Category: BGP

Once you decide to use BGP as the routing protocol in your DMVPN network, you face a few more design choices:

• Should you use IBGP or EBGP?
• Should you use a unique AS number for every DMVPN site, or the same AS number on all spoke sites?

The BGP Routing in DMVPN Access Networks ExpertExpress case study describes these dilemmas in more details.

Even though I questioned the wisdom of writing your own network programming applications, I know I would immediately jump into those waters if I were 20 years younger. If you’re like my younger self, you might want to keep a few guidelines in mind.

You did read my blog post on ThousandEyes, didn’t you? What I forgot to mention was that they have this cool API that allows you to extract measurement data (including BGP topology) from their system. Can we do something cool with that?

A group of researches presented an “interesting” result @ IETF 87: migrating from IBGP full mesh to IBGP reflectors can introduce temporary forwarding loops. OMG, really?

Don’t panic, the world is not about to become a Vogon hyperspace bypass. Let’s put their results in perspective.

One of the holy grails of data center SDN evangelists is controller-driven traffic engineering (throwing more leaf-and-spine bandwidth at the problem might be cheaper, but definitely not sexier). Obviously they don’t call it traffic engineering as they don’t want to scare their audience with MPLS TE nightmares, but the idea is the same.

Interestingly, you don’t need new technologies to get as close to that holy grail as you wish; Petr Lapukhov got there with a 20 year old technology – BGP.

TL&DR Summary: Yes (if you’re clumsy enough).

A while ago I read Impact of Graceful IGP Operations on BGP – an article that described how changes in IGP topology result in temporary (or sometimes even permanent) forwarding loops in networks using BGP route reflectors.

Is the problem real? Yes, it is. Could you generate a BGP RR topology that results in a permanent forwarding loop? Yes. It’s not that hard.

One of the Expert Express sessions focused on an MPLS/VPN-based WAN network using OSPF as the routing protocol. The customer wanted to add DMVPN-based backup links and planned to retain OSPF as the routing protocol. Not surprisingly, the initial design had all sorts of unexpectedly complex kludges (see the case study for more details).

Having a really smart engineer on the other end of the WebEx call, I had to ask a single question: “Why don’t you use BGP everywhere” and after a short pause got back the expected reply “wow… now it all makes sense.”

VMware gave me early access to NSX hands-on lab a few days prior to VMworld 2013. The lab was meant to demonstrate the basics of NSX, from VXLAN encapsulation to cross-subnet flooding, but I quickly veered off the beaten path and started playing with routing protocols in NSX Edge appliances.

Have you ever tried to implement a large-scale DMVPN or MPLS/VPN network using BGP as the routing protocol? If you tried to stitch more than ~1000 sites together you’re well aware of all the pain caused by a small range of private AS numbers defined in RFC 1930. We can kludge our way around the limitation by reusing the same AS number on multiple sites (and using allowas-in when we need full routing information on every site), but such a design clearly sucks.

The next small step in my MPLS/VPN update project: EIBGP load balancing – why is it useful, how it works, why can’t you use it without full-blown MPLS/VPN, and what the alternatives are.