Ivan Pepelnjak

Author Archives: Ivan Pepelnjak

Duplicate ARP Replies with Anycast Gateways

A reader sent me the following intriguing question:

I’m trying to understand the ARP behavior with SVI interface configured with anycast gateways of leaf switches, and with distributed anycast gateways configured across the leaf nodes in VXLAN scenario.

Without going into too many details, the core dilemma is: will the ARP request get flooded, and will we get multiple ARP replies. As always, the correct answer is “it depends” 🤷‍♂️

Combining BGP and IGP in an Enterprise Network

Syed Khalid Ali left the following question on an old blog post describing the use of IBGP and EBGP in an enterprise network:

From an enterprise customer perspective, should I run iBGP or iBGP+IGP (OSPF/ISIS/EIGRP) or IGP while doing mutual redistribution on the edge routers. I was hoping if you could share some thoughtful insight on when to select one over the another?

We covered tons of relevant details in the January 2022 Design Clinic, here’s the CliffNotes version. Keep in mind that the road to hell (and broken designs) is paved with great recipes and best practices, and that I’m presenting a black-and-white picture because I don’t feel like transcribing the discussion we had into an oversized blog post. People wrote books on this topic; I’m pretty sure you can search for Russ White and find a few of them.

Finally, there’s no good substitute for understanding how things work (which brings me to another webinar ;).

Video: Managed SD-WAN Services

Should service providers offer managed SD-WAN services? According to Betteridge’s law of headlines, the answer is NO, and that’s exactly what I explained in a short video with the same name.

Turns out there’s not much to explain; even with my usual verbosity I was done in five minutes, so you might want to watch SD-WAN Technical Challenges as well.

Both videos are accessible with the free ipSpace.net subscription

Beware: Ansible Reorders List Values in Loops

TL&DR: Ansible might decide to reorder list values in a loop parameter, resulting in unexpected order of execution and (in my case) totally borked device configuration.

A bit of a background first: I’m using an Ansible playbook within netsim-tools to deploy initial device configurations. Among other things, that playbook deploys configuration snippets for numerous configuration modules, and the order of deployment is absolutely crucial. For example, you cannot activate BGP neighbors in Labeled Unicast (BGP-LU) address family (mpls module) before configuring BGP neighbors (bgp module).

BGP Labeled Unicast on Cisco IOS

While researching the BGP RFCs for the Three Dimensions of BGP Address Family Nerd Knobs, I figured out that the BGP Labeled Unicast (BGP-LU, advertising MPLS labels together with BGP prefixes) uses a different address family. So far so good.

Now for the intricate bit: a BGP router might negotiate IPv4 and IPv4-LU address families with a neighbor. Does that mean that it’s advertising every IPv4 prefix twice, once without a label, and once with a label? Should that be the case, how are those prefixes originated and how are they stored in the BGP table?

As always, the correct answer is “it depends”, this time on the network operating system implementation. This blog post describes Cisco IOS behavior, a follow-up one will focus on Arista EOS.

MPLS/LDP Creation Myths

Hannes Gredler wrote an interesting comment to my Segment Routing vs LDP in Hub-and-Spoke Networks blog post:

In 2014 when I did the first prototype implementation of MPLS-SR node labels, I was stunned that just with an incremental add of 500 lines of code to the vanilla IPv4/IPv6 IS-IS codebase I got full any-to-any connectivity, no sync issues, no targeted sessions for R-LFA …. essentially labeled transport comes for free.

Based on that, one has to wonder “why did we take the LDP detour and all the complexity it brings?”. Here’s what Hannes found out:

Automating NSX-T Deployments

Nicholas Michel open-sourced an automation solution (video) that deploys the whole NSX-T infrastructure stack including:

  • NSX-T manager virtual machines
  • NSX-T uplink profiles and IP pools
  • Transport zones and transport nodes (NSX-T modules on ESXi hypervisors)
  • Edge clusters including BGP, EVPN and BFD

Once the infrastructure is set up, his solution uses a Terraform configuration file to deploy multiple tenants: external VLANs, tier-0 gateways, BGP neighbors, tier-1 gateways, and application segments.

While the infrastructure part of his solution might be fully reusable, the tenant deployments definitely aren’t, but they provide a great starting point if you decide to build a fully automated provisioning system.

So-Called Modern VPNs: Marketing and Reality

Someone left a “killer” comment1 after reading the Should We Use LISP blog post. It start with…

I must sadly say that your view on what VPN is all about is pretty rusty and archaic :( Sorry! Modern VPNs are all pub-sub based and are already turning into NaaS.

Nothing new there. I’ve been called old-school guru from an ivory tower when claiming TRILL is the wrong direction and we should use good old layer-3-based design2, but let’s unpack the “pub-sub” bit.

Hub-and-Spoke VPLS: Revenge of LDP

In the Segment Routing vs LDP in Hub-and-Spoke Networks blog post I explained why you could get into interesting scaling issues when running MPLS with LDP in a large hub-and-spoke network, and how you can use Segment Routing (MPLS edition) to simplify your design.

Sample hub-and-spoke network

Sample hub-and-spoke network

Now imagine you’d like to offer VPLS services between hubs and spokes, and happen to be using equipment that uses targeted LDP sessions to signal pseudowires. Guess what happens next…

netsim-tools Release 1.1.4

netsim-tools release 1.1.4 includes a number of seemingly unrelated goodies; here’s the the reasoning (or story) behind some of them:

netlab clab tarball creates a tar package that can be deployed with containerlab without netsim-tools

Julio Perez wanted to create ready-to-use labs running Arista cEOS on containerlab. Requiring the users of his labs to deploy netsim-tools and Ansible just to configure the lab devices is a clear overkill considering the startup-config support in containerlab. What he needed was:

Feedback: Ansible for Networking Engineers

One of ipSpace.net subscribers sent me the following feedback on Ansible for Networking Engineers webinar:

The “Ansible for Network Engineers” webinar is of the highest caliber. I’ve taken Ansible courses with your CCIE peers, and though they are good, I objectively feel, that I get more of a total comprehensive understanding with network automation here at ipSpace. Also, I enjoy your professional care-free tone, and how you pepper humor into the subject matter.

I’ve setup a virtual lab with Ubuntu 18.04 LTS server, and am using both Aruba and Cisco switches/routers. Ansible has lots of nuances that will take me time to fully get a grip-on– but, that’s why I subscribe with the network pros like ipSpace.

Video: Functions-as-a-Service Demo

Serverless computing (marketing term for code running on servers managed by other people) is one of the must-have terms if you’re playing a Buzzword Bingo, but what does it really mean and how does the whole thing work?

Matthias Luft and Florian Barth illustrated the concept during the Introduction to Cloud Computing webinar with a short demo in which they build a simple AWS Lambda function.

You need Free or Standard ipSpace.net Subscription to watch the video.

Should We Use LISP?

LISP started as yet-another ocean-boiling project focused initially on solving the “we use locators as identifiers” mess (not quite), and providing scalable IPv6 connectivity over IPv4-only transport networks by adding another layer of indirection and thus yet again proving RFC 1925 rule 6a. At least those are the diagrams I remember from the early “look at this wonderful tool” presentations explaining for example how Facebook is using LISP to deploy IPv6 (more details in this presentation).

Somehow that use case failed to gain traction and so the pivots1 started explaining how one can use LISP to solve IP mobility or IP multihoming or live VM migration, or to implement IP version of conversational learning in Cisco SD-Access. After a few years of those pivots, I started dismissing LISP with a short “cache-based forwarding never worked well” counterargument.

Segment Routing vs LDP in Hub-and-Spoke Networks

I got an interesting question that nicely illustrates why Segment Routing (the MPLS variant) is so much better than LDP. Imagine a redundant hub-and-spoke network with hundreds of spokes. Let’s settle on 500 spokes – IS-IS supposedly has no problem dealing with a link-state topology of that size.

Let’s further assume that all routers advertise only their loopbacks1 and that we’re using unnumbered hub-to-spoke links to minimize the routing table size. The global routing table thus contains ~500 entries. MPLS forwarding tables (LFIB) contain approximately as many entries as each router assigns a label to every prefix in the routing table2. What about the LDP table (LIB – Label Information Base)?

Flow-Based Packet Forwarding

In the Cache-Based Packet Forwarding blog post I described what happens when someone tries to bypass the complexities of IP routing table lookup with a forwarding cache.

Now imagine you want to implement full-featured fast packet forwarding including ingress- and egress ACL, NAT, QoS… but find the required hardware (TCAM) too expensive. Wouldn’t it be nice if we could send the first packet of every flow to a CPU to figure out what to do with it, and download the results into a high-speed flow cache where they could be used to switch the subsequent packets of the same flow. Welcome to flow-based packet forwarding.