Exam 350-401 topic 1 question 200 discussion

The correct answer is A.

No Answer is correct A) Med is not compered if As-Path is not the same or Always-compare-med is configured (nor is) B) would prefer the path BR2 -PE2 which is not the goal C) prefers the path only on BR1 local, does not change decision on BR2 D) you cant configure Local pref outbound to another AS

Option A, setting the MED to 1 on PE2 toward BR2 outbound, wouldn’t be as effective because the MED (Multi-Exit Discriminator) attribute is used to influence the path selection between different autonomous systems (AS) when multiple paths exist. However, MED is only considered when comparing routes from the same AS. In this scenario, since BR2 is choosing between paths through different ASes (AS200 and AS300), the MED attribute won’t have the desired impact on BR2’s path selection. The weight attribute, on the other hand, is a Cisco-specific attribute that is considered first in the BGP path selection process and is local to the router on which it is set. By setting the weight to 65,535 on BR1, you ensure that BR1’s path is preferred over any other paths, effectively directing BR2 to route traffic through BR1.

Option A would continue to prefer BR2>PE2. The lower the MED, the better.

Bot B and C are incorrect. Explanation: MED and weight attributes are not transitive, therefore setting the attributes on one iBGP router has no effect on the other iBGP router path selection , these attributes are propagated only to adjacent AS. B is the correct option: set origin {igp | egp as-system | incomplete}

It´s A By default, the MED attribute is set to 0 so by increasing the MED on PE2 toward BR2, BR2 would think the metric of its direct link to PE2 is higher than the path advertised by BR1. So BR2 would use BR1 to reach 209.165.201.0/27. B is incorrect, We can´t affect BR2 routing decision by modifying BGP advertisements from BR2 toward PE2 (inbound). Also if network 209.165.201.0/27 is advertised with “network” statement in BGP, BR2 will match it with origin IGP.

it should be set origin as Incomplete on the PE2 toward BR2: rtrb>sho ip bgp BGP table version is 5, local router ID is 172.12.0.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path * 172.16.23.0/24 10.0.13.1 100 0 300 ? *>i 10.0.0.1 0 100 0 200 i

neither A is correct :0|

D: Local pref has affect on the (inbound policy) outbound path selection so in this case it would effect PE1 outgoing traffic (we need other way around) C) Weight is just locally on the router takes affect and the answer is set the weight on BR1 so does not change anything regarding BR2 B) If the prefix from PE2 is incomplete type and we modify it to IGP (it cannot be done cause you can only modify from igp to egp or incomplete) we would choose the line towards PE2 what is the current situation so it is not choice we want A) weight is an (outbound policy) inbound path selection attribute so we can influence the incoming traffic --> this is a good solution for us cause the default med is 0 and lower is better so if we worsen the med on PE2 it means that BR2 will see a better med attribute from PE1 via BR1 so that direction will be the best during the path selection process

A. D is incorrect because the ASes are different on the PE2 so this rules out MED.

I can confirm A worked for me. After changing the metric for the route on PE2, BR2 changed its ip route to go via BR1. BR2#sh bgp ipv4 uni Network Next Hop Metric LocPrf Weight Path * 209.165.201.0/27 209.165.200.230 1 0 200 i *>i 172.24.109.2 0 100 0 200 i BR2#sh ip route 209.165.201.0 Routing entry for 209.165.201.0/27, 1 known subnets B 209.165.201.0 [200/0] via 172.24.109.2, 00:08:00

A is the correct answer only if the comment bgp Always-compare-med is configured. Synopsis bgp always-compare-med no bgp always-compare-med Configures BGP route selection Default Disabled Description This command allows the comparison of the multi-exit discriminator (MED) for paths, regardless of which autonomous system the path comes from.

MED is only applicable and useful when we are entering same AS. but here we have two different AS 200, 300 so none of the options are correct. FU$@ this question and person who made it.

A is not correct as for some people it may work who try in the lab and for others it might not. always compare MED is needed as other have said. B seems... correct, but weird C only on local router D local pref is only for the local AS... Pretty silly question, imo.

The answer in this case is A ONLY if you put the bgp always-compare-med command in BR2. A: RIGHT ONE B: nope... remember that the origin code means this: e: Exterior Gateway Protocol, you will never see this one 'cause it was the protocol before BGP. i: network announced via the network command ?: network redistributed in BGP So you they are saying that you need to put the origin code i for a BGP announce that for sure you are receiving via the network command,,,, Sounds bad right? It will not change nothing at all. C: Nope the weight attribute is only for the local router, so not good. D: Nope again, the local preference value can be propagated only to the local AS, so if they were saying to set the BGP local preference to 150 on BR1 in inbound direction it will be right but is not the case. (REMEBER: the LP is ONLY set in INBOUND DIRECTION!)

Answer B A is wrong. MED is only used when the first hop on the AS Path is identical. In this case, this is not true.

actually, this case is described here, see sec. #5 https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html and why MED "solution" is the wrong answer in this scenario (first AS nums are different) see next sec. #6