>With AS-PATH prepend you have no control on the choice of which ASN should do what action on your advertisements.

Robert- It is somewhat this problem we are trying to resolve.

>I was imagining something sexier, especially given how pretty "useless" AS_PATH prepending is nowadays.

I, too, am looking for something sexy (explained below). But can you explain why you think AS_PATH is "useless," Mark?

For background, and the reason I asked about DPA:

Currently, our routing carries user traffic to a single data center where it egresses to the Internet via three ISP circuits, two carriers. We are peering on a single switch stack, so we let L2 "load balance" user flows for us. We have now brought up another ISP circuit in a second data center, and are attempting to influence traffic to return the same path as it egressed our network. Simply, we now have two datacenters which user traffic can egress, and if one is used we want that traffic to return to the same data center. It is a problem of asymmetry. It appears the only tools we have are AS_Path and MED, and so I have been searching for another solution, that is when I came across DPA. In further looking at the problem, BGP Communities also seems to be a possible solution, but as the thread has explored, communities may/may not be scrubbed upstream. So, presently we are looking for a solution which can be used with our direct peers. Obviously, if someone has a better solution, I am all ears.

A bit more info: we are also looking at an internal solution which passes IGP metric into MED to influence pathing.

To avoid TL;DR I will stop there in the hopes this is an intriguing enough problem to generate discussion.

michael brooks
Sr. Network Engineer
Adams 12 Five Star Schools

michael.brooks@adams12.org

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

"flying is learning how to throw yourself at the ground and miss"

On Fri, Aug 18, 2023 at 1:39 AM Robert Raszuk <robert@raszuk.net> wrote:

Jakob,

With AS-PATH prepend you have no control on the choice of which ASN should do what action on your advertisements.

However, the practice of publishing communities by (some) ASNs along with their remote actions could be treated as an alternative to the DPA attribute. It could result in remote PREPEND action too.

If only those communities would not be deleted by some transit networks ....

Thx,
R.

On Thu, Aug 17, 2023 at 9:46 PM Jakob Heitz (jheitz) via NANOG <nanog@nanog.org> wrote:

"prepend as-path" has taken its place.

Kind Regards,

Jakob

Date: Wed, 16 Aug 2023 21:42:22 +0200
From: Mark Tinka <mark@tinka.africa>

On 8/16/23 16:16, michael brooks - ESC wrote:

> Perhaps (probably) naively, it seems to me that DPA would have been a
> useful BGP attribute. Can anyone shed light on why this RFC never
> moved beyond draft status? I cannot find much information on this
> other than IETF's data tracker
> (https://datatracker.ietf.org/doc/draft-ietf-idr-bgp-dpa/) and RFC6938
> (which implies DPA was in use,?but then was deprecated).

I've never heard of this draft until now, but reading it, I can see why
it would likely not be adopted today (not sure what the consensus would
have been back in the '90's).

DPA looks like MED on drugs.

Not sure operators want remote downstream ISP's arbitrarily choosing
which of their peering interconnects (and backbone links) carry traffic
from source to them. BGP is a poor communicator of bandwidth and
shilling cost, in general. Those kinds of decisions tend to be locally
made, and permitting outside influence could be a rather hard sell.

It reminds me of how router vendors implemented GMPLS in the hopes that
optical operators would allow their customers to build and control
circuits in the optical domain in some fantastic fashion.

Or how router vendors built Sync-E and PTP into their routers hoping
that they could sell timing as a service to mobile network operators as
part of a RAN backhaul service.

Some things just tend to be sacred.

Mark.