Outbound Route Filtering (ORF) vendor support

Jeffrey Haas jhaas at pfrc.org
Thu Aug 19 12:16:29 UTC 2021

> On Aug 19, 2021, at 12:18 AM, Douglas Fischer <fischerdouglas at gmail.com> wrote:
> I agree that without combining prefix-list and as-path, the effectiveness of ORF, considering its initial purpose, the pros and cons does not pay themselves.
> But (there is always a but), I was imagining a different use for ext-community-orf !
> Considering scenarios like:
>  -  Route-Servers of big IXPs, now days with almost 200K routes.
>  - Transit providers sending its own point of view of DFZ with almos 900K routes.
> On both cases, informative communities are an excelente way to decide "what is good for my ASN, and what is not".
> Yes, I know that is possible to filter based on that after receiving those routes.
> But it takes computational effort on both sides to do that.
> And I imagine that comparing to AS-Path Regex, the needed computational effort and the complexity of the logics to do filtering based on community-list are much smaller.
> So, if I could say:
>      "Hey Mr. Route-Server... how are you?
>      Could you please not send-me the routes that are tagged with the community <RS-ASN:xyz>?
>      And after that, send-me just the routes that are tagged with the community <RS-ASN:abc>?"
> In a Route-Server context, beyond reduce the number of BGP Messages that would be great for the CPU/Memory consumption both, RS and RS-Client.
> Or, in a Transit context...
> 1 - Customer opens a ticket with support team to set the export filter to send only default-route.
> 2 - Customer, 5 days later, opens a ticket with support team re-adjust the export filter, now sending full-routing.
> 3 - Customer, on next month, opens another ticket with support team to send only the cone at right of the ASN of ITP.
> With a good and public informative communities policy and ext-community-orf, the transit customer could change what his router will receive from the BGP transit Peer, depending only on himself.
> Well... I don't really know how complex is to deal with that again on a WG.
> But I would like to see that.

Once upon a time, people would register their filtering intent with a local IRR and the route server would simply construct the necessary view for them.  It seems like IXPs have gotten out of that habit.

As Robert notes elsewhere, RT-Constrain is something that might work fine if implementations support filtering vs. non-VPN famlies with it.  However, that's still somewhat problematic for the type of scenario you're discussing.

Rt-Constrain is intended to be a pull protocol for positive state.  Basically, "send me things that have X route-target extended community in it".  While it's possible that IXP process might map well to that semantic with some careful planning, much of the time the desire is for filtering out of stuff - the opposite semantic.  So, this becomes an awkward fit for Rt-Constrain.  Even for the previously discussed ORFs, this is awkward and that's part of the discussion about the RD-ORF proposal.

An example of something that would fit modestly well for Rt-Constrain is routes are tagged by the IXP with a route-target that has the AS of the IXP participant.  You then send in a Rt-Constrain route for each of the ASes you want to pull from the RS.  But as noted, this means applying Rt-Constrain to non-VPN families which not all implementations do.  (I keep intending to do the work in Juniper's implementation, but the round-tuit vs. fighting our process get in the way...)

-- Jeff

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