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Blog Posts in July 2010

TCP/IP is like a mainframe ... you can’t change a thing

Almost 30 years ago, I was lucky enough to work on one of the best systems of those days, VAX/VMS (BTW, it was able to run 30 interactive users in 2 MB of main memory), which had everything we’d wished for – it was truly interactive with hierarchical file system and file versioning (not to mention remote file access and distributed clusters). I couldn’t possible understand the woes of IBM mainframe programmers who had to deal with virtualized 132-column printers and 80-column card readers (ironically running in virtual machines that the rest of the world got some 20 years later). When I wanted to compile my program, I started the compiler; when they wanted to do the same, they had to edit a batch job, submit the batch job (assuming the disk libraries were already created), poll the queues to see when it completed and then open the editor to view the 132-column printout of compiler errors.

After a long discussion, I started to understand the problem: the whole system was burdened with so many legacy decisions that still had to be supported that there was nothing one could do to radically change it (yeah, it’s hard to explain that to a 20-year old kid full of himself).

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Data Center 3.0 webinar sold out

The very last tickets for the Data Center 3.0 webinar on August 25th were sold yesterday (I can’t tell you how delighted I am). I’ve pushed aside a few other commitments and scheduled another session on September 8th; if you register for that session and need some advance information, just let me know – I’ll send you the PDF around August 20th and a link to the recording of the first session as soon as it’s available.

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Use BitTorrent to update software in your Data Center

Stretch (@packetlife) shared an interesting link in a comment to my P2P traffic is bad for the network post: Facebook and Twitter use BitTorrent to distribute software updates across hundreds (or thousands) of servers ... another proof that no technology is good or bad by itself (Greg Ferro might have a different opinion about FCoE).

Shortly after I’ve tweeted about @packetlife’s link, @sevanjaniyan replied with an even better link to a presentation by Larry Gadea (infrastructure engineer @ Twitter) in which Larry describes Murder, Twitter’s implementation of software distribution on top of BitTornado library.

If you have a data center running large number of servers that have to be updated simultaneously, you should definitely watch the whole presentation; here’s a short spoiler for everyone else:

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P2P traffic and the Internet, part 2

As expected, my P2P traffic is bad for the network post generated lots of comments; from earning me another wonderful title (shill for Internet monopolies) that I’ll proudly add to my previous awards to numerous technical comments and even a link to a very creative use of BitTorrent to solve software distribution problems (thanks again, @packetlife).

Most of the commentators missed the main point of my post and somehow assumed that since I don’t wholeheartedly embrace P2P traffic I want to ban it from the Internet. Far from it, what I was trying to get across was a very simple message:

  • current QoS mechanisms allow P2P clients to get disproportionate amount of bandwidth;
  • per-session queuing needs to be replaced with per-user queuing;
  • few devices (usually dedicated boxes) can do per-user bandwidth management.

Not surprisingly, Petr Lapukhov was even more succinct: “The root cause of P2P QoS problem is the flow-fairness and congestion-adaption model that has been in the Internet since the very first days” and thus made a great introduction to a more fundamental problem: while we’re rambling about the “popular” P2P topic, we’re forgetting that Internet was never designed to cope with what we’re throwing at it.

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P2P traffic is bad for the network

I’m positive you all know that. I also hope that you’re making sure it’s not hogging your enterprise network. Service Providers are not so fortunate – some Internet users claim using unlimited amounts of P2P traffic is their birthright. I don’t really care what kind of content these users transfer, they are consuming enormous amounts of network resources due to a combination of P2P client behavior (which is clearly “optimized” to grab as much as possible) and the default TCP/QoS interaction.

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NAT444, DS-Lite, A+P and NAT64 explained

One of the biggest hurdles Internet Service Providers will face in the near future is access to legacy IPv4 content once we run out of globally routable IPv4 addresses. Although it’s easy to offer your content over IPv6 (assuming you have a properly designed network using load balancers from a company that understands the need for IPv6 in Data Center), a lot of the “long tail” content will remain reachable only over IPv4.

A while ago I’ve published a presentation I’d delivered at the Slovenian IPv6 summit; a few days ago has published my article describing various transition solutions in more details. In the first part, “IPv4 address exhaustion: Making the IPv6 transition work”, I’m describing the grim facts we’re facing and the NAT-PT fiasco. In the second part, “Comparing IPv6 to IPv4 address translation solutions”, you’ll find brief descriptions of LSN (also known as CGN – Carrier-Grade NAT), NAT444, DS-Lite, A+P and NAT64.

And don’t forget: if you’re looking for in-depth IPv6 information, consider registering for the Building IPv6 Service Provider Core webinar; we can also organize an Enterprise IPv6 Deployment workshop for you.

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Bridging and Routing, Part II

Based on the readers’ comments on my “Bridging and Routing: is there a difference? post (thanks you!), here are a few more differences between bridging and routing:

Cost. Layer-2 switches are almost always cheaper than layer-3 (usually combined layer-2/3) switches. There are numerous reasons for the cost difference, including:

  • Mass-market low-end switches are usually simple bridges. Low-cost high-speed bridging silicon is thus readily available.
  • MAC address lookup is simpler than IP table lookup and easier to implement in silicon. You need simple CAM (Content Addressable Memory) to perform MAC address lookup and TCAM (Ternary CAM) with additional output logic to perform longest-IP-prefix matching.
  • Layer-3 switches are expected to perform IP packet filtering. Implementing access lists in hardware (usually with even larger TCAM) is expensive.
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Why is TRILL not routing at layer-2

Peter John Hill made an interesting observation in a comment to my “The TRILLing brain split” post; he wrote “TRILL really is routing at layer 2.”

He’s partially right – TRILL uses a routing protocol (IS-IS) and the TRILL protocol used to forward Ethernet frames (TRILL data frames) definitely has all the attributes of a layer-3 protocol:

  • TRILL data frames have layer-3 addresses (RBridge nickname);
  • They have a hop count;
  • Layer-2 next-hop is always the MAC address of the next-hop RBridge;
  • As the TRILL data frames are propagated between RBridges, the outer MAC header changes.
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Book review: Tcl Scripting for Cisco IOS

Tcl scripting in Cisco IOS is a somewhat underdocumented topic (have you noticed I’m trying to use diplomatic language), so I was excited when I’d spotted the Tcl Scripting for Cisco IOS. I got my copy within a few days (thank you so very much, @jamieadams76) and ran through it in less than three hours. End result: slight disappointment.

If you have no idea what Tcl is, have never used Tcl on Cisco IOS, know only a little bit about Cisco IOS and would like to get started, this is exactly the book you need. If you’re slightly more advanced, continue reading my review.

To be fair, it’s extremely hard to write a good book covering this topic. Very few people know enough about programming and networking. Fluency in Tcl programming and Cisco router configuration is almost non-existent. It’s thus very important that you choose one or the other audience: do you want to teach IOS gurus how to program their boxes in a weird language or do you want to help programmers get control of Cisco IOS. The book tries to do a bit of both, resulting in my mixed feelings.

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Server virtualization has totally changed the Data Center networking

There’s an extremely good reason Brad Hedlund mentioned server virtualization in his career advice: it has fundamentally changed the Data Center networking.

Years ago, we’ve treated servers as oversized IP hosts. From the networking perspective, they were no different from other IP hosts. Some of them had weird clustering requirements, some of them had multiple uplinks that had to be managed somehow, but those were just minor details. Server virtualization is a completely different beast.

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Book review: NX-OS and Cisco Nexus Switching

If you’re a networking engineer familiar with Cisco IOS and you’re asked to migrate to the Nexus platform, the NX-OS and Cisco Nexus Switching: Next-Generation Data Center Architectures book from Cisco Press might be just what you need. It’s targeted at existing Cisco IOS users and covers a wide range of topics, including layer-2 and layer-3 configuration (covering all routing protocols), IP multicast, security, high availability, serviceability, unified fabric (in less details than I would appreciate) and even the weird world of Nexus 1000V.

If you’re new to networking, you should start somewhere else; this book assumes you know the theory and the configuration basics and concentrates primarily on the differences between Cisco IOS and NX-OS. Sometimes it helps if you have experience with the Catalyst branch of Cisco IOS. I was struggling with the layer-2 chapter due to its reliance on Catalyst IOS knowledge; spending a bit more time explaining various interface types and how they all fit together would not hurt.

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Extremely off-topic: Another recognition of my work

Regular readers of my blog might remember that I’ve hard-earned a prestigious “Certified Religious Bovine Professional” title after getting into the firewall holy cows debate.

This time, my bridging/TRILL-focused posts earned me the distinguished “old-school network guru from an Ivory Tower” designation, together with a bit of a career advice (“Dont be the guy saying 'No' to DC virtualization from your Ivory Tower”)... or it could be that Brad Hedlund had someone else in mind and I’m just a presumptuous GONER; after all, there are hundreds of people writing about DC bridging these days.

Anyhow, on the odd chance that I was included in that honored group, Brad didn’t need to worry about my career:

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Bridging and Routing: is there a difference?

In his comment to one of my TRILL posts, Petr Lapukhov has asked the fundamental question: “how is bridging different from routing?”. It’s impossible to give a concise answer (let alone something as succinct as 42) as the various kludges and workarounds (including bridges and their IBM variants) have totally muddied the waters. However, let’s be pragmatic and compare Ethernet bridging with IP (or CLNS) routing. Throughout this article, bridging refers to transparent bridging as defined by the IEEE 802.1 series of standards.

Design scope. IP was designed to support global packet switching network infrastructure. Ethernet bridging was designed to emulate a single shared cable. Various design decisions made in IP or Ethernet bridging were always skewed by these perspectives: scalability versus transparency.

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Data Center 3.0 for Networking Engineers

My next webinar is covering the topics I wanted to address for over half a year, but never found time to do it: the facts and the hype in the Data Center as seen from the perspective of a networking engineer. I was planning to run it sometime in autumn, but the recent TRILL-focused hype has prompted me to schedule it sooner.

In two hours (probably slightly more, my two-hour webinars are usually almost three hours long), we’ll cover:

  • Storage technologies and protocols, including iSCSI and FCoE;
  • Server virtualization, including virtual machine mobility and high-availability;
  • Differences between routing and bridging (including VLANs and PVLANs) and the need for layer-3 routing in Data Center;
  • Emerging technologies, including TRILL, DCB, L2MP and LISP;
  • Multi-site considerations and transport options (DWDM, VPLS, MPLS/VPN and OTV).

Follow this link to register for the Data Center webinar ... and let me conclude with excellent news: this is the first time I’ve got multiple registrations before even announcing the webinar.

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Bridges: a kludge that shouldn't exist

During the last weeks I tried hard to sort out my thoughts on routing and bridging; specifically, what’s the difference between them and why you should use routing and not bridging in any large-scale network (regardless of whether it happens to be cramped into a single building called Data Center).

My vague understanding of layer 2 (Data Link layer) of the OSI model was simple: it was supposed to provide frame transport between neighbors (a neighbor is someone who is on the same physical medium as you are); layer 3 (Network layer) was supposed to provide forwarding between distant end nodes. Somehow the bridges did not fit this nice picture.

As I was struggling with this ethereally geeky version of a much older angels-on-a-pin problem, Greg Ferro of (what a coincidence, isn’t it) shared a link to a GoogleTalk given by Radia Perlman, the author of the Spanning Tree Protocol and co-author of TRILL. And guess what – in her opening minutes she said “Bridges don’t make sense. If you do packet forwarding, you should do it on layer 3”. That’s so good to hear; I’m not crazy after all.

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Upcoming webinars (2010-07-11)

Date US, Canada,
Europe, Africa,
Middle East
Asia, Pacific
August 25th Data Center 3.0 for Networking Engineers
07:00 (San Francisco), 10:00 (New York), 16:00 (Paris)
August 30th Next-generation IP Services
07:00 (San Francisco), 10:00 (New York), 16:00 (Paris)
September 29th DMVPN: Advanced and Crazy Scenarios
10:00 (Paris), 12:00 (Moscow), 18:00 (Sydney)
Building IPv6 Service Provider Core
07:00 (San Francisco), 10:00 (New York), 16:00 (Paris)
October 13th Market Trends in Service Provider Networks
10:00 (Paris), 12:00 (Moscow), 18:00 (Sydney)
Choose the optimal VPN service
07:00 (San Francisco), 10:00 (New York), 16:00 (Paris)
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FeedFlares are gone from my blog

When I started using Blogger, it had no “share-me” buttons, so I had to use Feedburner’s FeedFlares to implement the sharing line at the bottom of the post. FeedFlares use JavaScript and each “share-me” line was an extra HTTP request, sometimes resulting in very long loading times of the blog’s home page.

In the meantime, Blogger has implemented sharing buttons and I developed some small bits of JavaScript code for individual articles. As of today, FeedFlares are gone and the first page should load significantly faster than before.

On a tangential note, if you like my articles, please share them. The more you tweet or blog about them, the easier it will be for other networking engineers to find them. Thank you!

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The summer is here!

This week’s webinars were the last ones before the summer break. I definitely need one, the last weeks were crazy, but I’ve also learned a lot about DMVPN (the need to revisit “old truths” and figure out odd details is what makes preparing for the webinars real fun).

I’ve also noticed that some of have already started you summer vacations. Last week’s blog traffic was way below the usual levels (Cisco Live and Independence Day were only two of the reasons) and this week is still below the average. Obviously it’s time to shift to summer schedule – I’ll write only a few posts per week and try to keep the reading light and not too technical ... the kind of summer campfire stories you’d hear from the geekiest granduncle you could imagine.

Enjoy the summer (while it lasts), have a great time and try to visit some truly spectacular spots; the Dolomites are never a bad choice.

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The TRILLing brain split

The split personality Cisco has exposed at Cisco Live 2010 is amazing: on one hand you have the Data Center team touting the benefits of Routing at Layer 2 (an oxymoron if I’ve ever seen one), on the other hand you have Russ White extolling the virtues of good layer-3 design in the CCDE training (the quote I like most: “It all meets at Layer 3 ... that’s why CCDE is layer-3 centric”). If you’re confused, you’re not the only one

Read more ... (this time @

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EIGRP offset lists

A simplistic explanation of EIGRP offset-list configuration command you might see every now and then is “it adjusts the RD/FD to influence route selection”. If that would be the case, the adjustment would not be propagated to upstream routers (remember: only the EIGRP vector metric is sent in the routing updates, not RD or FD) resulting in potential routing loops (it’s never a good idea to use one set of metrics and propagate another set of metrics to your neighbors).

In reality, the EIGRP offset lists adjust the delay portion of the EIGRP vector metric (which linearly influences the RD/FD value). You can increase or decrease the value of the delay metric for EIGRP updates received or sent through a specific interface (or all interfaces). You can also use an access list in the offset-list command, applying changes only to specific IP prefixes (very similar to what I described in the Scalable policy routing IP corner article). For more details, please read this technology note on Cisco’s web site.

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DMVPN: Fishing rod or grilled tuna?

Last days I was eating, drinking, breathing and dreaming DMVPN as I was preparing lab scenarios for my DMVPN webinar (the participants will get complete router configurations for 12 different scenarios implemented in an 8-router fully redundant DMVPN network).

Some of the advanced scenarios were easy; for example, I’ve found a passing reference to passive RIPv2 with IP SLA in the DMVPN/GETVPN Design & Case Study presentation. I knew exactly what Stephen Lynn had in mind and was able to create a working scenario in minutes. Unfortunately, 2-tier hub site with IPSec offload was a completely different beast.

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Interesting links (2010-07-04)

After my STP-is-like-hand-grenade tweet, several friends sent me links to All Systems Down: an epic STP fail. I am positive smaller STP failures happen on a daily basis, but are fixed too fast to be honored with an extensive case study. Nevertheless, vendors are furiously trying to persuade you that L2 switching (formerly known as bridging) is the sexiest thing since Paris Hilton, the last one being Cisco with its FabricPath announcement. Some of us will definitely enjoy the show ...

Another long-term grudge of mine got somewhat more fact-based: Nielsen study supposedly reported that 6% of mobile users cause 50% of the traffic. While this could cause some more people to believe that tiered data plans (or usage caps) might make sense, I am positive this issue will continue going the way global warming went years ago.

And, last but definitely not least, there’s another Packet Pushers podcast well worth listening to. Where else could someone start discussing new ASA 8.3 features only to realize a minute later that he’s praising the virtues of Cisco IOS Zone Based Firewalls? If only they would have remembered to mention my ZBF book ;)

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Interesting BGP/IGP interaction problem

I’ve stumbled across a really interesting BGP/IGP problem described by Jeremy Filliben that nicely illustrates the dangers of using more than one IGP in your network. You should read the original post for details, here’s a short summary:

  • The same IP prefix is received by two BGP border routers (A and D) and sent to a third IBGP-only router (E).
  • E can reach A via OSPF. It can reach D via EIGRP.
  • E receives two BGP paths to the target IP prefix from A and D. They are identical, so the IGP metric (taken from the IP routing table) is used as the tie-breaker.
  • EIGRP and OSPF metrics are totally incomparable and thus A (reachable via OSPF) is always preferred over D (reachable via EIGRP).

Lesson learned: use a single IGP in your AS (or at least in its BGP core).

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DNSSEC ... finally!

It looks like the signed DNS root zone might finally get deployed on July 15th and Geoff Huston celebrates the fact with a lengthy article on DNSSEC. Just in case you’re not aware what DNSSEC is all about, he’s providing this nifty summary:

A succinct summary of the problem that DNSSEC is intended to address is that DNSSEC is intended to protect DNS clients from believing forged DNS data.

Read the rest of the article on his blog.

DNSSEC deployment could cause some firewalls to hiccup. You might have to change your ASA configuration; zone-based firewall on IOS supposedly works just fine.

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