I'll get off my soap-box now and let you resume your observations that
complexity as a goal in and of itself is the olny path forward. What
a dismal world-view.
No-one is arguing that complexity is a goal. Opportunities to
introduce gratuitous complexity abound, and defending against them
while recognizing that the opportunity that represents genuine
progress (trading outhouses for indoor plumbing, for example) is quite
a challenge.
I'm all for using the cleanest, simplest, and most reliable means to
meet requirements. Not all IXPs have the same requirements driving
their business, though - an IXP that operates a distributed metro-area
fabric has additional concerns for reliability and cost-efficient use
of resources than an IXP that operates a single switch. If
requirements were such that I needed to buy and *use* a partial mesh
topology for a distributed IXP fabric in the most reliable fashion
possible, I'd much rather go the route described earlier than try to
cobble something together with PVST/MST L2 technologies, but that's
just me.
You can assert that the status quo gives you solid predictable
performance, but the reality is that you occasionally get sucked into
a vortex of operational issues arising from L2's failure modes. To
continue with my bad plumbing analogy, open sewers were a reliable
means of moving waste material, easy to see when they were failing,
but occasionally produced outbreaks of disease. Are open sewers still
in use in the world today? You bet.
The underlying hardware layer that IXPs use is capable of more than
IXPs use. Whether to turn on those features is driven by requirements,
from customers and from the economics of the business. I would argue,
though, that at today's level of robustness and penetration of the
technologies that we've been discussing, the customer "requirement" to
peer on a shared VLAN is much more about complacency than avoiding
risk (as you seem to be arguing).
When we were turning PAIX from a private interconnect location into a
public IXP, we questioned every assumption about what role IXPs played
in order to ensure that we weren't making decisions simply to preserve
the status quo. One of the things we questioned was whether to offer a
peering fabric at all, or simply rely on PNIs. Obviously we opted to
have a peering fabric, and I don't regret the decision despite the
many long nights dealing with migration from FDDI to Ethernet (and the
fun of translational bridge MTU-related issues during the migration),
and the failure modes of Ethernet L2 - so your assertion that Ethernet
L2 provides solid predictable performance needs to be modified with
"mostly". I'll counter with an assertion that some L2.5/L3 networks
are built and operated to more 9s than some IXP L2 networks that span
multiple chassis. Whether that additional reliability makes business
sense to offer, though, is a different question.
If lack of complexity was a *requirement* that trumped all others,
there would still be a DELNI at PAIX.