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3 & 5 Years Ago (June 2013)

Top hits of June 2008: explicit IPv4 address family in BGP configuration and static DHCP assignment for clients without client-id.

Two evergreens from June 2010 that still generate loads of traffic: Manipulating EIGRP metrics and GETVPN in 20 seconds.

And here are the other popular posts published a bit more than three and five years ago.

Update: Migrating my blog to

Update 2013-07-30 13:30 GMT - domain switched over, everything should be back to normal within an hour.

In the next 24 hours (to make sure all high-TTL DNS records expire) I'll migrate to The old domain will remain active with permanent redirects to the new domain so you shouldn't have to change anything on your end. However, if you experience any problems access the blog or the main web site ( please let me know. Thank you!

More private AS numbers

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.

Cloud-as-an-Appliance Design

The original idea behind cloud-as-an-appliance design came from Brad Hedlund’s blog post in which he described how he’d build a greenfield Hadoop or private cloud cluster with servers connected to a Clos fabric. Throw virtual appliances into the mix and you get an extremely simple and versatile architecture:

Live VM Mobility Network Requirements

Every time someone mentions how awesome new technologies solve live VM mobility across WAN networks, I start muttering unmentionables. Live VM mobility across disjoint layer-2 subnets works great in demos, but usually fails in real life due to stale ARP caches. The only way to solve this problem for good is to implement EC2-like layer-3 forwarding in hypervisor soft switches.

Update: LISP Host Mobility seems to be a potential exception; see the comment from Nico.

For more details, watch the VM Mobility Requirements video (part of Enterasys-sponsored DCI webinar), read the Hot and Cold VM Mobility blog post or watch the recording of NFD4 session with Cisco’s Victor Moreno.

Unreadable IPv6 Addresses Might Be Good For Us in the Long Run

One of the first arguments used by networking engineers living in IPv6 denial and trying to justify their stance is “IPv6 addresses are unreadable. We will never migrate to IPv6; it’s much easier to deal with IPv4 addresses.”

That’s absolutely true. If you use RFC 1918 addresses in a small(ish) network, the first two octets don’t change, and it’s easy to remember the remaining two numbers … but the unreadable IPv6 addresses just might change the way we approach network configuration and monitoring.

3 & 5 Years Ago (May 2013)

In May 2008 I wrote about asymmetric routing, OSPF across a PIX/ASA firewall and control plane protection.

The most popular article from May 2010 is still Tunneling VPNs and Zone-Based Firewalls with NAT64 and DNS64 in 30 minutes being close second. I also wrote about iSCSI and Moore’s Law and IPv6 myths.

Where’s the Revolutionary Networking Innovation?

In his recent blog post Joe Onisick wrote “What network virtualization doesn’t provide, in any form, is a change to the model we use to deploy networks and support applications. [...] All of the same broken or misused methodologies are carried forward. [...] Faithful replication of today’s networking challenges as virtual machines with encapsulation tunnels doesn’t move the bar for deploying applications.

Unicast-Only VXLAN Finally Shipping

The long-promised unicast-only VXLAN has finally shipped with the Nexus 1000V release 4.2(1)SV2(2.1) (there must be some logic behind those numbers, but they all look like madness to me). The new Nexus 1000V release brings two significant VXLAN enhancements: unicast-only mode and MAC distribution mode.

Downloadable Recording of Enterasys Data Center Interconnect Solutions Webinar

The recording of recent Enterasys Robust Data Center Solutions is available on demo web site.

You can watch (or download) the following videos:

First-Hop IPv6 Security Features in Cisco IOS

I wanted to figure out how to use IPv6 DAD proxy in PVLAN environments during my seaside vacations, and as I had no regular Internet access decided to download the whole set of IPv6 configuration guides while enjoying the morning cup of coffee in an Internet café. Opening the IPv6 First-Hop Security Configuration Guide was one of the most pleasant (professional) surprises I had recently.

One word summary: Awesome.

3 & 5 Years Ago (April 2013)

Not surprisingly, the top posts from April 2008 include BGP essentials: Non-transit AS, What is CLNS? and The impact of tx-ring-limit.

April 2010 was a bit more introspective – I wrote about how networking is like physics or math, not history and small steps to large complexity.

Doodling on a Napkin

Two more books you must read during your summer vacation: The Back of the Napkin and Unfolding the Napkin ... and here are a few drawing tools that work for me.

If you’re anything like me, your first results will be disastrous. Keep practicing; I’m able to draw understandable (if not pretty) diagrams by now (and I’m still trying to stay away from Pictionary).

Smart Fabrics Versus Overlay Virtual Networks

With the recent plethora of overlay networking startups and Cisco Live Dynamic Fabric Architecture announcements it’s time to revisit a blog post I wrote a bit more than a year ago, comparing virtual networks and voice technologies.

They say a picture is worth a thousand words – here are a few slides from my Interop 2013 Overlay Virtual Networking Explained presentation.

ProgrammableFlow Typical Use Cases

The last part of the ProgrammableFlow webinar described typical use cases including Cloud-as-an-Appliance, traffic steering (used by appliances like Radware DefenseFlow) and hypervisor switching with PF1000. Predictably, the use cases were followed by a lengthy Q&A session.

The Tools That I Use (Drawings)

Continuing from the previous Tools That I Use post, here’s what I’m using to generate the hand drawings in blog posts and case studies.

3 & 5 Years Ago (March 2013)

Summer seems to be the perfect time to revisit old posts. Here are a few from 2008 and 2010.

The top hit of March 2008 was the UDP flood in Perl closely followed by the mysteries of the “Internet” BGP community. I was also writing about tracking the DHCP default route and detecting router restarts.

March 2010 was way more interesting: I was writing about MPLS TE autoroute basics, the difference between CLNS and CLNP, QPPB in MPLS VPN networks and IPv6 myths.

Cloudbursting, the Wally Way

Priceless! (source:

iSCSI with PFC?

Nicolas VermandĂ© sent me a really interesting question: “I've been looking for answers to a simple question that even different people at Cisco don't seem to agree on: Is it a good idea to class IP traffic (iSCSI or NFS over TCP) in pause no-drop class? What is the impact of having both pauses and TCP sliding windows at the same time?

IPv6 Address Assignment and Tracking

One of the significant challenges of IPv6 is definitely the host address assignment and tracking (for logging/auditing reasons), more so if you use SLAAC or (even worse) SLAAC privacy extensions. Not surprisingly, Eric Vyncke and myself spent a significant amount of time addressing this topic in the IPv6 Security webinar.

Fallacies of GUI

I love Greg Ferro’s characterization of CLI:

We need to realise that the CLI is a “power tools” for specialist tradespeople and not a “knife and fork” for everyday use.

However, you do know that most devices’ GUI offers nothing more than what CLI does, don’t you? Where’s the catch?

Summer seems to have arrived

The current weather around Central Europe doesn’t exactly support this conclusion, but I do get many more “I’m on vacation” responses than usual, so it’s time to reduce the blogging frequency to keep your RSS reader from overloading (you did switch from Google Reader to something like Feedly, didn’t you?).

However, if you’re looking for some really heavy reading, do pick up The Hidden Reality and explore various multiverse proposals. There’s also a beach-friendly version of multiverse discussion: The Long Earth by the one-and-only Terry Pratchett.