAANY OTHER MODIFIERS....

A is correct. Local preference on South_A updates other Routers in the AS to prefer North_A path for the specified prefix. MED will influence incoming traffic from other AS

Local preference is applied within an AS, affecting the routing decisions of internal BGP (iBGP) routers. It is not advertised to external BGP (eBGP) peers. Admin has only access to his own AS 64502. A only work if we have access to ASN 64501 and modify LP between South_A and South_B routers not North_A and South_A routers

honestly.... a horrible question... but I would have to agree. C is the only thing that makes sense here... A - setting higher LP will only affect outgoing traffic from ASN64502 and not incoming traffic B - Setting a lower MED between NORTH_A and SOUTH_A.. MED is an optional non-transitive BGP attribute meaning it will not be sent to the eBGP neighbor... a lower MED is preferred, thus making the slow link path more preferred... C - What others are saying... by specifying more specific routes, you will choose those routes even before looking at BGP attributes. D - Lower weight is less preferred... and weight is also for outgoing traffic... Absolutely horrible question... they first mention "Networks" under 10.0.0.0/8... but later ask about traffic going to 10.0.0.0/8 (they should have kept their wording consistent).

option C is the only way to make sure you get the traffic in via desired path without having/requesting change in the neighboring AS.

After reading carefully, you find out the ISP is the ASN 64501, cause ISPs don't care if a network is reached through a backup or primary link, CE's network admins do. Then, you see you need to influence the incoming traffic towards your local "Network" 10.0.0..0/8 (The administrator of ASN 64502 wants traffic "FROM" ASN 64501 "TO" 10.0.0.0/8 to travel via the primary link, North_A.) Option D, is discarded immediately. Weight influences outgoing traffic, even worst a lower weight. Option A, would only influence the outgoing traffic towards the ASN 64501: https://networklessons.com/bgp/how-to-configure-bgp-local-preference-attribute Option C, does not make any sense in today's BGP setups. Yes, it resolves the problem, but is the same as shutting down the other link and kill redundancy - Why you want to do that? Option B is the correct answer!! Setting lower MED is the best option, when you don't have AS path prepand as an option. https://networklessons.com/bgp/how-to-configure-bgp-med-attribute

for ASN64510 -->to 10.0.0.0/8 --- option C for 10.0.0.0/8 --> to ASN64510 -- option A Question asked for 'to 10.0.0.0/8' so we should go with option C

C - the more specific or longest prefix match is always preferred, even before looking at the BGP metrics/attributes.

Answer - A ASN 64501 currently reaches the networks under the 10.0.0.0/8 prefix via the North_B router, which is a slow backup link. The administrator of ASN 64502 wants traffic from ASN 64501 to 10.0.0.0/8 to travel via the primary link, North_A We have an eBGP peering from South_A to North_A. We can apply a route-map of LP with LP of 200 for example, assuming this value will be higher, the route-map gets applied to the neighbor statement in the inbound direction. The 10/8 prefix advertised by 64502 to 64501 would be advertised by North_A and received by South_A. South_A has the route-map applied, applying the LP of 200 to the 10/8 route. Local Preference influences how traffic will leave the local AS. Traffic from 64501 towards 64502 is the egress path via South_A. A higher local preference would send the traffic the correct direction. Weight goes by the higher value. Lowest metric for MED. Technically both of these could be used, but other configurations would be needed to get traffic to South_A in the first place. Local Preference is advertised to all iBGP peers in the AS.

After reading the question and the comments again, I realized that C is indeed the right answer ! Local pref should be deployed on AS 64501and not the other AS.... The MED is non transitive... + they don't propose to do it the right way as they propose to lower the MED on the link we want to avoid

C is only reasonable answer.

A. Incorrect - Prefer the path with the highest LOCAL_PREF. But, local preference is an attribute that routers exchange in the same AS. B. Incorrect - Prefer the path with the lowest multi-exit discriminator (MED). D. Incorrect - Prefer the path with the highest WEIGHT. Perhaps, C is only reasonable answer.

C is correct..

A is not correct. Local Preference is not attached to eBGP updates. LP remain only within the AS for that reason A is not a possible answer for this question. For me, the most reasonable answer is C Weight has local meaning and lower MED make that traffic prefer the path with Lower MED value.

I believe this is the type of question that will be different on the actual exam, perhaps different answers o the absolute best answer will show up. Just think what BGP attributes influence incomming traffic from a foreign AS: AS PATH and MED, outgoing traffic would be: Local Pref and weight.

The administrator would use bgp communities and send these to other bgp as which would typically be used to match on route-map which in turn can change the local pref. So first choice A, 2nd choice would be C, but would say answer A is a better fit for sp exam.

C is correct. the two common way to influence the traffic entering your own AS is either to manipulate the AS_path ( AS prepending) or to use MED. local pref and WEIGTH are for the ongoing traffic.