NEXT-HOP.DEV

The Notebook

Field Notes

Practical network engineering write-ups, lab builds, automation notes, and reference material organized to be useful again later.

Sorted / Newest First

The path host-1 to LS-1 to SP-1 to LS-3 to host-2, with the VXLAN packet LS-1 builds: outer Ethernet, IP, UDP and VXLAN headers (50 bytes, VNI 10010) in front of host-1’s unchanged 98-byte frame, and BGP EVPN Type 2 and Type 3 routes carried via the spines.
  • Arista

DC Fabric Part 3: Building a Layer 2 EVPN Overlay

host-1 and host-2 share a subnet on different leaves of Part 1’s eBGP fabric. This post builds the VXLAN EVPN overlay that joins them, from VTEP loopbacks and EVPN sessions to the Type 2 and Type 3 routes and the packets on the wire.

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37 min read
host-1 connected to host-2 through VXLAN, the data plane, plus EVPN, the control plane
  • Arista

DC Fabric Part 2: VXLAN EVPN Theory

Part 1’s eBGP fabric can’t connect two hosts in the same subnet on different leaves. This post covers how VXLAN carries their frames across the underlay, how EVPN advertises MACs in BGP, and how symmetric IRB routes between subnets.

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19 min read
  • Arista, BGP

DC Fabric Part 1: Building an eBGP Spine-Leaf Underlay

Build and verify a five-node eBGP spine-leaf underlay using netlab and Arista cEOS, then explore route propagation, routing policy, ECMP forwarding, and path failure recovery.

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36 min read
  • BGP, Cisco

BGP Foundations Part 3: Building and Breaking Every Best-Path Tie

Build and break BGP best-path ties in a seven-router lab, testing Weight, Local Preference, AS_PATH, Origin, MED, eBGP vs. iBGP, IGP cost, multipath, route reflection, and the final tie-breakers that determine which route wins.

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89 min read
  • Arista

Building a Lightweight Network Lab with Netlab and Containerlab on Ubuntu 24.04

Build a lightweight network lab on Ubuntu using netlab, containerlab, Docker, and Cisco IOL for fast, repeatable topology deployment and testing.

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13 min read
  • BGP, Cisco

BGP Foundations Part 2: Following BGP Routes Across Multiple Autonomous Systems

Follow BGP routes across multiple autonomous systems and see how AS_PATH, route propagation, loop prevention, and best-path selection work in a real lab.

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13 min read