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BGP and IPv6 routing courses

Several times a year I teach two training courses, one about BGP and one about IPv6. The BGP course is half theory and half hands-on practice, and so is the new IPv6 routing course. Previously, we did an IPv6 course without a hands-on part.

The courses consists of a theory part in the morning and a practical part in the afternoon where the participants implement several assignments on a Cisco router (in groups of two participants per router).

The next dates are February 2 for the BGP course in Dutch and February 3 for the IPv6 routing course in Dutch. (There will be dates for the courses in English later in 2015.) Go to the NL-ix website to find more information and sign up. The location will be The Hague, Netherlands.

Interdomain Routing & IPv6 News

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  • Internet exchange renumbering: everything old is new again (posted 2014-10-15) article 149

    This week, the Amsterdam Internet Exchange is renumbering its peering LAN.

    An internet exchange (IX) is simply a very big Ethernet. Members connect a router port to that Ethernet, and can then exchange packets with each other. When you want to exchange traffic with many other networks, obviously this is more efficient than setting up dedicated connections with all these other networks.

    Until this week, AMS-IX used a /22 prefix, allowing for about a thousand connected routers. That was no longer enough, so they got a new /21 prefix, which can accommodate two thousand connected routers. This means that all the currently connected routers must get a new address. No big deal. This is why search-and-replace was invented.

    However, sometimes someone makes a mistake. Like configuring <new address>/22 instead of <new address>/21. And then letting that /22 propagate to other networks over BGP. Suppose:

    • is the prefix used on the IX peering LAN
    • peers with
    • advertises
    • wants to send a BPG keepalive packet to Normally this packet would go to the port where is configured. But the prefix is "more specific", so the packet is sent on its way to the source of the prefix: router
    • forwards the packet to router
    • All's well that ends well? Not so fast. In BGP, there are not supposed to be any intermediate routers between two BGP routers. sees the packet traversed and extra hop, rejects it and resets the BGP session.
    • can no longer send traffic to

    (A more specific prefix is a smaller range of IP addresses. is BGP talk for the address range - is the range - Because the latter identifies a smaller range of IP addresses, the packets are sent in that direction, just like you'd follow a sign "Paris" rather than a sign "France" if you were going to Paris, even though Paris is part of France so presumably following the sign "France" would also get you to Paris.)

    The sad thing is that the exact same thing happened in 2003, when the AMS-IX renumbered from a /24 to a /23. I always warn against this issue during my training courses, and tell students to filter the IX prefixes of internet exchanges they're connected to, as well as all possible subprefixes (more specifics) that fall within that IX prefix. For instance:

    ip prefix-list import deny le 32
    ip prefix-list import deny le 32
    ip prefix-list import permit le 24

    This prefix list will reject incoming updates with your own prefix and all possible more specifics (assuming your prefix is as well as the AMS-IX prefix and all possible subprefixes. It then allows all prefixes with a prefix length of no more than /24, which is common practice for IPv4.

    "le" means "less or equal" so " le 32" means:

    Hopefully, by the time AMS-IX connects more than 2000 routers, the issue is moot because we no longer use IPv4. But for now: happy renumbering!

  • IPv6 deployment has made a lot of progress in less than two years (posted 2014-09-15) article 148

    I was updating a presentation the other day, and I found something I wanted to share here. (Looks like old presentations are good blog fodder. Who knew.) The old presentation was the one I did at the 2013 ISOC New Year's event, where I put up some slides on Google's numbers of IPv6 users (from their vantage point) in various countries. That was less than two years ago, and a lot has changed.

    The global number of IPv6-capable users was 1% then (January 2013), now (September 2014) it's 4%. Some countries that were ahead of the curve extended their lead, others didn't, and a new world leader emerged out of nowhere. Click the link to see the old slides followed by the current situation. (1.4 MB PDF.)

    So the good news is that a lot can happen quickly when it comes to IPv6 deployment.

    Read the whole article

  • A decade of bandwidth growth (posted 2014-09-04) article 147

    Next week I'm doing a guest lecture about the infrastructure of the internet. Turns out that I already talked about that in 2003 (PDF). One slide in that presentation caught my eye:

    The image shows how bandwidth in research networks and ISP networks has grown, as well as the bandwidth provided by technologies such as Ethernet, Wi-Fi, mobile data, and consumer internet access. All of them show a healthy growth, although some start from extremely modest beginnings.

    So I wondered what happened in the intermediate decade. Which would be:

    Because research and ISP networks are so diverse, I didn't include these here. Of course this is all somewhat arbitrary, because few people use the highest available speeds. For instance, cable and fiber to the home internet access tops out at 100 Mbps or higher, but many people still use around 10 Mbps, either because of price or because they're limited to some form of DSL and a relatively long distance to the nearest telephone exchange. And LTE should be able to reach 300 Mbps, but I don't think that's available in commonly available handsets.

    My impression that Ethernet had run out of steam is not entirely correct, the progression to 100 Gbps is roughly in line with the earlier growth path. Wi-Fi looks a little less impressive than I had thought on this logarithmic scale. However, there's just one super star here: mobile. It's hard to believe that we went from 100 kbps or so EDGE to 100 Mbps LTE in about a decade. Not everyone may be using LTE just yet, but 14 Mbps UMTS HSPA is in very wide use, there is really no reason to use anything less.

  • → IPv6 adoption starting to add up to real numbers: 0.6 percent (posted 2014-08-28) article 146

    Last week a SIGCOMM paper was published with a bunch of IPv6 measurements in it, which concludes that IPv6 deployment is getting real. For instance, 0.6% of internet traffic was IPv6 at the beginning of 2014, which is 5 times more than in 2013 and that 5 times more than in 2012. Click the link to read my story about this on Ars Technica.

    Read the whole article Archives of all articles - RSS feed

My Books: "BGP" and "Running IPv6"

On this page you can find more information about my book "BGP". Or you can jump immediately to chapter 6, "Traffic Engineering", (approx. 150kB) that O'Reilly has put online as a sample chapter. Information about the Japanese translation can be found here.

More information about my second book, "Running IPv6", is available here. Apress, my new publisher, also has a sample chapter available: Chapter 5, The DNS.

"no synchronization"

When you run BGP on two or more routers, you need to configure internal BGP (iBGP) between all of them. If those routers are Cisco routers, they won't work very well unless you configure them with no synchronization.

The no synchronization configuration command tells the routers that you don't want them to "synchronize" iBGP and the internal routing protocol such as OSPF. The idea behind synchronizing is that when you have two iBGP speaking routers with another router in between that doesn't speak BGP, the non-BGP router in the middle needs to have the same routing information as the BGP routers, or there could be routing loops. The way to make sure that the non-BGP router is aware of the routing information in BGP, is to redistribute the BGP routing information into the internal routing protocol.

By default, Cisco routers expect you to do this, and wait for the BGP routing information to show up in an internal routing protocol before they'll use any routes learned through iBGP. However, these days redistributing full BGP routing into another protocol isn't really done any more, because it's easier to simply run BGP on any routers in the middle.

But if you don't redistribute BGP into internal routing, the router will still wait for the BGP routes to show up in an internal routing protocol, which will never happen, so the iBGP routes are never used.

The "no synchronization" configuration command tells the routers they shouldn't wait for this synchronization, but just go ahead and use the iBGP routes.

BGP Security

BGP has some security holes. This sounds very bad, and of course it isn't good, but don't be overly alarmed. There are basically two problems: sessions can be hijacked, and it is possible to inject incorrect information into the BGP tables for someone who can either hijack a session or someone who has a legitimate BGP session.

Session hijacking is hard to do for someone who can't see the TCP sequence number for the TCP session the BGP protocol runs over, and if there are good anti-spoofing filters it is even impossible. And of course using the TCP MD5 password option (RFC 2385) makes all of this nearly impossible even for someone who can sniff the BGP traffic.

Nearly all ISPs filter BGP information from customers, so in most cases it isn't possible to successfully inject false information. However, filtering on peering sessions between ISPs isn't as widespread, although some networks do this. A rogue ISP could do some real damage here.

There are now two efforts underway to better secure BGP:

  • Secure BGP (S-BGP) is developed by Bolt, Beranek and Newman (BBN). It has been around for several years and there is a proof-of-concept implementation. S-BGP tries to secure all aspects of the BGP protocol, and subsequently needs several signature checks for each BGP update, making the protocol relatively heavy-weight. You can see my earlier rants on S-BGP at the top of this page. Note that I'm not as anti-S-BGP as I used to be any more, although I still think implementing the protocol will be expensive because routers will need lots of extra memory (up to four times as much) and CPU power (possibly dedicated crypto hardware) and this aspect deserves some serious attention.

    Secure BGP (S-BGP) index at BBN.

  • Secure Origin BGP (soBGP) has surfaced fairly recently and hails from Cisco. There are no implementations so far. soBGP mainly focusses on securing the relationship between prefixes and the source AS number, and doesn't need as many computationally expensive checks as S-BGP. However, the protocol can easily be expanded to perform more checks.

    draft-ng-sobgp-bgp-extensions-00.txt (main soBGP draft)
    draft-white-sobgp-bgp-extensions-00.txt (deployment considerations)

    (If the links don't work, the drafts have expired; you'll have to use a search engine to find them.)

There is now also a different approach to increasing BGP security using an "Interdomain Routing Validation" service that works independent from the BGP protocol itself. See what I wrote about this in interdomain routing news on this site, or jump immediately to the Working Around BGP: An Incremental Approach to Improving Security and Accuracy of Interdomain Routing paper.

The IETF RPSEC (routing protocol security) working group is active in this area.


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What is is a website dedicated to Internet routing issues. What we want is for packets to find their way from one end of the globe to another, and make the jobs of the people that make this happen a little easier.

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Ok, but what is BGP?

Have a look at the "what is BGP" page. There is also a list of BGP and interdomain routing terms on this page.

BGP and Multihoming

If you are not an ISP, your main reason to be interested in BGP will probably be to multihome. By connecting to two or more ISPs at the same time, you are "multihomed" and you no longer have to depend on a single ISP for your network connectivity.

This sounds simple enough, but as always, there is a catch. For regular customers, it's the Internet Service Provider who makes sure the rest of the Internet knows where packets have to be sent to reach their customer. If you are multihomed, you can't let your ISP do this, because then you would have to depend on a single ISP again. This is where the BGP protocol comes in: this is the protocol used to carry this information from ISP to ISP. By announcing reachability information for your network to two ISPs, you can make sure everybody still knows how to reach you if one of those ISPs has an outage.

Want to know more? Read A Look at Multihoming and BGP, an article about multihoming I wrote for the O'Reilly Network.

For those of you interested in multihoming in IPv6 (which is pretty much impossible at the moment), have a look at the "IPv6 multihoming solutions" page.

Are you a BGP expert? Take the test to find out!

These questions are somewhat Cisco-centric. We now also have another set of questions and answers for self-study purposes.

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