Hello

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@jeff robins wrote:

Guys,

We have two ISP connections and we are advertising our public networks out both. ISP #1, we are not doing any as-prepending, but ISP#2 we are prepending out advertised network. Based on this we would assume all incoming traffic would be coming thru ISP#1, but its not, all inbound traffic is coming in on ISP#2.

 

I reached out to our ISP and he they said, "AS path prepending is not a reliable method for route selection on the internet. Generally we recommend using community values to communicate a route's priority instead"

 

Can someone please explain why as-prepending is not the reliable anymore?

 

Thanks

 

 

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My understanding is both can be negated by upstream ISP's is the need arose but i suppose because Pre-pending can be lost if the other ISP aggregates your prefix to an upstream peer communities are more of a preferred way to perform path selection with ISPs

Pre-pending manipulates the as-path attribute so its appended to the as-sequence of the bgp update it advertises to its neighboring peer/asn, but from thereon this information could be lost

Communities on the other hand can be group of prefixes /aggregates put together so that your isp has the option/derision on how to deal with these prefixes.

Unlike the aggregation that can lose the origin of the route, if the same aggregate was marked within a community then it would have its own path attribute (community) it could rely on.

res
Paul