The “discovery of the week” award goes to Terry Slattery for pointing out the dangers of bufferbloat while investigating TCP retransmissions (part 1 and part 2). BTW, in the end, he figured out it was just an overloaded Gigabit Ethernet linecard.

Two other interesting discoveries: PA /48 IPv6 prefixes are still filtered and BGP is more stable than we thought it would be.

Did you ever want to have a high-level overview of how 3G/4G mobile networks work? Where GGSN and SGSN fit in? What the PDP contexts are ... and why you need two for dual-stack connectivity? All that (and a lot more) is explained in very well written IETF draft IPv6 in 3GPP Evolved Packet System. Reading highly recommended.

An engineer attending my VMware Networking Deep Dive webinar has asked me a tough question that I was unable to answer:

What happens if a VM running within a vSphere host sends a BPDU? Will it get dropped by the vSwitch or will it be sent to the physical switch (potentially triggering BPDU guard)?

I got the answer from visibly harassed Kurt (@networkjanitor) Bales during the Networking Tech Field Day; one of his customers has managed to do just that.

Update 2011-11-04: The post was rewritten based on extensive feedback from Cisco, VMware and numerous readers.

Finally someone decided to make IPv6 flow label useful. First they had to justify why they want to change it, and then modify the definition (way too much work for a field nobody ever used). Planned use is to enhance ECMP load balancing, both in native IPv6 environments (where using the flow label is faster than digging deep into variable-length IPv6 extension headers) and (even more importantly) in tunneled environments, where the flow label propagates the entropy from the tunnel payload into the envelope header.

I hope you never believed the “OpenFlow networking nirvana” hype in which smart open-source programmable controllers control dumb low-cost switches, busting the “networking = mainframes” model and bringing the Linux-like golden age to every network. As the debates during the OpenFlow symposium clearly illustrated, the OpenFlow reality is way more complex than it appears at a first glance.

To make it even more interesting, at least four different models for OpenFlow deployment have already emerged:

A few weeks ago I delivered a short L2 DCI WebEx presentation to CCIE Club Poland. I took the L2 part of my Data Center Interconnect webinar and added 15 minutes of L2 DCI mythbusting.

That part of my presentation is on YouTube; for the rest, watch my Data Center Interconnect webinar.

Michael sent me an interesting question:

I work in a rather large enterprise facing a campus network redesign. I am in favor of using a routed access for floor LANs, and make Ethernet segments rather small (L3 switching on access devices). My colleagues seem to like L2 switching to VSS (distribution layer for the floor LANs). OSPF is in use currently in the backbone as the sole routing protocol. So basically I need some additional pros and cons for VSS vs Routed Access. :-)

The follow-up questions confirmed he has L3-capable switches in the access layer connected with redundant links to a pair of Cat6500s:

The last few days were exquisite fun: it was great meeting so many people focusing on a single technology (OpenFlow) and concept (Software-Defined Networking, whatever that means) that just might overcome some of the old obstacles (and introduce new ones). You should be at least a bit curious what this is all about, and even if you don’t see yourself ever using OpenFlow or any other incarnation of SDN in your network, it never hurts to enhance your resume with another technology (as long as it’s relevant; don’t put CICS programmer at the top of it).

We finished a fantastic Network Field Day (second edition) yesterday. While it will take me a while (and 20+ blog posts) to recover from the information blast I received during the last two days, here are the first impressions:

Explosion of innovation – and it’s not just OpenFlow and/or SDN. Last year we’ve seen some great products and a few good ideas (earning me the “grumpy old man that’s hard to make smile” fame), this year almost every vendor had something that excited me.

A while ago I got a set of MPLS/VPN-related questions from one of my long-time readers furiously working on a response to a large RFP. I answered the questions and (more as an afterthought) mentioned the ExpertExpress service I had been starting to consider. His response amazed me:

ExpertExpress is definitely a very very good idea!!! You know what? I think I will push the company to try to use it to get your advice on the current engagement. The company needs this "yesterday" so I would be able to verify my design and will feel safer with it and will deliver it on time and of course you will receive a fair payment for this.

Next question – when could we do it? Response: how about tomorrow? Sure, no problem (note: it doesn’t always work out that way).

Like every other blogger, I get occasional e-mails from people fishing for free consulting or second opinion (note: asking a serious technical question is a totally different story; as many people know, I always try to reply and help) and as I’m totally overloaded with OpenFlow symposium and Net Field Day these days, I decided to share one of the better ones.

Initial release of QFabric Junos can run STP only within the network node (see QFabric Control Plane post for more details), triggering an obvious question: “what happens if a server multihomed to a server node starts bridging between its ports and starts sending BPDUs?”. Some fabric solutions try to ignore STP (the diplomats would say “they are transparent to STP”) but fortunately Juniper decided to do the right thing.

While everyone deeply involved with OpenFlow agrees it’s just a low-level tool that can’t solve problems we couldn’t solve in the past (just like replacing Tcl with C++ won’t help you prove P = NP), occasionally you stumble across mindboggling ideas that are so simple you have to ask yourself: “were we really that stupid?” One of them that obviously impressed James Hamilton is the solution to load balancing that requires no load balancers.

Before clicking Read more, watch this video and try to figure out what the solution is and why we’re not using it in large-scale networks.

After more than a year, I’m back in California, anxiously waiting to meet my fellow bloggers and ask some tough questions to a fantastic lineup of vendors presenting at Net Field Day 2011. Stephen Foskett’s well-oiled organizing machinery is already in full gear; I’m typing this post from a WiFi-equiped car that picked me up @ SFO airport (you see, dear vendors, it’s so easy to make my inner geek happy ... all I need are some fantastic features that are actually usable and work as well as this WiFi connection).

InformationWeek has recently published an OpenFlow article by Jeff Doyle in which they graced me with a single grumpy quote taken out of three pages of excellent questions that Jeff asked me when preparing for the article. Jeff has agreed that I publish the original questions and my answers to them. Here they are (totally unedited):