SDN Internet Router (sir)

Mel Beckman mel at
Thu Jan 5 20:54:27 UTC 2023


I’m not sure I understand what you mean by “suboptimal“ routing. Even though the Internet uses AS path length for routing,  many of those path lengths are bogus, and don’t really represent any kind of path performance value. For example, a single AS might hide many hops in an MPLS network as a single hop, obscuring asymmetric routing and other uglies. Prepending also occurs when destinations are trying to enforce their own engineering  policies, which often conflict with yours or mine.

So what do you mean by “suboptimal“? Are you thinking that the “best” path in BGP tables actually meant you were getting a performance benefit? Because that’s definitely not the case in today’s Internet. Were were you thinking that you would be going along less congested paths? That’s really at the mercy of the traffic engineering of backbone providers over which we have no control.

I generally populate local router FIBs to merel choose an exit point for purposes of load balancing, and nothing more.


On Jan 5, 2023, at 12:38 PM, Mike Hammett <nanog at> wrote:

I guess I wasn't around for those days.

As far as running out, again, assuming the tooling works correctly, I'd think to target fewer routes than you could hold. Maybe 1k routes is all one would need to get a significant percent of the traffic. A lot of room to mess up if you can hold 100k, 500k routes.

Mike Hammett
Intelligent Computing Solutions<>
Midwest Internet Exchange<>
The Brothers WISP<>
From: "Joe Maimon" <jmaimon at>
To: "Mike Hammett" <nanog at>, "Christopher Morrow" <morrowc.lists at>
Cc: "NANOG" <nanog at>
Sent: Thursday, January 5, 2023 2:30:40 PM
Subject: Re: SDN Internet Router (sir)

Mike Hammett wrote:
> I'm not concerned with which technology or buzzword gets the job done,
> only that the job is done.
> Looking briefly at the couple of things out there, they're evaluating
> the top X prefixes in terms of traffic reported by s-flow, where X is
> the number I define, and those get pushed into the FIB. One
> recalculates every hour, one does so more quickly. How much is
> appropriate? I'm not sure. I can't imagine it would *NEED* to be done
> all of that often, given the traffic/prefix density an eyeball network
> will have. Default routes carry the rest. Default routes could be
> handled outside of this process, such that if this process fails, you
> just get some sub-optimal routing until repaired. Maybe it doesn't
> filter properly and sends a bunch of routes. Then just have a prefix
> limit set on the box. Maybe it sends the wrong prefixes. No harm, no
> foul. If you're routing sub-optimally internally, when it does hit a
> real router with a full FIB, it gets handled appropriately.

Unless it loops.

The rest sounds nice. But flow caching got a bad rap back in the early
worm days. But thats because the situation was a little worse back then.
Cache the wrong routes or run out of cache, router dies. So long as
thats not the case automating optimization is an extremely valuable goal.

> I would just be looking for solutions that influence what's in the FIB
> and let the rest of the router work as the rest of the router would.

The problem comes when the router wont work at all without the FIB
routes, like in the olden days.
> -----
> Mike Hammett
> Intelligent Computing Solutions <>
> <><><><>
> Midwest Internet Exchange <>
> <><><>
> The Brothers WISP <>
> <><>
> ------------------------------------------------------------------------
> *From: *"Christopher Morrow" <morrowc.lists at>
> *To: *"Mike Hammett" <nanog at>
> *Cc: *"Tom Beecher" <beecher at>, "NANOG" <nanog at>
> *Sent: *Thursday, January 5, 2023 12:27:08 PM
> *Subject: *Re: SDN Internet Router (sir)
> On Thu, Jan 5, 2023 at 11:18 AM Mike Hammett <nanog at
> <mailto:nanog at>> wrote:
>     Initially, my thought was to use community filtering to push just
>     IXes, customers, and defaults throughout the network, but that's
>     obviously still sub-optimal.
>     I'd be surprised if a last mile network had a ton of traffic going
>     to any more than a few hundred prefixes.
> I think in a low-fib box at the edge of your network your choices are:
>   "the easy choice, get default, follow that"
>   "send some limited set of prefixes to the device, and default, so
> you MAY choose better for the initial hop away"
> you certainly can do the second with communities, or route-filters
> (prefix-list) on the senders, or....
> you can choose what prefixes make the cut (get the community(ies))
> based on traffic volumes or expected destination locality:
>    "do not go east to go west!"
> these things will introduce toil and SOME suboptimal routing in some
> instances... perhaps it's better than per flow choosing left/right
> though and the support calls related to that choice.
> In your NOLA / DFW / ATL example it's totally possible that the
> networks in question do something like:
>   "low fib box in tier-2 city (NOLA), dfz capable/core devices in
> tier-1 city (DFW/ATL), and send default from left/right to NOLA"
> Could they send more prefixes than default? sure... do they want to
> deal with the toil that induces? (probably not says your example).
> SDN isn't really an answer to this, though.. I don't think. Unless you
> envision that to lower the toil ?

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the NANOG mailing list