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AS-Prepend Vs Community Strings

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

 

 

2 REPLIES
Community Member

Check out this page: http:/

Check out this page: http://hackingcisco.blogspot.com/2011/04/lab-100-bgp-hidden-feature-bestpath-as.html

 

It could just be that your ISP is ignoring the AS path (with your prepended AS numbers). Essentially they are receiving it and installing the route as if you never did an AS prepend at all.

 

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VIP Purple

Re: AS-Prepend Vs Community Strings

Hello

 


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

 

 


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

 

 

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