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Blog Posts in April 2008

The Impact of tx-ring-limit

Setting the size of the hardware output queue in Cisco IOS with the (then undocumented) tx-ring-limit (formerly known as tx-limit) has been a big deal when I was developing the first version of the QoS course that eventually became the initial release of the Implementing Cisco Quality of Service training.

However, while it's intuitively clear that the longer hardware queue affects the QoS, years passed before I finally took the time to measure the actual impact.

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Display operational IPv6 interfaces

The brief display of the state of IPv6 interfaces in the router (show ipv6 interface brief) is significantly different from the well-known show ip interface brief display as the IPv6 address might not fit in the same line as all the other data. To filter the printout and display only the operational interfaces, you have to replace the include filter with the section filter, which displays all the lines matching the regular expression as well as associated follow-up lines.
PE-A#show ipv6 interface brief | section up
Serial1/0 [up/up]
Serial1/1 [up/up]
Loopback0 [up/up]

The definition of the associated follow-up lines depends on the printout. Usually the indented lines are assumed to belong to a section, but you might be surprised.

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Almost-Dynamic Routing over ADSL Interfaces

Recently I had to implement Internet access using ADSL as the primary link and ISDN as the backup link. Obviously the most versatile solution would use the techniques described in my Small Site Multi-homing articles, but the peculiarities of Cisco IOS implementation of the ADSL technology resulted in a much simpler solution.

IOS implementation of PPPoE links uses dialer interfaces. However, the “dialing” on these interfaces is activated as soon as the underlying PPPoE session is active (before the first interesting packet is routed to the interface). When the simulated dial-out occurs, the router starts PPP negotiations including the IPCP handshake, which usually results in an IP address assigned to the dialer interface. Net result: if the dialer interface has an IP address, the PPPoE session is obviously active (and vice versa).

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Hot air party

Cisco recently announced a Linux-based add-on board for the ISR router models. It might not be the best thing ever invented, it's probably overpriced and we still have to see what we really get; the materials available on the Cisco's web site are another good example of a great marketing machinery (when I see a title "Our Strategy is Integration and Convergence"on the slide #11 of a Technical Overview presentation, I start wondering whether it's worth my time to continue looking at the presentation). But at least this time they talk about supporting Perl and Python, not Tcl :))

However, what really prompted me to start writing this post was the "wisdom" spread by industry journalists. Network world was still moderate; the gentleman at LinuxWorld had some strong opinions. It would be OK if they would stop at bashing the new module (and questioning the value-for-price is always fair), but of course it's more fun being all over the place, evangelizing the beauties of PC-based open-source routers and the demise of traditional router vendors. While there's (yet again) nothing wrong with open-source, let's bring a bit of the history into the picture:
  • 15 years ago, someone had a great idea to install WAN cards and routing software into PC servers. The journalists greeted that idea as the downfall of dedicated routers. Guess what ... it flopped and the router market continued to grow.
  • Cheap Layer-3 switches have been greeted as the next router killer. We still have routers and switches in our networks.
  • People have been using Linux as their home firewalls for years ... and it hasn't really impacted the low-end router market; SOHO users are still preferring to buy Linksys (or whatever other cheap low-end brand) over configuring firewall on Linux.
  • Public-domain BGP implementations have been around for as long as I can remember and they are not bad. Some people with very low budget use them for route servers ... but Cisco and Juniper are still selling high-end boxes.

In the real world of networks that have more than a few routers, if you have enough budget to buy yourself a good night's sleep, you usually install dedicated routing hardware ... but I guess this is not the sort of story that would sell the industry journals.

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IPv4 Forever?

One of the obscure facts of IPv6 OSPF (OSPFv3) is that it uses a 32-bit router ID like OSPFv2. It's a reasonable choice, I haven't seen an OSPF network with more than a billion routers yet. However, could you guess how this requirement is implemented in Cisco IOS? OSPFv3 searches for an IPv4 address (effectively the same algorithm used by OSPFv2) to get the router ID for the IPv6 routing process. Neat, isn't it?

You might wonder what happens if you want to configure an IPv6-only router. OSPF won't start unless you configure the router ID manually. And, no, you cannot enter a number (which would be the expected format, as the router ID is just a number in the IPv6 world), you have to enter an IPv4 address. Long live IPv4 :))

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AI (Artificial Intelligence) in Cisco IOS release 12.4T

Years ago, when the "fourth-generation computers" (looks like they've been renumbered to the fifth generation in the meantime) were as hot as Web 2.0 is today, we started talking about the generation after that which you could control with statements like “Do what I don't know how” ... and some of that mentality is already built into Cisco IOS.

In a comment to my recent NTP-related post mentioning OSPF configuration, Wan Tajuddin correctly stated that the OSPF network statement should contain the wildcard bits, not the subnet mask. But I was positive I had running networks with the network area 0 OSPF configuration, so it was time for one more lab test. As it turns out, at least the IOS release 12.4T accepts either the wildcard bits or the subnet mask and figures out which one you've used.
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BGP Essentials: Non-transit AS

One of the first things you have to do when configuring BGP with your ISP is to ensure you won’t become a transit AS. Decent ISPs filter out things that don’t belong to you from your updates, but not everyone cares (including some really big names), and so small organizations manage to bring down large parts of the Internet just with a few fat fingers.

Here’s the BGP configuration you should use on Cisco IOS: apply AS-path access-list to outbound updates with neighbor filter-list command:

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Cisco IOS NTP Essentials

A while ago I've been involved in an interesting discussion focusing on NTP authentication and whether you can actually implement it reliably on Cisco IOS. What I got out of it (apart from a working example :) was the feeling that NTP and it's implementation in Cisco IOS was under-understood and under-documented, so I planned to write an article about it.

However, as I did my research, I figured out there's so much I didn't know about NTP (do you know what's the essential difference between a peer and a server?) that I decided to write It’s Good to be on Time article – you’ll find it somewhere in this list.

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Use UDP flood to increase router's CPU load

If you want to test the ERM policies in a controlled environment, it's almost mandatory to have tools that allow you to overload the router. One way to overload a router is to flood it with UDP packets. Flooding a router's IP address, you're guaranteed to raise the CPU to 100%, with majority of the process CPU being used by the IP Input process (the interrupt CPU load will also be significant).

This phenomenon illustrates very clearly why it's so important to have inbound access lists protecting the router's own IP addresses on all edge interfaces.

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Do bootcamps make sense?

My recent post about a CCNP bootcamp program I was involved with generated interesting comments, most of them focusing on the question: “Does it make sense to attend a bootcamp?”

The answer depends on how you got to the stage where you want to (or need to) attain the CCxP certification. Before going into discussions on “experience” versus “knowledge retention” (potentially “aided” by brain dumps), please read The Top 10 Problems with IT Certification in 2008 article published by InformIT. My potential disagreements with this article are so minor that I will not even try to document them.

OK, now that we're on the same page, let's analyze why someone would want to pursue CCxP certification:

  • To increase the salary or have better job options (as HR departments ask for people with specific set of certifications). From what I hear, this reason is more viable in US than the rest of the world (in most of Europe we can still test the technical skills of the candidates in any way we want without running the risk of being sued). Bootcamps might not be the best option for these candidates, as they tend to be priced similarly to the regular classes. Reading books or e-learning material (not to mention certain not-so-very-legitimate activities) will get you through the exams as long as they don't have the hands-on part ... and of course you'll end up having certification with zero experience;
  • To learn something new and valuable resulting in a formal recognition of the effort. Don't even think about attending the bootcamps. If you're learning completely new concepts, go through the regular courses (or use e-learning combined with hands-on lab exercises). Highly intensive format of the bootcamps (after all, we're trying to squeeze almost two weeks worth of material into a single week) will fly way over your head.
  • To formalize your experience ... either because you want to or because your employer needs certified head count (very common with Cisco partners trying to get better discount based on their partner status). In this case, a condensed bootcamp is usually the best option. For example, we had very successful bootcamp program a few years ago running back-to-back with the exams ... and, mind you, we used no cheating or brain dumps, the fact that the students took the exam right after the course obviously helped.

Last but definitely not least, it's worth mentioning that not all five-day courses have five days worth of content. In these cases, condensing them into bootcamps makes even more sense.

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RFC 3514 implemented by the ASR series of routers

The information on the IOS XE software used by the recently launched ASR 1000 router is pretty scarce (there is still no link to the documentation available on CCO), but obviously some backdoor links already exist, as I was able to find some IOS XE-related documents with Google. One of the most amazing features I've found is the support for the security-oriented RFC 3514 which allows you to mark the security level of an IP packet.

The RFC 3514 requires the end host to participate in the process, but as most operating system vendors still don't have a trusted computing platform, a transparent proxy has to be implemented on the network edges to properly tag the ingress packets. ASR 1000 has the first high-speed implementation of the RFC 3514 proxy thanks to its non-deterministic parallel QuantumFlow processors.

The configuration of the RFC 3514 proxy is extremely simple: all you need to do is to configure auto-secure mark on the ingress interfaces of the ASR 1000. Once the security bit has been set, you can use the match ip security-bit 0|1 command in a class-map or a route-map on any router running IOS release 12.4(11)T or later (the command is still hidden).
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