Topic: 300-410 topic 1 question 292
Which BGP attribute can be used to influence the path that outgoing traffic takes from your AS to other Autonomous Systems? (Choose two.)
A.
MED
B.
AS_Path
C.
Weight
D.
Local Preference
Re: 300-410 topic 1 question 292
The BGP attributes that can be used to influence the path that outgoing traffic takes from your AS to other Autonomous Systems are AS_Path and Local Preference.
The Weight attribute is useful for influencing the path of traffic within a single router, it cannot be used to influence the path of outgoing traffic from your AS to other Autonomous Systems.
Re: 300-410 topic 1 question 292
Inbound Route Policy is applied for Outbound PATH SELECTION:
Weight
Local Pref
Outbound Route Policy is applied for Inbound PATH SELECTION:
AS PATH (prepend)
MED
Re: 300-410 topic 1 question 292
Answers : C & D
Both local pref and weight will be influencing routing when the route is LEAVING the local router. These will use “in” in the route map statement as they will modify the local pref or weight attribute after the BGP message is received. Once the modification is done, the new local pref or weight value will be applied when the traffic LEAVES the router.
Wording of the question is confusing but logically only these are correct options.
Tested in gns3 as well. AS path will be used as “out” in the route map statement and the AS path prepend will take place in the neighbor router hence the AS path prepend value will be visible when the traffic comes in.
Hope its somewhat clear now!!
Re: 300-410 topic 1 question 292
weight doesn't get advertised to other routers in your AS. Local preference does. So if you have 2 or more exit routers you set local preference so internal AS routers choose the appropriate exit router and final AS. With only one exit router you set Weight to choose the final AS.
Re: 300-410 topic 1 question 292
And AS_pAth is obvious. I will go with B and D.
Re: 300-410 topic 1 question 292
Correction: C and D is best.
Re: 300-410 topic 1 question 292
Think B,C and D are all valid.
Local preference is shared between router in the same OS whereas Weight and AS path manipulation need to be done per router via route maps/neighbor settings.
Re: 300-410 topic 1 question 292
CD
Practically B,C and D will do the job, however, using the BGP path selection order, I would go for C and D
Priority Attribute
1 Weight
2 Local Preference
3 Originate
4 AS path length
5 Origin code
Re: 300-410 topic 1 question 292
C, D is correct.
https://networklessons.com/bgp/bgp-attributes-and-path-selection
Re: 300-410 topic 1 question 292
C and D easily
Re: 300-410 topic 1 question 292
C, D is correct. AS-PATH with the prepend I can use polcy traffi inside AS
Re: 300-410 topic 1 question 292
per long time reserch, correct anwer is B D
Weight is only right when in some special situation.
You can read offical book P519-521, Then you can find the answer.
Re: 300-410 topic 1 question 292
I am still not sure whether it's B or C. OCG for ENARSI says:
"Weight can be set for specific routes with an inbound route map or for all routes learned from a specific neighbor. Weight is not advertised to peers and only influences outbound traffic from a router or an AS." Then there is a picture with a topology where weight is set on two routers of the same AS, and outgoing path is only manipulated by weight from that AS. Which part are you referring to in OCG?
Re: 300-410 topic 1 question 292
I voted C, D, because weight (C) can be used for dual-home network scenarios. Weight is only locally relevant, however, if there is only one border router for eBGP peering it can be used to manipulate outbound traffic out of one AS towards other ASs.
https://community.cisco.com/t5/other-network-architecture-subjects/bgp-inbound-and-outbound-traffic/td-p/337728
I do not doubt that B (AS-path prepend) is correct, too. I just can't choose between B and C. As-path prepend used outbound is also possible. Example:
https://blog.ipspace.net/2009/03/as-path-prepending-technical-details.html
Re: 300-410 topic 1 question 292
https://community.cisco.com/t5/routing/bgp-weight-local-preference-attributes-question/td-p/738421
https://community.cisco.com/t5/networking-knowledge-base/understanding-bgp-best-path-selection-manipulation/ta-p/3150576
Re: 300-410 topic 1 question 292
Local preference is Never Shared Between eBGP Peers, Any BGP router that receives a LOCAL_PREF attribute from an eBGP peer must ignore it (except in the case of BGP confederations)
Re: 300-410 topic 1 question 292
you are wrong my friend. We have policy outside our AS, with we can use LOCAL PREFERENCE for that
Re: 300-410 topic 1 question 292
C and D are correct. ASpath is only for manipulating incoming traffic to your AS, not outgoing.
Re: 300-410 topic 1 question 292
However not a typical case, as-path prepend used outbound is also possible.
https://blog.ipspace.net/2009/03/as-path-prepending-technical-details.html
Re: 300-410 topic 1 question 292
C and D are correct. Weight is locally significant, but if you have one router that is dual-homed, you are still influencing egress traffic. Local pref is used for influencing egress traffic from your entire autonomous system, I.e you have two separate routers running iBGP with an uplink to a different ISP each.
Re: 300-410 topic 1 question 292
Provided answer is correct, its Weigth and Local preference. AS Path is used to influence incoming traffic not outgoing. You can use it to influence incoming traffic but is not recomended.
@Titini "The Weight attribute is useful for influencing the path of traffic within a single router, it cannot be used to influence the path of outgoing traffic from your AS " - you dont understand what tha 'AS' is, you can have huge autonomous system network and olny one BGP router. So still you cant use Weigth?
Re: 300-410 topic 1 question 292
Provided answer is correct, its Weigth and Local preference. AS Path is used to influence incoming traffic not outgoing. You can use it to influence outgoing* traffic but is not recomended.
Re: 300-410 topic 1 question 292
B and D are correct answer