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.
The field guide for GRE failures on Cisco IOS XE, rebuilt around a real CML capture of a recursive-routing flap. The %ADJ-5-PARENT looped chain that lands just before %TUN-5-RECURDOWN, the administrative distance comparison that causes it, and three fixes: a static /32, filtering the transport...
Multipoint GRE, NHRP, and IPsec combined to form DMVPN. The three phases, hub and spoke config for Phase 3, verification commands, and design tradeoff
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 compared: encryption, multicast, routing protocols, overhead, and a decision framework for site-to-site tunnel design.
GRE tunnel MTU and fragmentation explained: ip mtu 1400, ip tcp adjust-mss 1360, df-bit ping diagnostics, IPv6 considerations, and PMTUD black holes.
Cisco GRE keepalives explained: mechanism, configuration, IPsec rekey interactions, vendor interop, and the 30-second failover default that just works
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.