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Jonathan Forbes




I have 2 ISP in BGP config on 1 Cisco Router. I am taking a full Table from both ISP's 


What i am looking to do it have a ASN go IN and OUT a different ISP than the default path. The current path is ISP 1 but i would like to have this go in and out ISP 2. 



Can someone help me on this as i am new to bgp. 



I have the config up and running on both service providers.




paul driver
VIP Expert VIP Expert
VIP Expert


the easiest way is to use weight prefferance for egress and as path pretending for ingress traffic


I would also make sure you don't become a transit path for either ISP also


ip as-path access-list 10 permit ^$

route-map ASN-prepending permit 10

set as-path 20 20 20


Router bgp 20

neighbour (isp2) weight 400000

neighbour (isp2) filter-list 10 out

neighbour (isp1) weight 200000 ( optional)

neigbour (isp1) filter-list 10 out

neigbour (isp1) route-map ASN-pending out




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Kind Regards

The problem i have is that i have a single asn that i want to use isp2


I use Cogent for ISP 1 and HE for ISP 2 when i run a traceroute to anything that is on the comcast network it goes out the HE ISP 2 pipe i want all traffic for comcast to use my cogent as this is a 10gb burst and the HE is single gig connection. 



Is there a way i can tell the comcast asn to use the cogent in and out and allow all the other traffic to go best path. the reason i want to do this is because from our DC here in the US we are seeing the comcast traffic use up our he gig connection. with moving this over we can allow more traffic over the cogent connection. 





I would start by pointing out that when you are using BGP with 2 ISP that you must deal separately with outbound traffic and inbound traffic. The things that control outbound have no effect on inbound and things that control inbound do not effect outbound.


Outbound is easier so let us start with this. I suggest that the first thing is to figure why traffic for Comcast network is using HE. Please check your BGP table (not the IP route table but the table that shows what you neighbors have advertised to you before your router has chosen the best path). Are Cogent and HE advertising the Comcast routes in exactly the same way? Do they use the same prefix and the same prefix length? If they are advertised differently for some reason they you will have to deal with this in some way. If they are advertised the same then it should be possible to configure a route map to work with the BGP advertisements and in the route map to set either weight or local preference so that your router prefers ISP1 for the Comcast traffic.


Influencing the inbound traffic is more difficult because that depends on the routing policy of the AS originating the traffic. In this case how does Comcast select the path toward your AS? The results of your traceroute show only how you send outbound traffic and tell you nothing about the path used for return traffic. It might even be possible that Comcast already chooses the path through ISP 1 and that you do not need to do anything else for inbound traffic. So what you should do is to try to determine how Comcast does get from their AS to your AS. If they are choosing the path through HE I can not think of much that you can do other than to use prepending as Paul suggested.






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