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Blog Posts in November 2022

Azure Networking Update (Phase 1)

Last week I completed the first part of the annual Azure Networking update. The Azure Firewall section is already online; hope you’ll find it useful. I already have the materials for the Private Link and Gateway Load Balancer services, but haven’t decided whether to schedule another live session to cover them, or just create a short video.

Then there are a half-dozen smaller things I found while processing a year worth of Azure networking News. You’ll find them (and links to documentation) in New Azure Services and Features document.

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Integrated Routing and Bridging (IRB) Design Models

Imagine you built a layer-2 fabric with tons of VLANs stretched all over the place. Now the users want to exchange traffic between those VLANs, and the obvious question is: which devices should do layer-2 forwarding (bridging) and which ones should do layer-3 forwarding (routing)?

There are four typical designs you can use to solve that challenge:

  • Exchange traffic between VLANs outside of the fabric (edge routing)
  • Route on core switches (centralized routing)
  • Route on ingress (asymmetric IRB)
  • Route on ingress and egress (symmetric IRB)

This blog post is an overview of the design models; we’ll cover each design in a separate blog post.

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Network Automation: a Service Provider Perspective

Antti Ristimäki left an interesting comment on Network Automation Considered Harmful blog post detailing why it’s suboptimal to run manually-configured modern service provider network.


I really don’t see how a network any larger and more complex than a small and simple enterprise or campus network can be developed and engineered in a consistent manner without full automation. At least routing intensive networks might have very complex configurations related to e.g. routing policies and it would be next to impossible to configure them manually, at least without errors and in a consistent way.

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Worth Reading: Another Hugo-Based Blog

Bruno Wollmann migrated his blog post to Hugo/GitHub/CloudFlare (the exact toolchain I’m using for one of my personal web sites) and described his choices and improved user- and author experience.

As I keep telling you, always make sure you own your content. There’s absolutely no reason to publish stuff you spent hours researching and creating on legacy platforms like WordPress, third-party walled gardens like LinkedIn, or “free services” obsessed with gathering visitors' personal data like Medium.

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Multihoming Cannot Be Solved within a Network

Henk made an interesting comment that finally triggered me to organize my thoughts about network-level host multihoming1:

The problems I see with routing are: [hard stuff], host multihoming, [even more hard stuff]. To solve some of those, we should have true identifier/locator separation. Not an after-thought like LISP, but something built into the layer-3 addressing architecture.

Proponents of various clean-slate (RINA) and pimp-my-Internet (LISP) approaches are quick to point out how their solution solves multihoming. I might be missing something, but it seems like that problem cannot be solved within the network.

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BGP in ipSpace.net Design Clinic

The ipSpace.net Design Clinic has been running for a bit over than a year. We covered tons of interesting technologies and design challenges, resulting in over 13 hours of content (so far), including several BGP-related discussions:

All the Design Clinic discussions are available with Standard or Expert ipSpace.net Subscription, and anyone can submit new design/discussion challenges.

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BGP Unnumbered Duct Tape

Every time I mention unnumbered BGP sessions in a webinar, someone inevitably asks “and how exactly does that work?” I always replied “gee, that’s a blog post I should write one of these days,” and although some readers might find it long overdue, here it is ;)

We’ll work with a simple two-router lab with two parallel unnumbered links between them. Both devices will be running Cumulus VX 4.4.0 (FRR 8.4.0 container generates almost identical printouts).

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BGP Route Reflectors in the Forwarding Path

Bela Varkonyi left two intriguing comments on my Leave BGP Next Hops Unchanged on Reflected Routes blog post. Let’s start with:

The original RR design has a lot of limitations. For usual enterprise networks I always suggested to follow the topology with RRs (every interim node is an RR), since this would become the most robust configuration where a link failure would have the less impact.

He’s talking about the extreme case of hierarchical route reflectors, a concept I first encountered when designing a large service provider network. Here’s a simplified conceptual diagram (lines between boxes are physical links as well as IBGP sessions between loopback interfaces):

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Using EVPN/VXLAN with MLAG Clusters

There’s no better way to start this blog post than with a widespread myth: we don’t need MLAG now that most vendors have implemented EVPN multihoming.

TL&DR: This myth is close to the not even wrong category.

As we discussed in the MLAG System Overview blog post, every MLAG implementation needs at least three functional components:

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netlab Release 1.4.0: EVPN Asymmetric IRB, Anycast Gateways, VRRP

The big three features of the netlab release 1.4.0 are:

  • EVPN asymmetric IRB on Arista EOS, Cumulus Linux, Dell OS10, Nokia SR Linux, Nokia SR OS and VyOS
  • Anycast gateway on Arista EOS, Cumulus Linux, Nokia SR OS and Nokia SR Linux
  • VRRP on Arista EOS, Cisco IOSv/CSR, Cisco Nexus OS, Cumulus Linux and Nokia SR OS

We also added tons of new functionality, including:

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Video: EVPN Multihoming Deep Dive

After starting the EVPN multihoming versus MLAG presentation (part of EVPN Deep Dive webinar) with the taxonomy of EVPN-based multihoming, Lukas Krattiger did a deep dive into its intricacies including:

  • EVPN route types needed to support multihoming
  • A typical sequence of EVPN updates during multihoming setup
  • MAC multipathing, MAC aliasing, split horizon and mass withdrawals
  • Designated forwarder election
You need Free ipSpace.net Subscription to watch the video. To watch the whole webinar, buy Standard or Expert ipSpace.net Subscription.
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Rant: Cloudy Snowflakes

I could spend days writing riffs on some of the more creative (in whatever dimension) comments left on my blog post or LinkedIn1. Here’s one about uselessness of network automation in cloud infrastructure (take that, AWS!):

If the problem is well known you can apply rules to it (automation). The problem with networking is that it results in a huge number of cases that are not known in advance. And I don’t mean only the stuff you add/remove to fix operational problems. A friend in one of the biggest private clouds was saying that more than 50% of transport services are customized (a static route here, a PBR there etc) or require customization during their lifecycle (e.g. add/remove a knob). Telcos are “worse” and for good reasons.

Yeah, I’ve seen such environments. I had discussions with a wide plethora of people building private and public (telco) clouds, and summarized the few things I learned (not many of them good) in Address the Business Challenges First part of the Business Aspects of Networking Technologies webinar.

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Scalability Aspects of SR-MPLS

Henk Smit left a wonderful comment discussing various scalability aspects of SR-MPLS. Let’s go through the points he made:

When you have a thousand routers in your networks, you can put all of them in one (IS-IS) area. Maybe with 2k routers as well. But when you have several thousand routers, you want to use areas, if only to limit the blast-radius.

Absolutely agree, and as RFC 3439 explained in more eloquent terms than I ever could:

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