Heads-up: RFC 8212 on default EBGP route handling behavior

Job Snijders job at ntt.net
Thu Jul 6 12:34:45 CST 2017


Dear NANOG,

After a bit of tug-of-war common sense prevailed and RFC 8212
"External BGP (EBGP) Route Propagation Behavior without Policies"
was published: https://tools.ietf.org/html/rfc8212

This industry has a long history of improving default behavior: DEC MOP
is no longer enabled by default, telnet was swapped out in favor of SSH,
and SHA-1 is now deprecated, so I'm confident we can manage this one
too. 

TL;DR This mail offers advice on test scenarios to add to your
evaluation checklist and a call to action to ask your vendor to
implement RFC 8212. Please share this message with other communities.

Background
----------

Prior to RFC 8212, the default behavior of BGP implementations (when no
policy is configured on an EBGP session) was undefined, this resulted in
a myriad of vendor defaults: some implementations not accepting routes
and not advertising anything; some would accept anything, but announce
nothing; some would announce only internal routes and accept anything;
and some would indiscriminately accept everything and announce everything.
The latter mode of operation is of course the most harmful one.

An example minimal configuration:

    !
    router bgp 15562
      neighbor 192.0.2.1 remote-as 174
      neighbor 192.0.2.5 remote-as 2914
    !

Most of us have learned (the hard way) that on many platforms the above
configuration will not only bring up two BGP sessions, but also
immediately result in a "Lateral ISP-ISP-ISP Leak" [2], simply because
no routing policy was associated with these EBGP sessions and an
implicit 'permit-any' is assumed. The above configuration is of course
an oversimplification of what happens in real life: operators may
attempt to change a BGP peer to a peer-group which has missing
configuration, or a copy+paste of a snippet of configuration only
partially succeeds, or in an attempt to debug a BGP session some
configuration is removed without realizing the full ramifications of
doing so (until you see smoke coming off the circuit ;-).

RFC 8212 updates the core BGP specification (RFC 4271) to specify that
the above behavior is _incorrect_, and an implicit deny-all must be
associated with the EBGP session. In other words: "fail closed" rather
then "fail open and oops you tripped max-prefix all over the place (or
worse [3])". 

The document purposefully does not cover IBGP, nor does it proscribe
what the contents of configured policy on EBGP sessions should be. If an
operator explicitly configures a 'permit-any' style policy, that is
perfectly fine, it was a conscious choice to do so.

Evaluation checklist
--------------------

Going forward when you evaluate a BGP implementation or a new software
release, it is advisable to take note of the default behavior of that
specific release. As vendors come into compliance with RFC 8212 it may
be beneficial to track this. I also strongly recommend to audit your
network configurations for instances which depend on implicit 'permit-any'
behavior and reconfigure those instances to be an explicit 'permit-any'.
This way software upgrades are less likely to cause surprises, and as a
bonus the readability of the device's configuration is improved!

Call to action
--------------

Some vendors will need encouragement to take their implementation from
EBGP "fail open" to "fail closed", we are keeping track of the
industry's current state of affairs here: https://github.com/bgp/RFC8212

Please contact your account management team and express your interest in
them supporting RFC 8212. Also, make sure to include RFC 8212 in your
next round of "Request for Proposals" (RFPs) as a 'must have'.
Purchasing usually is an excellent opportunity to have meaningful
dialogue with the vendor.

Some vendors may argue "but our customers depend on our unsafe
behavior!", but this only holds true if we don't speak up collectively
and show them otherwise. EBGP is an internet-wide shared resource, we
all benefit from sane defaults.

Hat tip to Jared Mauch for initiating this project and to Greg Hankins
for demonstrating change is possible [4].

Kind regards,

Job

[1]: https://mailarchive.ietf.org/arch/msg/idr/mqPltvvgEhpxBgAXET1y1Xow6t8
[2]: https://tools.ietf.org/html/rfc7908#section-3.2
[3]: https://bgpmon.net/massive-route-leak-cause-internet-slowdown/
[4]: https://mailarchive.ietf.org/arch/msg/idr/kgl6etbjUuR3jLHVeDSi4LLIs50


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