Category: Design

Inspired by The Zen of Python, Dinesh Dutt wrote The Zen of Routing Protocols:

Beautiful is better than ugly.

Simple is better than complex.

Complex is better than complicated.

So just because you can, don't.

In the market overview section of the introductory part of data center fabric architectures webinar I made a recommendation to use larger number of fixed-configuration spine switches instead of two chassis-based spines when building a medium-sized leaf-and-spine fabric, and explained the reasoning behind it (increased availability, reduced impact of spine failure).

One of the attendees wondered about the “right” number of spine switches – does it has to be four, or could you have three or five spines. In his words:

After explaining the basics of SD-WAN, Pradosh Mohapatra, the author of SD-WAN Overview webinar focused on SDWAN reference network design.

Yves Haemmerli, Consulting IT Architect at IBM Switzerland, sent me a thoughtful response to my we need product documentation rant. Hope you’ll enjoy it as much as I did.

Yes, whatever the project is, the real added value of an IT/network architect consultant is definitely his/her ability to create models (sometimes meta-models) of what exists, and what the customer is really looking for.

The next set of Leaf-and-Spine Fabric Myths listed by Evil CCIE focused on BGP:

BGP is the best choice for leaf-and-spine fabrics.

I wrote about this particular one here. If you’re not a BGP guru don’t overcomplicate your network. OSPF, IS-IS, and EIGRP are good enough for most environments. Also, don’t ever turn BGP into RIP with AS-path length serving as hop count.

A while ago I had the dubious “privilege” of observing how my “beloved” airline Adria Airways deals with exceptions. A third-party incoming flight was 2.5 hours late and in their infinite wisdom (most probably to avoid financial impact) they decided to delay a half-dozen outgoing flights for 20-30 minutes while waiting for the transfer passengers.

Not surprisingly, when that weird thingy landed and they started boarding the outgoing flights (now all at the same time), the result was a total mess with busses blocking each other (this same airline loves to avoid jet bridges).

As I explained in a previous blog post, most leaf-and-spine best-practices (as in: what to do if you have no clue) use BGP as the IGP routing protocol (regardless of whether it’s needed) with the same AS number shared across all spine switches to implement valley-free routing.

This design has an interesting consequence: when a link between a leaf and a spine switch fails, they can no longer communicate.

You might have noticed that almost every BGP as Data Center IGP design uses the same AS number on all spine switches (there are exceptions coming from people who use BGP as RIP with AS-path length serving as hop count… but let’s not go there).

There are two reasons for that design choice:

Here’s a question I got from an attendee of my Building Next-Generation Data Center online course:

As far as I understood […] it is obsolete nowadays to build a new DC fabric with routing on the host using BGP, the proper way to go is to use IGP + SDN overlay. Is my understanding correct?

Ignoring for the moment the fact that nothing is ever obsolete in IT, the right answer is it depends… this time on answer(s) to two seemingly simple questions “what services are we offering?” and “what connectivity problem are we trying to solve?”.

Most blog posts generate the usual noise from the anonymous peanut gallery (if only they’d have at least a sliver of Statler and Waldorf in them), but every now and then there’s a comment that’s pure gold. The one made by Tony Przygienda (of RIFT fame) on Valley-Free Routing post is so good and relevant that I decided to republish it as a separate blog post. Enjoy!