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
GRE
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.
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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 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.