Network Security

The default Layer 2 posture is wide open. This checklist ties every switch-hardening control to the specific attack it stops, with live CML proof where possible: 8 spoofed ARPs dropped, an intra-VLAN ping killed, and more.

Private VLANs isolate hosts that share a subnet, at Layer 2, without a subnet per host. Primary, isolated, and community secondary VLANs on Cisco IOS XE, with the classic DMZ use case and real show output.

A router ACL only sees routed traffic, so it can never filter two hosts in the same VLAN. A VACL can. Real CML before/after: intra-VLAN ping goes 0% to 100% loss while the gateway ping still works.

DAI needs DHCP snooping bindings to validate ARP. On a static-IP segment there are none, so you supply the legitimate IP-to-MAC pairs with a static ARP ACL. Proven in CML: 8 spoofed ARPs dropped.

Harden a Cisco router by disabling unused services, proven with a real before-and-after Nmap scan taking Telnet from open to closed, plus syslog design.

Configure authenticated NTP on Cisco IOS XE, why IOS flags MD5 as weak, and how ntp authenticate and trusted-key stop a rogue server moving your clock.