Generic Routing Encapsulation, the simple tunneling protocol that sits behind countless enterprise overlays. Articles tagged GRE cover how it works, packet format, Cisco IOS XE configuration, keepalives, MTU, GRE over IPsec, and routing protocol support.
Small pings and HTTP passed, but three 2 MiB downloads stalled in a deliberate GRE/IPsec PMTUD fault. A DF sweep found a 1302-byte inner limit. Measured captures explain why MSS 1360 failed and what MTU 1302 plus MSS 1262 repaired.
IPsec over GRE and GRE over IPsec stack the same two protocols in opposite order, and the order decides what gets encrypted on the wire. This post explains both designs, contrasts them cleanly, and shows when each one belongs.
Configure a manual IPv6-over-IPv4 tunnel on Cisco IOS XE 17.18, prove it with a real ping across an IPv4-only core, and compare the three tunnel types you need to know: manual, GRE, and 6to4.
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.
We broke a healthy Phase 3 DMVPN lab three ways on purpose: a spoke that cannot register, an up/up tunnel eating packets, and shortcuts that never form. Real IOS XE 17.18 output shows each failure, including when the hub logs nothing.
DMVPN sends everything cleartext until you add tunnel protection. This post builds the IKEv2 and IPsec profile stack on two Catalyst 8000v routers running IOS XE 17.18, with every show command captured live.
A DMVPN hub relaying routes between spokes trips over split horizon, next-hop handling, and OSPF network types. Run EIGRP named mode and OSPF point-to-multipoint over the same live Phase 3 lab and compare what lands in the RIB.
A complete DMVPN Phase 3 build: one hub, two spokes, a simulated internet underlay, and EIGRP over the top. Every command and every verification step captured from live IOS XE 17.18 devices.
One DMVPN lab reconfigured live from Phase 1 to Phase 2 to Phase 3 on IOS XE 17.18, with the routing table, NHRP cache, and traceroutes captured at each stop. What actually changes in configuration, routing requirements, and the RIB.
NHRP registration, resolution, and redirects are everything interesting DMVPN does. Walk all three message flows with real debugs and cache states from an IOS XE 17.18 lab with one hub and two spokes.
DMVPN is three well-understood pieces: mGRE, NHRP, and a routing protocol, each covering a gap the others leave. This walkthrough uses real output from an IOS XE 17.18 lab with a hub, two spokes, and an underlay router.
Configure GRE tunnels on Linux with ip tunnel add. Cisco-to-Linux interop, MTU, MSS clamping, IPsec, gretap (L2), and persistence with systemd-network