Category: networking fundamentals
Feedback: How Networks Really Work
A few weeks ago I asked my subscribers which webinar they’d like to see in November (thanks a million to everyone who replied!). Not surprisingly, network automation got the top spot, but I was a bit sad to see my long-term pet project at the bottom of the list:
Graceful Restart and BFD
The whole High Availability Switching series started with a question along the lines of “does it make sense to run BFD together with Graceful Restart”. After Non-Stop Forwarding 101, Graceful Restart 101, and Graceful Restart and Convergence Speed we finally have enough information to answer that question.
TL&DR: Most probably not.
A more nuanced answer depends (as always) on a gazillion implementation details.
Lessons Learned: Complexity Will Kill Your System
You wouldn’t believe the intricate network designs I created decades ago until I learned that having an uninterrupted sleep is worth more than proving I can get the impossible to work (see also: using EBGP instead of IGP in a 4-node data center fabric).
Once I started valuing my free time, I tried to design things to be as simple as possible. However, as my friend Nicola Modena once said, “Consultants must propose new technologies because they must be seen as bringing innovation,” and we all know complexity sells. Go figure.
Graceful Restart and Routing Protocol Convergence
I’m always amazed when I encounter networking engineers who want to have a fast-converging network using Non-Stop Forwarding (which implies Graceful Restart). It’s even worse than asking for smooth-running heptagonal wheels.
As we discussed in the Fast Failover series, any decent router uses a variety of mechanisms to detect adjacent device failure:
- Physical link failure;
- Routing protocol timeouts;
- Next-hop liveliness checks (BFD, CFM…)
Video: Theoretical View of Network Addressing
After explaining the basics of (network) names, addresses and routes, I wasted a few minutes of everyone’s time discussing the theoretical aspects of layered addressing, and then got back to practical issues like address scopes, namespaces, and address provisioning.
The video ends with a simple (and unappreciated) truth: if you have a point-to-point link between two nodes you don’t need data-link-layer addresses. The consequences of that fact are left as an exercise for the viewer (or you can wait till the next video ;)
Graceful Restart and Other Control Plane Protocols
In the Graceful Restart 101 blog post, I promised to discuss the ugly parts of this concept in a follow-up post. It turns out we’ll need more than one; today, we’ll focus on other control plane protocols in an access network scenario.
Imagine an access router with multiple uplinks serving a bunch of non-redundantly-connected customers:
Graceful Restart (GR) 101
In the Non-Stop Forwarding (NSF) article, I mentioned that the routers adjacent to the device using NSF have to play along to make the idea work. That capability is called Graceful Restart. Today we’ll explore its intricate details, be diplomatic, and leave the shortcomings and tradeoffs for the next blog post.
Imagine an access (provider edge) router providing connectivity services to its clients and running a routing protocol with one or more upstream devices.
Stateful Switchover (SSO) 101
Stateful Switchover (SSO) is another seemingly awesome technology that can help you implement high availability when facing a
broken non-redundant network design. Here’s how it’s supposed to work:
- A network device runs two copies of the control plane (primary and backup);
- Primary control plane continuously synchronizes its state with the backup control plane;
- When the primary control plane crashes, the backup control plane already has all the required state and is ready to take over in moments.
Delighted? You might be disappointed once you start digging into the details.
Lessons Learned: Fundamentals Haven't Changed
Here’s another bitter pill to swallow if you desperately want to believe in the magic powers of unicorn dust: laws of physics and networking fundamentals haven’t changed (see also: RFC 1925 Rule 11).
Whenever someone is promising a miracle solution, it’s probably due to them working in marketing or having no clue what they’re talking about (or both)… or it might be another case of adding another layer of abstraction and pretending the problems disappeared because you can’t see them anymore.
Non-Stop Forwarding (NSF) 101
Non-Stop Forwarding (NSF) is one of those ideas that look great in a slide deck and marketing collaterals, but might turn into a giant can of worms once you try to implement them properly (see also: stackable switches or VMware Fault Tolerance).
Video: Introduction to Network Addressing
A friend of mine pointed out this quote by John Shoch when I started preparing the Network Stack Addressing slide deck for my How Networks Really Work webinar:
The name of a resource indicates what we seek, an address indicates where it is, and a route tells us how to get there.
You might wonder when that document was written… it’s from January 1978. They got it absolutely right 42 years ago, and we completely messed it up in the meantime with the crazy ideas of making IP addresses resource identifiers.
Video: Typical Large-Scale Bridging Use Cases
In the previous video in the Switching, Routing and Bridging section of How Networks Really Work webinar we compared transparent bridging with IP routing. Not surprisingly (given my well-known bias toward stable solutions) I recommended using IP routing as much as possible, but there are still people out there pushing large-scale transparent bridging solutions.
In today’s video we’ll look at some of the supposed use cases and stable solutions you could use instead of stretching a virtual thick yellow cable halfway across a continent.
Video: Comparing Routing and Bridging
After covering the basics of transparent Ethernet bridging and IP routing, we’re finally ready to compare the two. Enjoy the ride ;)
Lessons Learned: Technology Still Matters
In June 2020, a friend of mine asked me to do a short presentation on lessons learned during my 35 years of being a networking engineer. It went reasonably well, so I decided to turn it into a webinar, starting with regardless of what the disruptive marketers tell you, technology still matters.
… updated on Monday, July 12, 2021 18:00 UTC
Unnumbered Ethernet Interfaces, DHCP Edition
Last week we explored the basics of unnumbered IPv4 Ethernet interfaces, and how you could use them to save IPv4 address space in routed access networks. I also mentioned that you could simplify the head-end router configuration if you’re using DHCP instead of per-host static routes.
Obviously you’d need a smart DHCP server/relay implementation to make this work. Simplistic local DHCP server would allocate an IP address to a client requesting one, send a response and move on. Likewise, a DHCP relay would forward a DHCP request to a remote DHCP server (adding enough information to allow the DHCP server to select the desired DHCP pool) and forward its response to the client.