BGP is widely used as an IGP in the underlay of modern DC fabrics. This series argues this is not the best long-term solution to the problem of routing in fabrics because BGP is not ideal for this use case. This post will consider the potential harm we are doing to the larger Internet by pressing BGP into a role it was not originally designed to fulfill—an underlay protocol or an IGP.
My last post described the kinds of configuration required to make BGP work on a DC fabric—it turns out that the configuration of each BGP speaker on the fabric is close to unique. It is possible to automate configuring each speaker—but it would be better if we could get closer to autonomic operation.
To move BGP closer to autonomic operation in a DC fabric, there are several things we can do. First, we can allow a BGP speaker to peer with any other BGP speaker it receives an open message from—this is often called promiscuous mode. While each router in the fabric will still need to be configured with the right autonomous system, at least we won’t need to configure the correct peers on each router (including the remote AS).
Note, however, that using this kind of promiscuous peering does come with a set of tradeoffs (if you’re reading this blog, you know there will be tradeoffs). BGP speakers running in promiscuous mode open a large attack surface on the control plane of the network. We can close this attack surface by configuring authentication on all BGP speakers … but we are now adding complexity to reduce complexity. We could also reduce the scope of the attack surface by never permitting BGP to peer beyond a single hop, and then filtering all BGP packets at the fabric edge. Again, just a bit more complexity to manage—but remember that the road to highly fragile and complex systems is always paved with individual steps that never, on their own, seem to add “too much complexity.”
The second thing we can do to move BGP closer to autonomic operation is to advertise routes to every connected peer without any policy configured. This does, again, introduce some tradeoffs, particularly in the realm of security, but let’s leave that aside for the moment.
Assume we can create a version of BGP that has these modifications—it always accepts any peer from any other AS, and it advertises all routes without any policy configured. Put these features behind a single knob which also includes setting the MRAI to 0 or 1, tightens up the dampening parameters, and adjusts a few other things to make BGP work better in a DC fabric.
As an experiment, let’s enable this DC fabric knob on a BGP speaker at the edge of a dual-homed “enterprise customer.” What will happen?
The enterprise network will automatically peer to any speaker that sends an open message—a huge security hole on the open Internet—and it will advertise every route it learns even though there is no policy configured. This second issue—advertising routes with no policy configured—can cause the enterprise network to become a transit between two much larger provider networks, crashing out some small corner of the Internet.
This might seem like a trivial issue. After all, just don’t ever enable the DC fabric knob on an eBGP peering session upstream into the DFZ, or any other “real” internetwork. Sure, and just don’t ever hit the brakes when you mean to hit the accelerator, or the accelerator when you mean to hit the brakes. If I had a dime for every time we “just don’t ever make that mistake …” Well, I wouldn’t be blogging, I’d be relaxing in the sun someplace (okay, I’m not likely to ever stop working to sit around and “relax” all the time, but you get the picture anyway).
Maybe—just maybe—it would really be better overall to use two different protocols for IGP and EGP work. Maybe—just maybe—it’s better not to mix these two different kinds of functions in a single protocol. Not only is the single resulting protocol bound to be really complex (most BGP implementations are now over 100,000 lines of code, after all), but it will end up being really easy to make really bad mistakes.
No tool is omnicompetent. If you found a tool that was, in fact, omnicompetent, it would also be the most dangerous tool in your toolbox.