Relationships between Layer-2 (VLAN) and Layer-3 (Subnet) Segments

Sometimes it takes me years to answer interesting questions, like the one I got in a tweet in 2021:

Do you have a good article describing the one-to-one relation of layer-2 and layer-3 networks? Why should every VLAN contain one single L3 segment?

There is no mandatory relationship between multi-access layer-2 networks and layer-3 segments, and secondary IP addresses (and subnets) were available in Cisco IOS in early 1990s. The rules-of-thumb1 claiming there should be a 1:1 relationship usually derive from the oft-forgotten underlying requirements. Let’s start with those.

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Network Automation Expert Beginners

Some network automation skeptics came to that place the hard way: they got burned by half-baked semi-tested systems. This is what one of my good friends had to say in a LinkedIn comment:

I am suspicious of automation, as I’ve unfortunately seen too many outages caused by either human error or faulty automation. Every time it required human CLI/GUI intervention to correct it. The problem is that the more automation we push, the fewer people know how to use the “old school” way to administer stuff.

Network automation is not the only IT discipline that could cause hard-to-correct errors requiring manual intervention. I’m positive everyone knows at least one horror story resulting in manual tweaking of the Windows registry, or a sequence of arcane SQL commands1.

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netlab Release 1.4.3: Cisco IOS XRv, MPLS on FRR

I had tons of plans to implement new netlab features during the last week of December, but then (fortunately) reality intervened and I spent my time relaxing and enjoying the break. I still managed to add IOS XRv support to netlab release 1.4.3 though ;). Other new features include:

To upgrade, execute pip3 install --upgrade networklab.

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Worth Reading: Routing Protocol Implementation Evaluation

In 2018 I tried to figure out whether the rush to deploy new routing protocols in leaf-and-spine fabrics is anything more than another blob of hype (RIFT, OpenFabric, BGP), considering OSPF got the job done for AWS. Those discussions probably sounded like a bunch of smart kids trying to measure outside temperature with a moist finger, so the only recommendation I could give in 2021 was “use the best tool for the job, keeping in mind you’re not Google or Microsoft

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State of LDPv6 and 6PE

One of my readers successfully deployed LDPv6 in their production network:

We are using LDPv6 since we started using MPLS with IPv6 because I was used to OSPF/OSPFv3 in dual-stack deployments, and it simply worked.

Not everyone seems to be sharing his enthusiasm:

Now some consultants tell me that they know no-one else that is using LDPv6. According to them “everyone” is using 6PE and the future of LDPv6 is not certain.

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Updates: AWS Networking

It didn’t make sense to update Amazon Web Services Networking webinar before the re:Invent conference – even though AWS introduced only a few networking features during the conference, at least one of them made a significant impact on the materials.

However, once the conference was over, I went over the to-do list that has been slowly accumulating for months and spent days updating over a dozen videos1. The major changes include:

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