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Simulations in certification tests

Anyone who has ever had the “privilege” of interviewing a certified individual with purely theoretical knowledge appreciates the value of hands-on tests. The creators of certifications in the IT industry (including Cisco Systems) have responded by including more and more hands-on exercises in the certification exams. Unfortunately, Cisco decided not to use the real equipment, but rather simulations. While this is definitely better than relying exclusively on multiple-choice tests, students can still work their way through the simulations without having a decent level of hands-on experience.

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Connecting a switch to itself: does it hurt?

I’ve got an unusual question a few days ago:

Does a loop (cable returning back to same switch) in one switch affect other switches? How can I detect that there is such a problem in a particular switch?

The correct answer to the first question is obviously it depends. To start with, it depends on whether the two ports will be able to communicate. With a crossover (switch-to-switch) cable (and assuming there are no negotiation issues), the physical layer will work correctly. If you’re using a standard RJ-45 patch cable, you’re “out of luck” unless the switch is too smart and has auto-MDI sensing (like the Linksys switches, now well hidden under obscure part numbers like Cisco SRW248G4). In this case, the two ports will become active even connected with a patch cable.

What happens next depends on how smartly you’ve configured the ports. If you haven’t touched the default configuration, the spanning tree (STP) will discover a loop and disable one of the ports. You can identify a disabled port(s) with the show spanning-tree commands.

If you’ve configured spanning-tree portfast on a port but forgot to configure BDPU guard (or maybe you have an older switch or a low-end router with a switch module that does not support BPDU guard), you’ve just created a nice temporary loop in your bridged network. If you want something more permanent, use BPDU filter or a low-end switch that does not support STP

I’ve seen interesting network meltdowns following a layer-2 loop. Broadcasts or multicasts are most likely to be looped, so they will affect the whole network, including all connected workstations.

This article is part of You've asked for it series.

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Oversized AS paths: Cisco IOS bug details

Numerous articles describing the widespread routing instabilities caused by sloppy parser of a small router vendor (including posts at BGPmon, Renesys, Arbor Security and my blog) hinted that the unusual BGP update caused so many problems because the ISPs were using outdated Cisco IOS releases. This is definitely not the case; all classic IOS releases were affected.

Rodney Dunn from Cisco and myself were quickly able to reproduce so far unknown bug in Cisco IOS that occurs only when the inbound AS-path contains close to 255 AS numbers and the router does inbound or outbound AS-path prepending. The new bug is tracked as CSCsx73770 and affects downstream EBGP or IBGP sessions as follows:

  • When you do inbound AS-path prepending and receive a BGP update where the total length of inbound AS-path and prepending exceeds 255, the AS-path in your BGP table is completely mangled. The path is sporadically advertised to IBGP or EBGP peers and kills downstream BGP sessions (remote BGP peer sends BGP notification due to invalid UPDATE message).
  • The results of inbound AS-path prepending are very hard to reproduce, so they might be completely random

  • When you do outbound AS-path prepending and send a BGP update where the total length of the AS-path in your BGP table, prepended AS-path and your own AS-number exceed 255, the outbound EBGP update is incorrect and the downstream EBGP peer sends BGP notification, resulting in BGP session reset. IBGP peers are not affected, as IOS does not perform AS-path prepending on IBGP sessions.
  • If you don’t prepend, you’re safe. IOS marks BGP paths with AS-path length greater or equal to 254 as invalid and does not propagate them, so the AS-path length in the outbound update can never exceed 255 without prepending.

All the problems associated with CSCsx73770 can be avoided if you limit the AS-path length with the bgp maxas-limit global configuration command.

Obviously, if a downstream device is running a very old IOS release that is vulnerable to CSCdr54230, it has no chance of surviving incoming BGP update.

When I was testing the IOS behavior in my lab, I was not aware how long the offending AS-path was, so my initial analysis was incorrect. With the input from NANOG mailing list supplying to the actual length of the AS-path, this is the most probably scenario: the offending AS-path length was close enough to 255 that the outbound AS-path prepending performed between Tier-1 providers (or their peers) that did not limit the AS-path length caused EBGP sessions to reset (if someone performs inbound AS-path prepending, the IBGP sessions would suffer). Those providers that implemented AS-path length limiting successfully protected their downstream peers and their customers (unless, of course, the customers were running really old IOS releases).

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Root cause analysis: oversized AS paths

The Tuesday's BGP experiment has generated quite a splash: Cisco has discovered a new BGP bug that can be triggered only if you have a long enough AS-path and do outbound AS-path prepending (and a few of us learned more BGP intricacies we never wanted to know), lots of people have (hopefully) discovered the importance of the bgp maxas-limit configuration command and at least some ISPs have implemented inbound prepending filters that I wrote about almost a year ago. However, most of us thought that the original problem arose due to inexperienced operators of a leaf AS.

Mikael Abrahamsson was the first to notice that the number of prepends matches the low-order 8 bits of the offending AS number. Further contributors to NANOG mailing list confirmed that two autonomous systems with very long prepends are using BGP routers from Mikrotik. You configure those boxes with commands that have syntax deceptively close to Cisco's, but expect the number of AS numbers to prepend, not the AS-path. Obviously no range checking is done on the configuration parameter and the high-order 8 bits are ignored.

So it looks like the incident started with a box that accepts invalid configuration parameter used in an AS with very high value in low-order 8 bits (very improbable, but obviously not impossible). Numerous ISPs that did not limit the BGP updates they were propagating and an IOS bug did the rest.

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How useless is UMTS?

One of my fellow engineers was working with a customer who wanted to have UMTS backup for his primary link. They’ve got it to work, including IPSec+GRE running over UMTS to provide the necessary privacy/security. However, it turned out that all wireless solutions (at least as offered by nearby Service Providers) are ridden with ridiculous round-trip times. For example, UMTS transmission delay is equivalent to the time it takes to send an IP packet from Central Europe to US West coast.

Obviously UMTS technology is not ready to provide backup for VoIP traffic (which prompts an interesting question: how do the Service Providers plan to provide reasonable quality IP-based voice calls over UTMS?). The reality is way worse than that: a few hundred millisecond round-trip time can kill web browsing performance if you have awfully-designed web sites (and a lot of intranets are not well-designed because the developers never considered the delay impact).

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Video: Layer-3 switching in MPLS VPN sites

What could you do if your MPLS VPN Service Provider offers BGP between PE and CE routers, while the layer-3 switches within your site support only OSPF or RIP? If you’re studying for your CCIE lab exam, you should be able to give me at least two answers in three milliseconds. Unfortunately, most enterprise engineers are not so familiar with BGP or multiple routing protocols in a single network.

To help them, I’ve prepared the next video in my “Introduction to BGP” series. It discussed two design options: two-way redistribution or default routing in the OSPF part of the network. The “Layer-3 switching in an MPLS VPN site” article in the CT3 wiki contains the high-quality video and the network diagram as well as the initial and final router configurations. You can also watch the video served from Vimeo.

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Using MPLS Traffic Engineering to avoid core network congestion

If you’ve ever tried to design QoS in a large Service Provider network, you’ve probably realized that QoS is a zero-sum game: you can give some traffic preferential treatment only if you decide to drop or delay some other traffic. Furthermore, QoS is not linked to IP routing; interface queues cannot apply backpressure to IP routing tables or influence load sharing ratios.

The only technology that can shift the excess traffic in high-speed IP-only networks from congested links to underutilized alternate paths is MPLS traffic engineering. The “Using MPLS TE to avoid core network congestion” article I’ve written for SearchTelecom describes the basic interactions of MPLS TE and QoS and introduces autotunnel and autobandwidth concepts.

Read the article on SearchTelecom

You can find the list of all articles I wrote for SearchTelecom in the CT3 wiki.

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Update: oversized AS paths

Update @ 2009-02-20: Root cause analysis, Detailed Cisco IOS bug description

I was wrong about the details of yesterday's Internet brownout: older IOS releases don't recognize AS-paths having more than 128 AS numbers due to improper handling of extended length flag in the BGP UPDATE message (CSCdr54230).

However, quick stress tests indicate that classic IOS releases (including 12.2SRC) can't handle AS-paths having more than 255 AS numbers. IOS is able to accept (and properly process) inbound updates with two AS_SEQUENCE segments, but does not generate valid AS-path attribute in outbound update when there are more than 255 AS numbers in the AS-path, resulting in a NOTIFICATION message and continuously flapping BGP session. The only global protection you have against this behavior is the bgp maxas-limit router configuration command.

I've also updated the Wiki article.

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Protect your network with BGP maxas-limit

Update @ 2009-02-17 18:42UTC: more IOS bug details
Update @ 2009-02-20: Root cause analysis, Detailed Cisco IOS bug description

Have you noticed how slow the Internet was yesterday? I almost blamed my kids (sometimes they manage to overload my WAN link), but it turned out to be a global problem. It looks like a greenhorn ISP (they joined RIPE less than four months ago) in central Europe managed to generate a BGP update with too many AS numbers in the AS path, confusing older routers. It’s my wild guess that those routers did not anticipate two AS_SEQUENCE attributes in the BGP update message. You can find the details in the Renesys blog; at the peak of the instability, they were receiving over 100.000 BGP updates per second.

It’s very easy to protect yourself (and your downstream neighbors) from an operational error like this one. Cisco has implemented the AS-path length limiting code in IOS release 12.2. One would hope that the major ISPs would have started using this feature years ago; obviously that’s not the case. I wrote an article in the CT3 Wiki describing the “intricate” details of this obviously ignored IOS feature just to make sure everyone understands what the bgp maxas-limit command does (and hopefully implements it in this millennium)

Read the article in the CT3 wiki

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Display relevant configuration sections while configuring the router

One of the readers of our forums was looking for an interesting solution: he would like to be able to display interface configuration while configuring the same interface. Obviously you could always use the do show running interface name command, but he was looking for a single command without parameters that would display the configuration of the currently selected interface.

Boštjan Šuštar devised a brilliant solution based on EEM Tcl policies and expanded it to include the routing protocol configuration. After installing two EEM policies and adding a few aliases, you can use the config command in interface or routing protocol configuration modes to see the current configuration of the object you’re configuring.

Download the Tcl EEM policies from the CT3 wiki

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HTTP PUT is broken in IOS release 12.4(15)T

Isaac was trying to implement the HTTP PUT solution I’ve described in the IP Corner article Using a Web Server to Manage Your Router Configurations. He couldn’t get IIS or Apache to work and finally discovered that he was facing an IOS problem. When he’s upgraded the routers to IOS release 12.4(20)T or 12.4(22)T, the copy running-config http://host/path command worked as expected.

Isaac, thanks for the feedback!

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Writing good exam questions

Readers who commented on some of my previous certification-related posts have complained about the vagueness of exam questions. I have to agree with them; I’ve seen my fair share of dubious questions in the exams I’ve taken. For example, when I was developing EIGRP and BGP courses for Cisco, my lowest scores on the CCIE recertification exams were in those two categories. I knew too many details and was confused by the vagueness of the questions.

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Yellow journalism at work: Previously Unknown DNS Attacks

When I’ve stumbled across the headline Porn site feud spawns new DNS attack on NetworkWorld’s web site, the urge to read the article was simply irresistible. The article starts with the following paragraph (emphasis mine):

A scrap between two pornographic Web sites turned nasty when one figured out how to take down the other by exploiting a previously unknown quirk in the Internet's DNS.

The link in the paragraph points to another article documenting a completely different DNS attack. The next paragraph contradicts the first one (emphasis yet again mine):

The attack is known as DNS Amplification. It has been used sporadically since December, but it started getting talked about last month when ISPrime, a small New York ISP, started getting hit hard with what's known as a distributed denial of service (DDoS) attack.

So an attack that has been known to be used since December is previously unknown in February. Maybe the time runs backwards for some people?

But the most interesting part of the story is the attack itself: it has almost nothing to do with DNS; any UDP application where the reply is significantly longer than the request (for example, TFTP) would do. DNS just happens to be the most widespread one. Calling this a quirk in the Internet’s DNS is at least an exaggeration.

Here is how it works:

  • It helps if you have a zombie network, but even a single workstation with 100 Mbps connection to the Internet (there are plenty of them in corporate and academic networks) would do.
  • The site to which the workstation is connected as well as the ISP has to be careless enough to allow source IP address spoofing. A large percentage of ISPs still fit this criterion.
  • You start sending large number of DNS requests that generate long replies to DNS servers around the world. The source IP address of the request should be the IP address of the server you want to attack.
  • The DNS servers send their replies to the spoofed IP address, swamping the Internet connection of the attack target.

The effectiveness of this attack depends on the amplification factor: the ratio between the DNS query and DNS response sizes (including the IP and UDP header). For example, the amplification factor of the root nameserver query is more than five. Wireshark on my Linux workstation reports that the IP packet size of the DNS query is 45 bytes and the packet size of the DNS reply is 256 bytes.

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Poly-Labs from Internetwork Expert

A few hours ago, Internetwork Expert launched an ingenious solution: the Poly-Lab Assessments. The basic idea is brilliantly simple: if you’re using a common hardware and L2/L3 topology for all labs (and hundreds of tasks within these labs), it’s possible to mix individual tasks into a customized lab tailored to the needs of a CCIE candidate. The mix-and-match approach allows you to customize the difficulty of selected tasks (and technologies from the CCIE blueprint) to the candidate’s capabilities.

Initially, the candidates have to guess their competence level; after the first grading, the system gives them a more realistic estimate, along with the recommended actions they need to take to improve their skills. Using the Poly-Labs continuously gives you a nice progress indication, as you can watch whether you’ve improved in the most problematic areas. Once you’re satisfied with your knowledge level, you just switch to another well-known web site and schedule the actual CCIE lab.

I’ve particularly liked the grading system: finally there’s a vendor (apart from the assessment engine I wrote for NIL more than five years ago) that understands you should examine the show printouts (not just the configurations) and check whether they confirm the task has been accomplished. Obviously, this could lead to some nasty surprises: even though your configuration is correct, you might still lose points because another feature you configured has demolished your previous work (for example, inbound ACL dropping OSPF hello packets). Anyhow, that’s life; this grading system is more realistic than simplistic configuration matching.

All in all, the Poly-Labs look extremely promising. Brian McGahan has offered me a test drive; I’ll definitely keep you posted.

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DMVPN: Basics, Resilience, Performance and Scalability

The fourth article in the IPSec series written by Boštjan Šuštar deals with Dynamic Multipoint VPN (DMVPN). Boštjan describes the design and implementation aspects of DMVPN without going into too many unnecessary details, behavior of RIP, OSPF and EIGRP over DMVPN clouds and the resilience (multiple hubs), performance and scalability considerations of DMVPN.

Read the DMVPN article in the IP corner

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Off-topic: Workstation vulnerability — FUD at its best

Reading an interestingly-titled article on InformIT, I’ve stumbled across the following text:

The survival time is an estimate of how long an un-patched computer will remain uncompromised once it’s connected to the Internet. While the actual time varies, historically it tends to run between 4 and 20 minutes.

This is such an obvious nonsense that I had to check the source, which is also full of alarming messages, but admits at the end that the problems described largely disappeared with XP SP2. Just to put things in perspective: XP SP2 was released in August 2004 and the graph in the alarming blog post displays data from 2008.

Next step: investigate the source of the graph. The »average survival time« is defined as the time between probes on numerous TCP or UDP ports, regardless of whether the port was actually enabled in the workstation and whether the probe was successful or not.

My personal conclusion: as most workstations include some sort of rudimentary firewall these days, the whole approach is bogus. More precisely, it measures an important parameter (average time between probes), but claims it represents something completely different (average survival time). Would you agree with my conclusion?

Lesson learned: Never trust alarming over-simplifying statements based on misunderstood data.

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Tutorial: Tclsh on Cisco IOS

Finally I found time to organize the various interesting Tclsh bits-and-pieces I’ve blogged about in a comprehensive Tclsh on Cisco IOS tutorial.

It’s not a classic tutorial; I’m assuming you know what Tcl is and how to write Tcl programs. The articles in the tutorial document the implementation details and discrepancies that are not (to my knowledge) documented anywhere else.

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Running IOS commands from Tclsh: the details you never wanted to know

You might be aware that there are two ways of executing IOS CLI commands from IOS tclsh: either you insert the IOS commands directly in the Tcl code (tclsh passes any unknown commands to the CLI parser) or you execute them with the exec command. There are subtle differences between the two methods, described in the “Executing IOS commands from Tcl shell article” in the CT3 wiki.

Read the full article in the CT3 wiki

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Dance around IOS bugs with Tcl and EEM

Recently, on an IPSec-based customer network, we installed one of the brand new platforms introduced by Cisco Systems. The initial software release had memory leaks (no problem, we all know these things happen), so we upgraded the box to the latest software. It works perfectly … until you reload it. The software we’re forced to use cannot get IPSec to work if the startup configuration includes interface-level crypto-maps. Interestingly, you can configure crypto-maps manually and they work … until you save them into the startup configuration and reload the box.

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Things you cannot do with Tclsh

What would you think if you’d receive three queries about the same (somewhat obscure) feature within six hours? It started with a nice e-mail from an engineer that I’ve corresponded with in the past. He wanted to send a Wake-on-LAN packet to a PC in a remote office. Usually you could use the ip directed-broadcast feature, but he wanted to use the remote office router to generate the packet.

A few hours later an engineer from another country that had sent me a few interesting configuration tips a while ago sent an almost identical question:

Since you're a whizz in tcl on IOS, how about a few lines of code to implement sending wake-on-lan magic packets from an IOS router !

To top it off, the following line came not long afterwards from an almost-anonymous source:

Hi Ivan, I have read several articles on your tcl-ios very interesting, I'm learning to use the IOS. Could you say if there is any way to generate a magic packet on ios to boot pc by Wake on LAN? Would be a good entry for your page.

I don't believe in coincidences, so I'm guessing these three engineers were trying to solve very similar problems ... and I keep wondering what happened in their networks in the last week.

Unfortunately, I was not able to help them. The Wake-on-LAN packet has to be sent to the broadcast address and it has to contain pretty long payload, so it's usually sent as a UDP packet (ICMP echo packet would also be an option). Core Tcl supports only socket API and thus does not support UDP (or any other protocol on top of IP but TCP). While various Tcl implementations support UDP with help of external libraries, you cannot use the same libraries in Cisco IOS. So it looks like there's no mechanism to generate a UDP packet with desired payload from Cisco IOS. Am I missing something?

Oh, and there's yet another option: if you can afford to invest $3500 per site, you can install AXP in your router to send the Wake-on-LAN packets.

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Display all parts of a VRF configuration

Very recent IOS releases (12.2SRC and 12.4(22)T) have a handy command: show running vrf name, which displays VRF, interface and routing protocol configurations of the specified VRF. It does not, however, include referenced access-lists or route-maps.

Davor Koncic solved this problem years ago: he wrote a Tcl script that does the same job better; his script displays most configuration parameters related to a VRF (he’s missing the MQC or IPSec parts). Great job (and a nice illustration of the power of Tclsh on IOS).

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