CompTIA Exam Prep - ITF+, A+, Network+, Security+, CySA+

Fibre carries light rather than electricity, which removes the distance limits, the interference problems and the power issues of copper — and introduces a set of failure modes of its own, almost all of which come down to the connector end faces. Single mode and multimode The fundamental split. Single mode has a very small core, around nine microns, so light travels essentially one path. That eliminates modal dispersion and allows distances measured in tens of kilometres. It uses laser sources and is the choice for anything long. Multimode has a larger core, fifty or 62.5 microns, so light takes multiple paths that arrive at slightly different times. That dispersion limits distance to hundreds of metres, and the larger core makes alignment easier and the optics cheaper. It dominates inside data centres and buildings. Multimode is graded — OM1 through OM5 — with higher grades supporting greater distance at a given speed. The practical point is that grade and spee...

A power distribution unit takes one high-capacity feed into a rack and splits it across outlets for the equipment inside. A managed PDU adds a network interface, so it can be monitored and controlled remotely. That one addition turns a power strip into an availability tool and, less obviously, into a device that needs securing. The four types Basic PDUs distribute power and nothing else. No network port, no display, no data. They are cheap and appropriate where power is monitored upstream at the panel. Metered PDUs measure current draw and report it, either on a local display or over the network. This is what lets you answer "how much headroom is left in this rack" before installing another server, instead of finding out when a breaker trips. Switched PDUs add remote control of individual outlets. You can power cycle one device without touching anything else, and you can define which outlets energize in what order when power returns after an outage. Switched and met...

This article assumes you know what VXLAN is — the encapsulation, the 24-bit VNI, the VTEP endpoints. If those are unfamiliar, start with VXLAN fundamentals . What follows is the operational layer: how the fabric learns where things are, where the tunnel endpoints should live, and what breaks in practice. Flood-and-learn, and why it was replaced The original VXLAN specification had no control plane. A VTEP learned remote MAC addresses the way a switch always has — by flooding unknown traffic and observing the replies — with multicast in the underlay used to deliver broadcast, unknown unicast, and multicast traffic to every VTEP in the segment. Two problems followed. Multicast in the underlay is operationally unpopular, requiring rendezvous points and a level of multicast expertise many teams do not have. And flooding scales badly: as tenants and endpoints grow, broadcast traffic replicated to every VTEP consumes an increasing share of the fabric. Head-end replicati...

VXLAN — Virtual Extensible LAN — exists because traditional VLANs ran out of room. The VLAN ID field in the 802.1Q header is 12 bits, which allows 4,094 usable VLANs . That was generous in 1998 and is nowhere near enough for a modern multi-tenant data centre. VXLAN uses a 24-bit identifier instead of 12, raising the ceiling from about 4,000 segments to roughly 16 million — and it carries layer 2 traffic across a layer 3 network. Those two properties are the whole exam answer, and the second one is arguably more important than the first. The Problems VXLAN Solves Scale. 4,094 VLANs is a hard limit. A cloud provider with thousands of customers, each needing several isolated segments, exhausts it immediately. Layer 2 across layer 3. Traditional VLANs cannot cross a router. But virtual machine live migration generally requires the VM to keep its IP address, which means the destination host must be in the same layer 2 domain. Stretching VLANs across a data centre with large sp...