BGP Session and Address Family Parameters

As I was doing the final integration tests for netlab release 1.9.0, I stumbled upon a fascinating BGP configuration quirk: where do you configure the allowas-in parameter and why?

A Bit of Theory

BGP runs over TCP, and all parameters related to the TCP session are configured for a BGP neighbor (IPv4 or IPv6 address). That includes the source interface, local AS number (it’s advertised in the per-session OPEN message that negotiates the address families), MD5 password (it uses MD5 checksum of TCP packets), GTSM (it uses the IP TTL field), or EBGP multihop (it increases the IP TTL field).

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Arista cEOS Got Working MPLS Data Plane

Urs Baumann brought me a nice surprise last weekend. He opened a GitHub issue saying, “MPLS works on Arista cEOS containers in release 4.31.2F” and asking whether we could enable netlab to configure MPLS on cEOS containers.

netlab already had MPLS configuration templates for Arista EOS but reported an error message if you tried to use MPLS with the cEOS containers because the containers did not have a working MPLS data plane.

After a few configuration tweaks and a batch of integration tests later, I had the results: everything worked. You can use MPLS on Arista cEOS with netlab release 1.9.0 (right now @ 1.9.0-dev2), and I’ll be able to create MPLS labs running in GitHub Codespaces in the not-too-distant future.

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Layer-3-Only EVPN: Behind the Scenes

In the previous blog post, I described how to build a lab to explore the layer-3-only EVPN design and asked you to do that and figure out what’s going on behind the scenes. If you didn’t find time for that, let’s do it together in this blog post. To keep it reasonably short, we’ll focus on the EVPN control plane and leave the exploration of the data-plane data structures for another blog post.

The most important thing to understand when analyzing a layer-3-only EVPN/VXLAN network is that the data plane looks like a VRF-lite design: each VRF uses a hidden VLAN (implemented with VXLAN) as the transport VLAN between the PE devices.

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Response: The Usability of VXLAN

Wes made an interesting comment to the Migrating a Data Center Fabric to VXLAN blog post:

The benefit of VXLAN is mostly scalability, so if your enterprise network is not scaling… just don’t. The migration path from VLANs is to just keep using VLANs. The (vendor-driven) networking industry has a huge blind spot about this.

Paraphrasing the famous Dinesh Dutt’s Autocon1 remark: I couldn’t disagree with you more.

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Migrating a Data Center Fabric to VXLAN

Darko Petrovic made an excellent remark on one of my LinkedIn posts:

The majority of the networks running now in the Enterprise are on traditional VLANs, and the migration paths are limited. Really limited. How will a business transition from traditional to whatever is next?

The only sane choice I found so far in the data center environment (and I know it has been embraced by many organizations facing that conundrum) is to build a parallel fabric (preferably when the organization is doing a server refresh) and connect the new fabric with the old one with a layer-3 link (in the ideal world) or an MLAG link bundle.

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Interesting: Crafting Endless AS Paths in BGP

Vincent Bernat documented a quirk I hope you’ll never see outside of a CCIE lab: combining BGP confederations with AS-override can generate endless AS paths.

I agree entirely with his conclusions (avoid both features). However, I still think that replacing an AS within the confederation part of an AS path (which should belong to a single well-managed AS) is not exactly the most brilliant idea I’ve seen.

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Using netlab Reports

Did you know you can use netlab to generate reports describing your lab topology, IP addressing, BGP details, or OSPF areas? The magic command (netlab report) was introduced in August 2023, followed by netlab show reports to display the available reports a few months later.

You can generate the reports in text, Markdown, or HTML format. The desired format is selected with the report name suffix. For example, the bgp-asn.md report will create Markdown text.

Let’s see how that works.

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BGP, EVPN, VXLAN, or SRv6?

Daniel Dib asked an interesting question on LinkedIn when considering an RT5-only EVPN design:

I’m curious what EVPN provides if all you need is L3. For example, you could run pure L3 BGP fabric if you don’t need VRFs or a limited amount of them. If many VRFs are needed, there is MPLS/VPN, SR-MPLS, and SRv6.

I received a similar question numerous times in my previous life as a consultant. It’s usually caused by vendor marketing polluting PowerPoint slide decks with acronyms without explaining the fundamentals1. Let’s fix that.

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Worth Reading: AI Is Still a Delusion

Here’s another AI rant to spice your summer: AI Is Still a Delusion, including an excellent example of how the latest LLMs flunk simple logical reasoning. I particularly liked this one-line summary:

The real danger today is not that computers are smarter than us but that we think computers are smarter than us and consequently trust them to make decisions they should not be trusted to make.

It might be worth remembering that quote when an AI-powered management appliance messes up your network because of a false positive ;)

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