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Category: networking fundamentals

Response: Is Switching Latency Relevant?

Minh Ha left another extensive comment on my Is Switching Latency Relevant blog post. As is usual the case, it’s well worth reading, so I’m making sure it doesn’t stay in the small print (this time interspersed with a few comments of mine in gray boxes)

I found Cisco apparently manages to scale port-to-port latency down to 250ns for L3 switching, which is astonishing, and way less (sub 100ns) for L1 and L2.

I don’t know where FPGA fits into this ultra low-latency picture, because FPGA, compared to ASIC, is bigger, and a few times slower, due to the use of Lookup Table in place of gate arrays, and programmable interconnects.

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Fundamentals: Is Switching Latency Relevant?

One of my readers wondered whether it makes sense to buy low-latency switches from Cisco or Juniper instead of switches based on merchant silicon like Trident-3 or Jericho (regardless of whether they are running NX-OS, Junos, EOS, or Linux).

As always, the answer is it depends, but before getting into the details, let’s revisit what latency really is. We’ll start with a simple two-node network.

The simplest possible network

The simplest possible network

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Video: Path Discovery in Transparent Bridging and Routing

In the previous video in this series, I described how path discovery works in source routing and virtual circuit environments. I couldn’t squeeze the discussion of hop-by-hop forwarding into the same video (it would make the video way too long); you’ll find it in the next video in the same section.

The video is part of How Networks Really Work webinar and available with Free Subscription.
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Video: High-Level Technology Guidelines

I concluded the Focus on Business Challenges First presentation (part of Business Aspects of Networking Technologies webinar) with a few technology guidelines starting with:

  • Be vendor-agnostic (always look around to see what others are doing);
  • Try to understand how the technology you’re evaluating really works (it will help you spot the potential problems before they crash your network);
  • Always select what’s best for your business, not for the sales quota of your friendly $vendor account manager.

For more guidelines, watch the video (available with Free Subscription).

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Link-State Routing Protocols Are Eventually Consistent

One of my readers sent me this interesting question:

Assuming we are running a very large OSPF area with a few thousand nodes. If we follow the chain reaction of OSPF LSA flooding while the network is converging at the same time, how would all routers come to know that they all now have same view of area link states and there are no further updates or convergence?

I have bad news: the design requirements for link state protocols effectively prevent that idea from ever working well.

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OMG, It's Graphs Everywhere

One of the subscribers watching the Graph Algorithms in Networks webinar found the webinar had an interesting impact on his perspective (according to his feedback):

This is genuine content that I haven’t seen anywhere else. It helps to get up to speed on computer science topics that are relevant to network professionals. After attending this webinar, I couldn’t unsee the graphs anymore that are almost everywhere in networking.

This webinar is available with Standard Subscription as are other webinars by Rachel Traylor including Network Connectivity, Graph Theory, and Reliable Network Design, Queuing Theory and Reliability Theory: Networking through a Systems Analysis Lens. She’ll be back next week starting a series of deep dives into reliability topics. Hope you’ll enjoy them as much as our subscriber did the Graph Algorithms webinar.

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Video: Finding Paths Across the Network

Regardless of the technology used to get packets across the network, someone has to know how to get from sender to receiver(s), and as always you have multiple options:

  • Almighty controller
  • On-demand dynamic path discovery (example: probing)
  • Participation in a routing protocol

For more details, watch Finding Paths Across the Network video.

The video is part of How Networks Really Work webinar and available with Free Subscription.
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Video: Multi-Layer Switching and Tunneling

After discussing the technology options one has when trying to get a packet across the network, we dived deep into two interesting topics:

  • How do you combine packet forwarding at multiple layers of OSI stack (multi-layer switching)?
  • What happens when you do layer-N forwarding over layer-M transport core where N <= M (example: IPv6 packets over IPv4 packets) aka tunneling?

You’ll find more details (including other hybrids like Loose Source Routing) in Multi-Layer Switching and Tunneling video.

The video is part of How Networks Really Work webinar and available with Free Subscription.
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IBGP, IGP Metrics, and Administrative Distances

TL&DR: If you run multiple IGP protocols in your network, and add BGP on top of that, you might get the results you deserve. Even better, the results are platform-dependent.

One of my readers sent me a link to an interesting scenario described by Jeremy Filliben that results in totally unexpected behavior when using too many routing protocols in your network (no surprise there).

Imagine a network in which two edge routers advertise the same (external) BGP prefix. All other things being equal, it would make sense that other routers in the same autonomous system should use the better path out of the autonomous system. Welcome to the final tie-breaker in BGP route selection process: IGP metric.

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What Exactly Happens after a Link Failure?

Imagine the following network running OSPF as the routing protocol. PE1–P1–PE2 is the primary path and PE1–P2–PE2 is the backup path. What happens on PE1 when the PE1–P1 link fails? What happens on PE2?

Sample 4-router network with a primary and a backup path

Sample 4-router network with a primary and a backup path

The second question is much easier to answer, and the answer is totally unambiguous as it only involves OSPF:

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Learning Networking Fundamentals at University?

One of my readers sent me this interesting question:

It begs the question in how far graduated students with a degree in computer science or applied IT infrastructure courses (on university or college level or equivalent) are actually aware of networking fundamentals. I work for a vendor independent networking firm and a lot of my new colleagues are college graduates. Positively, they are very well versed in automation, scripting and other programming skills, but I never asked them what actually happens when a packet traverses a network. I wonder what the result would be…

I can tell you what the result would be in my days: blank stares and confusion. I “enjoyed” a half-year course in computer networking that focused exclusively on history of networking and academic view of layering, and whatever I know about networking I learned after finishing my studies.

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Video: Should You Build or Buy a Solution?

After figuring out what business problem you’re trying to solve and what the users expect to get from you it’s time for the next crucial question: should you buy a shrink-wrapped product/solution or build your own? I addressed that question in the third part of Focus on Business Challenges First presentation.

Not surprisingly, the same dilemma applies to network automation solutions, and is often the source of endless time-wasting discussions that I really should have stopped engaging in, but sometimes duty calls ;)

The video is part of Business Aspects of Networking Technologies webinar and available with Free Subscription.
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Chasing CRC Errors in a Data Center Fabric

One of my readers encountered an interesting problem when upgrading a data center fabric to 100 Gbps leaf-to-spine links:

  • They installed new fiber cables and SFPs;
  • Everything looked great… until someone started complaining about application performance problems.
  • Nothing else has changed, so the culprit must have been the network upgrade.
  • A closer look at monitoring data revealed CRC errors on every leaf switch. Obviously something was badly wrong with the whole batch of SFPs.

Fortunately my reader took a closer look at the data before they requested a wholesale replacement… and spotted an interesting pattern:

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Video: Know Your Users' Needs

After explaining why you should focus on defining the problem before searching for a magic technology that will solve it, I continued the Focus on Business Challenges First presentation with another set of seemingly simple questions:

  • Who are your users/customers?
  • What do they really need?
  • Assuming you’re a service provider, what are you able to sell to your customers… and how are you different from your competitors?
The video is part of Business Aspects of Networking Technologies webinar and available with Free Subscription.
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