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Which BGP attribute can you set to allow traffic that originates in ASN 64523 to exit the ASN through ISP B?

Refer to the exhibit.

ASN 64523 has a multihomed BGP setup to ISP A and ISP B. Which BGP attribute can you set to allow
traffic that originates in ASN 64523 to exit the ASN through ISP B?

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A.
origin

B.
next-hop

C.
weight

D.
multi-exit discriminator

21 Comments on “Which BGP attribute can you set to allow traffic that originates in ASN 64523 to exit the ASN through ISP B?”

  2. AmitO says:

    MED is for influencing incoming traffic i.e routes that we send out to peers. Isn’t weight more appropriate as it can be used to influence traffic going out i.e incoming routing updates from peers.




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  5. Vignesh says:

    The answer definetly can’t be Weight as the question asks to influence the path selection of the whole AS. Weight wont do that, it can alter only for a single router. Origin can be the correct answer as if we tweak that using some route map we can change the outbound path from AS 64523.




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    1. Max says:

      The external routes will be advertised internally between the iBGP peers with an origin of i, hence you won’t be able to use Origin to influence the exit. MED is the right answer here.




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  6. JJ says:

    MED isn’t correct, since that only influences inbound traffic. Recall that both ASPath pre-pending and MED are the only options available to influence incoming traffic.

    Weight isn’t correct, since that attribute is locally significant to the router where it is set. The value isn’t communicated to any other router, period. Additionally, Weight only makes sense if the router in question itself has two separate ISP connections. This is the not the case here.

    Origin doesn’t make sense. The origin is what the origin is. No matter how you manipulate it, either G or F will have the same view of it. This can’t be right.

    This leaves Next-Hop. You could create a route-map on Router G and say for traffic that originates from the ASN, the next hop will be Router F. Next-Hop is certainly the correct answer.




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  7. Tron says:

    This is ridiculous.
    Correct answer is LP, but it’s not there.
    MED is not it (neighbours are not even the same AS) but you could use any attribute (origin or NH) to then use policy. You could also use WEIGHT in both routers. Question does not ask for only one router config.




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  8. Theo says:

    The answer is LP but of course that answer isn’t there. If on Router G you weight the interface pointing to Router F for all traffic destined for the ISP and on Router F you weight the interface pointing to the ISP for all traffic destined for the ISP you can achieve the desired result.




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  9. Rob says:

    It’s weight and you don’t need to configure it on both routers.
    When Router G does bestpath calculation it will have to choose between an eBGP route and an iBGP route. The eBGP will win (assuming all other attributes are equal) (step 7- http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html). However, if you use weight (step 1) you can influence that choice. On router G you’d configure this:

    router bgp 64523
    neighbor 209.165.201.2 remote-as
    neighbor 209.165.201.2 weight 0 (this would be the default)
    neighbor 10.10.1.21 remote-as 64523
    neighbor 10.10.1.21 weight 200

    For any parallel paths learned from both Router 1 (ISP A) and Router F, the higher weight of the path through Router F would be preferred. By default, router F will make a bestpath decision where the routes have the same weight so it will choose the eBGP path.




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  10. j says:

    MED can be right. you can set MED inbound on router G to 200 for example. It does not affect the decision on router Gs routes, but routers behind it in that AS would be affected.

    The question doesn’t state WHERE to do the configuration either. So you could set MED out on the ISP side.

    You can not influence G or As local decision with any of the provided options i don’t believe. But you can influence routers behind them, which is what I assume the question is trying to say but, shocker, cisco has a bad tricky question.




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