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Question 461

Refer to the exhibit. The Customer Edge router wants to use AS 100 as the preferred ISP for all external routes and SP2 as a backup.

Customer-Edge -

route-map SETAS

set as-path prepend 111

!

router bgp 64555

neighbor 192.168.1.1 remote-as 100

neighbor 192.168.2.2 remote-as 200

neighbor 192.168.2.2 route-map SETAS in

After this configuration, all the backup routes have disappeared from the BGP table on the Customer Edge router. Which set of configurations resolves the issue on the Customer Edge router?

    Correct Answer: C

    In BGP routing, influencing the path selection by using AS path prepending is typically done on outbound route advertisements to the neighbor. To achieve the goal of preferring ISP-1 (AS 100) for all external routes, and using ISP-2 (AS 200) as a backup, you need to make the routes through ISP-2 less preferred by prepending the AS path multiple times when advertising routes to ISP-2. This can be done by configuring the route-map to prepend AS 200 and applying it in the outbound direction. Hence, the correct configuration would be to set the AS path prepend to 200 and apply this route-map on the outbound direction for the ISP-2 neighbor.

Discussion
inteldarvidOption: C

for me option corerct is "C". I need out Prepending wit 200 200

HungarianDishOption: A

Inbound is correct for sure. Only answer "A" uses correct neighbor + direction. neighbor 192.168.2.2 route-map AS in Probably this explains, why they want to prepend AS 200 (remote AS number): https://blog.ipspace.net/2009/03/as-path-prepending-technical-details.html Inbound AS-Path Prepending ... to add an extra copy of the REMOTE AS number to inbound updates received from (neighbor)

spada05Option: A

Inbound, as the CE we presumably cannot touch the ISP and so have to manipulate inbound.

drxzOption: B

When you want to prepend a route, you need the route-map to be "out" , not in.

potato_inet0

not true, you can preepend AS-path as the route is coming in, so the inbound policy will preepend the AS then the route will be added to Local-RIB with the AS-path preepended, the question seems wrong in my opinion, none of the answers makes semse

9410480Option: A

Answer is A

Tim303Option: B

A is wrong because you have to apply the route-map to the outbound direction, Google " BGP deep Dive" and watch Kevin Wallace

yefrimartOption: A

When prepending an AS to a AS-PATH, you need to either prepend your own AS or the neighbor AS. Any other AS may cause problems. For this question, because we want to influence outbound traffic, we need to prepend received prefixes. The answer that fulfill both conditions is answer A.

HungarianDish

The original configuration (set as-path prepend 111) did not influence the path-selection at all. Plus, none of the routes disappeared. The question seems to be incorrect.

chinoplaOption: B

Prepending is applied outbound. You want to make the backup path to appear as the longer path by advertising additional AS on this path and that is done outbound.

asansOption: A

Something wrong with this question. The original route map config is the same as answer A, save for the different AS. And both works without issues

HungarianDishOption: B

Prepending as-number 111 inbound worked. It needs to be added two times, though (111 111). CE#sh run | sec route-map neighbor 192.168.2.2 route-map AS in route-map AS permit 10 set as-path prepend 111 111 CE#

HungarianDish

This had the same effect: set as-path prepend 200 200

HungarianDishOption: B

Sorry, did not wait enough. Now it's OK. Using as-path prepend 111 111. Result: CE is choosing ISP1 as primary. CE#sh ip bgp BGP table version is 3, local router ID is 192.168.2.1 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, t secondary path, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path * 1.1.1.1/32 192.168.2.2 0 111 111 200 100 i *> 192.168.1.1 0 0 100 i * 2.2.2.2/32 192.168.2.2 0 0 111 111 200 i *> 192.168.1.1 0 100 200 i CE#

HungarianDishOption: B

The issue came after prepending AS 111 two times: CE#sh ip bgp BGP table version is 1, local router ID is 192.168.2.1 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, t secondary path, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path * 1.1.1.1/32 192.168.1.1 0 0 100 i * 192.168.2.2 0 111 111 200 100 i * 2.2.2.2/32 192.168.1.1 0 100 200 i * 192.168.2.2 0 0 111 111 200 i CE#

HungarianDish

Ignore this please, I just did not give enough time to bgp to complete the bgp table.

HungarianDish

I labbed it, and do not see the issue with the disappearing routes. Original configuration. All routes are there in bgp table. CE#sh ip bgp BGP table version is 4, local router ID is 192.168.1.2 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, t secondary path, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path * 1.1.1.1/32 192.168.2.2 0 111 200 100 i *> 192.168.1.1 0 0 100 i * 2.2.2.2/32 192.168.1.1 0 100 200 i *> 192.168.2.2 0 0 111 200 i CE#

HungarianDish

CE#sh run | sec router bgp router bgp 64555 bgp log-neighbor-changes neighbor 192.168.1.1 remote-as 100 neighbor 192.168.2.2 remote-as 200 neighbor 192.168.2.2 route-map AS in CE#sh run | sec route-map neighbor 192.168.2.2 route-map AS in route-map AS permit 10 set as-path prepend 111 CE#

HungarianDish

Adding an as to the as-path only once did not change the path selection at all. They need to prepend two times: "set as-path prepend 111 111" or "set as-path prepend 200 200"