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I recently started to look into BGP load balancing, explicitly, I was wondering if it there is a way in BGP to perform load balancing on two paths whose advertisement came from two external BGP speakers from to distinct ASes. When looking into the corresponding Cisco and Juniper documentation, it is said that if the functionality is enabled, then load balancing is applied if the decision process results in a tie, which is only the case if the advertised routes come from the same external AS.
Could someone explain me why a tie is always implying that the advertisements came from the same AS, and whether it is possible to do load balancing over two different ASes.
Thank you in advance.

Devrim Celik
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The reason is that in order to avoid potential routing loops, ECMP requires that the "competing" IGP costs be equal (per the "fish" problem -- see https://routingfreak.wordpress.com/tag/traffic-engineering/ for an explanation). Given that each AS, almost by definition, has its own independent IGP, the IGPs are not comparable (and would likely be unequal anyway). Also, typically, each prefix will have different AS path lengths from the two ASes. It could work; for example, with static routes to reach the bgp next hop.

Andrew
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