Category: Tags

high availability

High availability refers to the ability of a system or application to keep running even if there is a problem or failure. This is important because if a system or application goes down, it can cause problems for those who rely on it. To achieve high availability, multiple copies of the system or application are set up in different locations so that if one fails, the others can take over and keep things running smoothly. This helps ensure that the system or application is always available when needed.

ChatGPT explaining application high availability to a high school kid

Before going into the details, it’s worth figuring out what the application (or system) users need as opposed to what they think they need:

Not surprisingly, IT vendors sell magic infrastructure solutions as the high-availability panacea based on the assumption that redundant infrastructure cannot fail. Nothing could be further from the truth:

High Availability Concepts, Technologies, and Solutions

You can use a plethora of approaches depending on your availability targets:

  • Disaster recovery is the right tool for the job if you’re OK with the system being down for a few hours.
  • Automatic restart of application instances combined with disaster recovery is acceptable if you can accept your system to be down ~0.1% of the time (99.9% availability)
  • Availability targets higher than 99.9% can only be reached reliably with proper application design supported by well-designed infrastructure.

I wrote over 130 blog posts on these topics. It would be impossible to list all of them on a single page; major high-availability technologies or concepts thus have dedicated pages:

One of the prerequisites for highly available services is also redundant networking infrastructure:

Regardless of your approach, the only sustainable way to get highly available services is the correct design of the application stack. For more details, watch the Designing Active-Active and Disaster Recovery Data Centers webinar; I also wrote a few blog posts on the topic:

Notable Outages

Finally, here are a few notable outages. TL&DR: it can happen to the big guys and will eventually happen to you.

Other High Availability Blog Posts

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video

We published hundreds of public videos covering dozens of technologies on ipSpace.net. Networking technologies covered in free videos include:

Contents

Artificial Intelligence and Machine Learning

More in the AI/ML in Networking: The Good, the Bad and the Ugly webinar (with more videos coming soon).

Border Gateway Protocol (BGP)

More in the Network Security Fallacies part of the How Networks Really Work webinar and the Internet Routing Security webinar.

Business Aspects of Networking Technologies

More in the Business Aspects of Networking Technologies webinar.

Cloud Networking

Cumulus Linux

Ethernet VPN (EVPN)

FRRouting

IPv6 Security

More in the IPv6 Security webinar.

Kubernetes

More in the Kubernetes Networking Deep Dive webinar (with more videos coming soon).

Leaf-and-Spine Fabrics

Networking Fundamentals

More in the How Networks Really Work webinar (with more videos coming soon).

Networking Labs

More in the Network Automation Tools webinar (with more videos coming soon).

Software-Defined WAN (SD-WAN)

More in Software-Defined WAN (SD-WAN) Overview, Cisco SD-WAN and Business Aspects of Networking Technologies webinars (with more videos coming soon).

Switching and ASICs

Other Videos or Video-Related Blog Posts

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EIGRP

EIGRP was the best choice for an interior gateway protocol in late 1990s – it was fast, efficient, and easy to deploy. OSPF and IS-IS implementations improved in the intervening 30 years, slowly turning EIGRP into a forgotten technology.

On a more serious note, I wouldn’t deploy EIGRP in new network designs for compatibility reasons (no major networking vendor apart from Cisco implemented it), and I’d use BGP in designs where a single router has to deal with hundreds of adjacent routers (the only scenario where EIGRP still outshines OSPF and IS-IS).

While the ultimate sources of EIGRP wisdom remain the EIGRP Network Design Solutions Cisco Press book and RFC 7868, you might want to read these articles and blog posts describing EIGRP implementation details and deployment guidelines.

The Basics

Implementation Details

EIGRP Deployment Scenarios

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DMVPN

DMVPN is an old1 Cisco-proprietary technology that combines NHRP, IPsec, IKEv2 and multipoint GRE tunnels to build dynamically-provisioned multi-access VPNs.

The easiest way to master DMVPN is to watch the ipSpace.net DMVPN webinars, and every now and then someone still finds them somewhat useful:

I also wrote dozens of DMVPN-related blog posts. Hope you’ll enjoy them!

The Basics

DMVPN always relies on a hub-and-spoke topology, but enables direct communication between spokes (Phase-2 DMVPN) and simplified routing with NHRP redirects (Phase-3 DMVPN).

Routing Protocols in DMVPN Networks

Routing protocols face significant challenges in DMVPN networks due to very large number of directly-connected neighbors, with EIGRP faring better than OSPF, and BGP being the only viable solution in deployments with a very large hub-to-spoke ratio.

Typical DMVPN Designs

DMVPN Deployment Guidelines

Integration with Other Network Technologies

DMVPN Alternatives

Quirks and Implementation Details

I wrote numerous blog posts documenting DMVPN quirks while preparing the materials for the DMVPN webinars. Most of these blog posts were written in early 2010s and might no longer be relevant.

  1. As in: created around 2010. For more details, listen to the History of DMVPN with Mike Sullenberger. ↩︎

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