IOS X-Treme!


As a nerd, I’m a huge fan of science fiction. One of my favorite shows was Stargate SG-1. Inside the show, there was a joke involving an in-universe TV program called “Wormhole X-Treme” that a writer unintentionally created based on knowledge of the fictional Stargate program. Essentially, it’s a story that’s almost the same as the one we’re watching, with just enough differences to be a totally unique experience. In many ways, that’s how I feel about the new versions of Cisco’s Internetwork Operating System (IOS) that have been coming out in recent months. They may look very similar to IOS. They may behave similarly to IOS. But to mistake them for IOS isn’t right. In this post, I’m going to talk about the three most popular IOS-like variants – IOS XE, IOS XR, and NX-OS.


IOS XE is the most IOS-like of all the new IOS builds that have been released. That’s because the entire point of the IOS XE project was to rebuild IOS to future proof the technology. Right now, the IOS that runs on routers (which will henceforth be called IOS Classic) is a monolithic kernel that runs all of the necessary modules in the same memory space. This means that if something happens to the routing engine or the LED indicator, it can cause the whole IOS kernel to crash if it runs out of memory. That may have been okay years ago but today’s mission critical networks can’t afford to have a rogue process bringing down an entire chassis switch. Cisco’s software engineers set out on a mission to rebuild the IOS CLI on a more robust platform.

IOS XE runs as a system daemon on a “modern Linux platform.” Which one is anyone’s guess. Cisco also abstracted the system functions out of the main kernel and into separate processes. That means that if one of them goes belly up it won’t take the core kernel with it. One of the other benefits of running the kernel as a system daemon is that you can now balance the workload of the processes across multiple processor cores. This was one of the more exciting things to me when I saw IOS XE for the first time. Thanks to the many folks that pointed out to me that the ASR 1000 was the first device to run IOS XE. The Catalyst 4500 (the first switch to get IOS XE) is using a multi core processor to do very interesting things, like the ability to run inline Wireshark on a processor core while still letting IOS have all the processor power it needs. Here’s a video describing that:

Because you can abstract the whole operation of the IOS feature set, you can begin to do things like offer a true virtual router like the CSR 1000. As many people have recently discovered, the CSR 1000 is built on IOS XE and can be booted and operated in a virtualized environment (like VMware Fusion or ESXi). The RAM requirements are fairly high for a desktop virtualization platform (CSR requires 4GB of RAM to run), but the promise is there for those that don’t want to keep using GNS3/Dynamips or Cisco’s IOU to emulate IOS-like features. IOS XE is the future of IOS development. It won’t be long until the next generation of supervisor engines and devices will be using it exclusively instead of relying on IOS Classic.


In keeping with the sci-fi theme of this post, IOS XR is what the Mirror Universe version of IOS would look like. Much like IOS XE, IOS XR does away with the monolithic kernel and shared memory space of IOS Classic. XR uses an OS from QNX to serve as the base for the IOS functions. XR also segments the ancillary process in IOS into separate memory spaces to prevent system crashes from an errant bug. XR is aimed at the larger service provider platforms like the ASR 9000 and CRS series of routers. You can see that in the way that XR can allow multiple routing protocol processes to be executed at the same time in different memory spaces. That’s a big key to the service provider.

What makes IOS XR so different from IOS Classic? That lies in the configuration method. While the CLI may resemble the IOS that you’re used to, the change methodology is totally foreign to Cisco people. Instead of making live config changes on a live system, the running configuration is forked into a separate memory space. Once you have created all the changes that you need to make, you have to perform a sanity check on the config before it can be moved into live production. That keeps you from screwing something up accidentally. Once you have performed a sanity check, you have to explicitly apply the configuration via a commit command. In the event that the config you applied to the router does indeed contain errors that weren’t caught by the sanity checker (like the wrong IP), you can issue a command to revert to a previous working config in a process known as rollback. All of the previous configuration sets are retained in NVRAM and remain available for reversion.

If you’re keeping track at home, this sounds an awful lot like Junos. Hence my Mirror Universe analogy. IOS XR is aimed at service providers, which is a market dominated by Juniper. SPs have gotten very used to the sanity checking and rollback capabilities provided by Junos. Cisco decided to offer those features in an SP-specific IOS package. There are many that want to see IOS XR ported from the ASR/CSR lines down into more common SP platforms. Only time will tell if that will happen. Jeff Fry has an excellent series of posts on IOS XR that I highly recommend if you want to learn more about the specifics of configuration on that platform.


NX-OS is the odd man out from the IOS family. It originally started life as Cisco’s SAN-OS, which was responsible for running the MDS line of fibre channel switches. Once Cisco started developing the Nexus switching platform, they decided to use SAN-OS as the basis for the operating system, as it already contained much of the code that would be needed to allow networking and storage protocols to interoperate on the device, a necessity for a converged data center switch. Eventually, the new OS became known as NX-OS.

NX-OS looks similar to the IOS Classic interface that most engineers have become accustomed to. However, the underlying OS is very different from what you’re used to. First off, not every feature of classic IOS is available on demand. Yes, a lot of the more esoteric feature sets (like the DHCP server) are just plain unavailable. But even the feature sets that are listed as available in the OS may not be in the actual running code. You need to active each of these via use of the feature keyword when you want to enable them. This “opt in” methodology ensures that the running code only contains essential modules as well as the features you want. That should make the security people happy from an exploit perspective, as it lowers the available attack surface of your OS.

Another unique feature of NX-OS is the interface naming convention. In IOS Classic, each interface is named via the speed. You can have Ethernet, FastEthernet, GigabitEthernet, TenGigabit, and even FortyGigabit interfaces. In NX-OS, you have one – Ethernet. NX-OS treats all interfaces as Ethernet regardless of the underlying speed. That’s great for a modular switch because it allows you to keep the same configuration no matter which line cards are running in the device. It also allows you to easily port the configuration to a newer device, say from Nexus 5500 to Nexus 6000, without needed to do a find/replace operation on the config and risk changing a line you weren’t supposed to. Besides, most of the time the engineer doesn’t care about whether an interface is gigabit or ten gigabit. They just want to program the second port on the third line card.

Tom’s Take

No software program can survive without updates. Especially if it is an operating system. The hardware designed to run version 1.0 is never the same as the hardware that version 5.0 or even 10.0 utilizes. Everything evolves to become more efficient and useful. Think of it like seasons of sci-fi shows. Every season tells a story. There may be some similarities, but people overall want the consistency of the characters they’ve come to love coupled with new stories and opportunities to increase character development. Network operating systems like IOS are no different. Engineers want the IOS-like interface but they also want separated control planes, robust sanity checking, and modularized feature insertion. Much like the writers of sci-fi, Cisco will continue to provide new features and functionality while still retaining the things to which we’ve grown accustomed. However, if Cisco ever comes up with a hare-brained idea like the Ori, I can promise there’s no way I’ll ever run IOS-Origin.


8 thoughts on “IOS X-Treme!

  1. Excellent article, I always enjoy your view on upcoming technology. Especially the side of skepticism and snark that we all share with new vendor implementations. Do you think if the ASR 9000 series, as well as other IOS XR platforms are unsuccessful that we will see Cisco abandon IOS-XR?

    • That all depends on how you look at XR. If XR is merely a bridge to get Juniper people more familiar with IOS devices, then we will eventually see the important stuff rolled into IOS XE and NX-OS, which apparently has already happened according to Andy Sholomon. XR will be relegated to the same bin as CatOS when the ASR gen 2 arrives.

      If XR isn’t a transitional mechanism and instead really is the SP way going forward, I think it will stick around in the next revision and eventually displace IOS XE on the ASR 1k and all other “carrier grade” platforms. That does create some contention around fragmentation of the code base and gives Juniper a great marketing vector to put up beside their One Junos campaigns.

      If I had to put money on it for the long term, I’d be that you see IOS XE and IOS XR merge to a unified OS sometime in the next two years.

  2. XR not user friendly? I’ve not had a single issue when using it. I think it’s great and Cisco would do themselves a great disservice if they were to kill it off

  3. Pingback: CCIE Version 5: Out With The Old | The Networking Nerd

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s