GRE on Linux: ip tunnel add Commands and Examples
Configure GRE tunnels on Linux with ip tunnel add. Cisco-to-Linux interop, MTU, MSS clamping, IPsec, gretap (L2), and persistence with systemd-network
Go Deeper on Enterprise Networking
In-depth tutorials on OSPF, BGP, 802.1X, and more- for engineers who want to understand the why, not just the how.
OSPF
Open Shortest Path First tutorials covering LSA types, area design, neighbor states, route summarization, authentication, and OSPF tuning for enterprise and service provider networks.
34 posts
BGP
Border Gateway Protocol tutorials covering eBGP, iBGP, path selection, route filtering, communities, and BGP design for enterprise and service provider networks.
35 posts
802.1X
802.1X port-based network access control tutorials covering EAP methods, RADIUS integration, MAB, dynamic VLAN assignment, and Cisco ISE deployment for wired and wireless environments.
32 posts
VLAN
VLAN tutorials covering VLAN creation, 802.1Q trunking, inter-VLAN routing, VTP, private VLANs, and VLAN design for enterprise campus and data center switching environments.
25 posts
Wireless
Wireless networking tutorials covering 802.11 standards, Wi-Fi 6/6E/7, RF fundamentals, WLAN design, roaming, security, and Cisco Catalyst 9800 wireless LAN controller configuration for enterprise Wi-Fi deployments.
45 posts
STP
Spanning Tree Protocol tutorials covering classic STP, RSTP, and MSTP operation, port roles and states, PortFast, BPDU Guard, root bridge election, and loop prevention in switched networks.
27 posts
Latest posts
GRE Tunnel Troubleshooting Guide
Step-by-step GRE tunnel troubleshooting on Cisco IOS XE. Five-minute triage, recursive routing, MTU drops, keepalive flap, debug commands, and EPC cap
mGRE and DMVPN Introduction
Multipoint GRE, NHRP, and IPsec combined to form DMVPN. The three phases, hub and spoke config for Phase 3, verification commands, and design tradeoff
Routing Protocols Over GRE: OSPF, EIGRP, BGP
Run OSPF, EIGRP, and BGP across GRE tunnels on Cisco IOS XE. Recursive routing fix, OSPF network types, EIGRP metric tuning, and BGP update-source.
GRE vs IPsec vs GRE-over-IPsec: Which Tunnel Type?
GRE vs IPsec vs GRE-over-IPsec compared: encryption, multicast, routing protocols, overhead, and a decision framework for site-to-site tunnel design.
GRE MTU and Fragmentation: Fixing Tunnel Packet Loss
GRE tunnel MTU and fragmentation explained: ip mtu 1400, ip tcp adjust-mss 1360, df-bit ping diagnostics, IPv6 considerations, and PMTUD black holes.
GRE Tunnel Keepalives Explained
Cisco GRE keepalives explained: mechanism, configuration, IPsec rekey interactions, vendor interop, and the 30-second failover default that just works
GRE over IPsec: When and How to Combine Them
Configure GRE over IPsec on Cisco IOS XE with IKEv2 and IPsec profiles. Modern and legacy configs, MTU planning, OSPF over the tunnel, and gotchas.
GRE Tunnel Configuration: Step-by-Step Cisco IOS XE Lab
Build a GRE tunnel between two Cisco IOS XE routers step by step: tunnel interface, MTU, keepalives, OSPF over the tunnel, and recursive-routing fix.
IPv6 Header Format Explained
The IPv6 header byte by byte: 40 bytes fixed, 8 fields, extension headers, Next Header chaining, comparison to IPv4, and what it looks like in Wireshark.
IPv6 Configuration on Cisco IOS XE
IPv6 configuration on Cisco IOS XE: enabling IPv6, static and SLAAC addresses, RA options, OSPFv3, BGP for IPv6, EIGRP for IPv6, VRRPv3, dual-stack, and first-hop security.
IPv6 Address Types: Link-Local, Global, Unique Local
The six IPv6 address types: global unicast, link-local, ULA, multicast, anycast, and special. Prefixes, scope, when to use which, and design implications.