Reading academic articles about Internet-wide routing challenges you might stumble upon valley-free routing – a pretty important concept with applications in WAN and data center routing design.

If you’re interested in the academic discussions, you’ll find a pretty exhaustive list of papers on this topic in the Informative References section of RFC 7908; here’s the over-simplified version.

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!

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:

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.