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Category: IP routing

Fast Failover: The Challenge

Sometimes you’re asked to design a network that will reroute around a failure in milliseconds. Is that feasible? Maybe. Is it simple? Absolutely not.

In this series of blog posts we’ll start with the basics, explore the technologies that you can use to reach that goal, and discover one or two unexpected rabbit holes.

Fast failover is just one of the topics we’ll discuss in Advanced Routing Protocol Features part of How Networks Really Work webinar.
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Do We Need LFA or FRR for Fast Failover in ECMP Designs?

One of my readers sent me a question along these lines:

Imagine you have a router with four equal-cost paths to prefix X, two toward upstream-1 and two toward upstream-2. Now let’s suppose that one of those links goes down and you want to have link protection. Do I really need Loop-Free Alternate (LFA) or MPLS Fast Reroute (FRR) to get fast (= immediate) failover or could I rely on multiple equal-cost paths to get the job done? I’m getting different answers from different vendors…

Please note that we’re talking about a very specific question of whether in scenarios with equal-cost layer-3 paths the hardware forwarding data structures get adjusted automatically on link failure (without CPU reprogramming them), and whether LFA needs to be configured to make the adjustment happen.

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Grasp the Fundamentals before Spreading Opinions

I should have known better, but I got pulled into another stretched VLANs for disaster recovery tweetfest. Surprisingly, most of the tweets were along the lines of you really shouldn’t be doing that and that would never work well, but then I guess I was only exposed to a small curated bubble of common sense… until this gem appeared in my timeline:

Networking Needs ZIP codes

Interestingly, that’s exactly how IP works:

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Weird: Wrong Subnet Mask Causing Unicast Flooding

When I still cared about CCIE certification, I was always tripped up by the weird scenario with (A) mismatched ARP and MAC timeouts and (B) default gateway outside of the forwarding path. When done just right you could get persistent unicast flooding, and I’ve met someone who reported average unicast flooding reaching ~1 Gbps in his data center fabric.

One would hope that we wouldn’t experience similar problems in modern leaf-and-spine fabrics, but one of my readers managed to reproduce the problem within a single subnet in FabricPath with anycast gateway on spine switches when someone misconfigured a subnet mask in one of the servers.

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Musings on IP Packet Reordering

During the Comparing Transparent Bridging and IP Routing part of How Networks Really Work webinar I said something along the lines of:

While packets should never be reordered in transit in transparent bridging, there’s no such guarantee in IP networks, and IP applications should tolerate out-of-order packets.

One of my regular readers who designs and builds networks supporting VoIP applications disagreed with that citing numerous real-life examples.

Of course he was right, but let’s get the facts straight first:

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Can We Trust BGP Next Hops (Part 2)?

Two weeks ago I started with a seemingly simple question:

If a BGP speaker R is advertising a prefix A with next hop N, how does the network know that N is actually alive and can be used to reach A?

… and answered it for the case of directly-connected BGP neighbors (TL&DR: Hope for the best).

Jeff Tantsura provided an EVPN perspective, starting with “the common non-arguable logic is reachability != functionality”.

Now let’s see what happens when we add route reflectors to the mix. Here’s a simple scenario:

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Can We Trust BGP Next Hops (Part 1)?

Aldrin sent me an interesting question as a comment to one of my EVPN blog posts:

How does the network know that a VTEP is actually alive? (1) from the point of view of the control plane and (2) from the point of view of the data plane? And how do you ensure that control and data plane liveness monitoring has the same view? BFD for BGP is a possible solution for (1) but it’s not meant for 3rd party next hops, i.e. it doesn’t address (2).

Let’s stop right there (or you’ll stop reading in the next 10 milliseconds). I will also try to rephrase the question in more generic terms, hoping Aldrin won’t mind a slight detour… we’ll get back to the original question in another blog post.

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The First Networking Fundamentals Videos are Online

In mid-June I started another pet project - a series of webinars focused on networking fundamentals. In the first live session on June 18th we focused on identifying the challenges one has to solve when building an end-to-end networking solution, and the role of layered approach to networking.

Not surprisingly, we quickly went down the rabbit holes of computer networking history, including SCSI cables, serial connections and modems… but that’s where it all started, and some of the concepts developed at that time are still used today… oftentimes heavily morphed by recursive application of RFC 1925 Rule 11.

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If You Worry About 768K Day, You’re Probably Doing Something Wrong

A few years ago we “celebrated” 512K day - the size of the full Internet routing table exceeded 512K (for whatever value of K ;) prefixes, overflowing TCAMs in some IP routers and resulting in interesting brownouts.

We’re close to exceeding 768K mark and the beware 768K day blog posts have already started appearing. While you (RFC 2119) SHOULD check the size of your forwarding table and the maximum capabilities of your hardware, the more important question should be “Why do I need 768K forwarding entries if I’m not a Tier-1 provider

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