[afnog] please announce your prefixes at IXPs

[Frank Habicht]

Hello all,

I have come here to beg.

All of you, if you're connected to your local IXP,
if you announce your beautifully aggregated prefix to the IXP,
if you need to "balance" the amount of traffic on your internet transit
("upstream") links,
if for that you're breaking up your prefixes [1] into smaller subnets,
and announcing the smaller subnets to upstream providers,
 (nothing broken until now)
and then your provider joins the same IXP,
where you're also getting a lot of traffic from a peer,
then your upstream would announce your prefix at the IXP (same city as the
connection of the customer link to you),
and all the big traffic from your peer would suddenly prefer the more
specific prefix you're advertising to your upstream.
And come to you over a link you're paying $$$ for,
and that's maybe not even big enough for all that traffic suddenly rushing in.

So:
aggregate your prefixes.
if you need to deaggregate (split up), then take a moment to think of all
your other options first. (maybe deaggregate with NO-Export community ?)

If finally you announce deaggregated prefixes to your upstreams and/or the
internet in general,
then _please_please_please_ also announce the same ones at your IXP.
Only then you get the IXP connection to do what you want it to do.


The deciding factor here is that a more specific prefix always wins.


I want to give a (slightly anonymised) example.

You are ISP $I
you have an AfriNIC allocation x.xx.240.0/20
You're advertising your x.xx.240.0/22 at your IXP
You're advertising x.xx.240.0/23 to your upstream.
All working. From peer $P you're getting a lot of traffic at the IXP. Good!
Your upstream joins the IXP.
Advertises x.xx.240.0/23 at the IXP - and you don't advertise the same
prefix there.
Lots of traffic that you once got directly and for free at the IXP will go
to your upstream. And from your upstream over the link that you pay a lot
of money for to you. Maybe the link is even too small.

Please advertise all of your prefixes you advertise to any outsiders also
at the IXP.
You want your traffic to come in over the cheap link, right?
If you advertise the same prefix at the IXP, then (all other things being
equal) routers should decide that the AS_PATH via your peer is longer and
prefer your direct connection - Done.


That's a real-world problem.
I'm the "upstream". Joining the IXP. Going to cause pain when I start
announcing the customers' prefixes at the IXP. Ready to do that any minute.


Thanks,
Frank

[1] always first think of other ways to achieve that goal,
like different allocation to different uplinks, and as backup announcing
the other with a community to not prefer it... ?

[2] can provide the real numbers to replace "x.xx" - to those concerned.
    will work with them to explain more, or to fix