Request for comment -- BCP38

John Levine johnl at
Mon Sep 26 16:04:33 UTC 2016

>If you have links from both ISP A and ISP B and decide to send traffic out 
>ISP A's link sourced from addresses ISP B allocated to you, ISP A *should* 
>drop that traffic on the floor.  There is no automated or scalable way for 
>ISP A to distinguish this "legitimate" use from spoofing; unless you 
>consider it scalable for ISP A to maintain thousands if not more 
>"exception" ACLs to uRPF and BCP38 egress filters to cover all of the cases 
>of customers X, Y, and Z sourcing traffic into ISP A's network using IPs 
>allocated to them by other ISPs?

I gather the usual customer response to this is "if you don't want our
$50K/mo, I'm sure we can find another ISP who does."

>From the conversations I've had with ISPs, the inability to manage
legitimate traffic from dual homed customer networks is the most
significant bar to widespread BCP38.  I realize there's no way to do
it automatically now, but it doesn't seem like total rocket science to
come up with some way for providers to pass down a signed object to
the customer routers that the routers can then pass back up to the
customer's other providers.


PS: "Illegitimate" is not a synonym for inconvenient, or hard to handle.

More information about the NANOG mailing list