Exam 350-401 topic 1 question 381 discussion

A. R1(config)#route-map RM_LOCAL_PREF permit 10 R1(config-route-map)#set local-preference 101 R1(config-route-map)#exit R1(config)#router bgp 100 R1(config-router)#neighbor 13.13.13.3 route-map RM_LOCAL_PREF in R1(config-router)#end R1#clear ip bgp 13.13.13.3 soft in B. R1(config)#route-map RM_AS_PATH_PREPEND R1(config-route-map)#set as-path prepend 200 200 R1(config-route-map)#exit R1(config)#router bgp 100 R1(config-router)#neighbor 12.12.12.2 route-map RM_AS_PATH_PREPEND in R1(config-router)#end R1#clear ip bgp 12.12.12.2 soft in C. R1(config)#router bgp 100 R1(config-router)#neighbor 13.13.13.3 weight 1 R1(config-router)#end D. R2(config)#route-map RM_MED permit 10 R2(config-route-map)#set metric 1 R2(config-route-map)#exit R2(config)#router bgp 200 R2(config-router)#neighbor 12.12.12.1 route-map RM_MED out R2(config-router)#end R2#clear ip bgp 12.12.12.1 soft out

This is the explanation why A is the ONLY Correct answer. A: This will prefer both router to choose R1 - R3 - R4, as Local Preference is shared within the same AS. B: AS-PREPEND is adding 200 200 - that's is NOT allowed as this is the neighbor AS and you can only prepend the local AS. C: This will not work because Weight attribute requires a route map - The Router won't allow to set the weight directly like this - TESTED. D. The MED attribute is only affecting how inbound traffic will reach us and NOT how we go out.

A)WRONG. Local preference is valid only for I-BGP updates. B)WRONG. The Rout-Map miss the "permit" (route-map RM_AS_PATH_PREPEND R1) C) WRONG. It Misses the "R1#clear ip bgp 13.13.13.3 soft in". Without this command, the BGP table is not updated D) Correct Answer

B is right,local preference and weight will be used mostly in locally and usr for set inbound traffic ,but AS-path can advertise to another AS, so R 1 to R2 advertise 2 times AS 200 So R1 to R3 has shorter AS 200 and traffic goes R1-R3-R4

Route selection in BGP priority goes to: routes with higher weight (default is 0); with higher local preference (default is 100); with shorter AS path length (number of AS passed through to reach destination); and with less MED (metric, default is 0) value. Our objective is making R2's BGP routes more desirable to R1 than routes learned from R3. Option A will set the local preference for BGP routes learned from R2 to 101, (R2 101 LP > R3 100 LP) making these routes more preferable. Option B is incorrect, you can only prepend you local AS. Also, prepending AS path to routes advertised by R2 will make the path look longer and less desirable to take, when compared to R3. Option C is incorrect, we need to assign a higher weigh to R2 (default weight is 0) not R3. Option D is incorrect, we need to advertise routes with less MED value to make R2 more desirable to R1 than R3.

Correct answer is D. All others are wrong: A. Local preference can be used for iBGP only ( if the routers resided in the same AS ) B. AS_PATH can't use 200 to prepend AS_PATH C. Weight is lower than default D. Is correct

it´s A

A and B would work according to this document: https://community.cisco.com/t5/networking-knowledge-base/understanding-bgp-best-path-selection-manipulation/ta-p/3150576

Just labbed this question on 4 routers in EVE-NG running I86BI_LINUX-ADVENTERPRISEK9-M 15.7(3) M2 A: WORKS. No more comments as it already is the Most Voted answer B: WORKS. I see that there are some answers saying 'you can't prepend foreign/neighbor ASs to AS_PATH.' I couldn't find any Cisco documentation on such a limitation, so where did you get this info? The only relevant info I could find is that YOU SHOULDN'T DO IT as it might have unintended effects https://www.noction.com/blog/as-path-and-as-path-prepending but YOU CAN DO IT. C: DOESN'T WORK: As others already pointed out, without " #clear ip bgp <neighbor> soft in ", the next hop via R2 stays as it is. However, not that this configuration also works, if "clear ip bgp" is also issued. For all the answers saying "you can't set the weight like this", you are wrong. D: WORKS: R2 sets metric of 1 for the routes outbound towards R1, so R1 will choose the route from R3 as best path.

Another interesting one. Firs of all - B is wrong! One cannot prepend a foreign AS 200. Not to mention that prepanding ASes influences the traffic flow in the other way around it is required in this case. i checked options A & D as they both made sense and... they actually did. Both of them wil do the trick as A is modyfing a local pref from 100 to 101 making the path via R3 more desirable while D is adding +1 to the MED which makes the route via R2 less desirable which is also fine. IOU1#sh bgp ipv4 uni * 172.16.10.0/24 12.12.12.12 1 0 200 300 i *> 13.13.13.13 101 0 200 300 i Since LP is the 2nd criteria and MED is lower I would go with A.

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13759-37.html

Its A for me Regards

I rechecked in GNS3. Although both A and B work but simplest and easy way is C.

Option A tested in GNS3 and it works as per requirement. Therefore correct answer is A.

This question is based on this article: https://community.cisco.com/t5/networking-knowledge-base/understanding-bgp-best-path-selection-manipulation/ta-p/3150576 The article lists all the solutions A, B, and D as appropriate. C is missing "clear ip bgp ...", so the attribute weight is out. B is okay, too, because it is an as-path prepend inbound. You can set the neighbor's as or operator "last-as ..." for inbound updates. See: https://blog.ipspace.net/2009/03/as-path-prepending-technical-details.html It is not clear to me why answer A is better than the other answers, unless we consider the order of the best path selection. Local preference will be checked before the other possible solutions (as-path prepend and MED).

Correct answer is A.

AS path prepend is only used in OUTGOING BGP updates, to influence incoming traffic. Weght default value is 1 and could have been right, but clear soft session is not set. Weight has local meaning (scope just inside R1 router). Local_pref is used to select an output path, a BGP AS scope (iBGP updates).