OSPF has a lot of moving parts. The state machine. The LSA types. The area types and what each one filters. The exact words to type when adjacency is stuck
BGP has a lot of moving parts. The state machine. The 13-step path-selection algorithm. The dozen knobs that influence which path wins. The exact words to type when adjacency is
Configure GRE tunnels on Linux with ip tunnel add. Cisco-to-Linux interop, MTU, MSS clamping, IPsec, gretap (L2), and persistence with systemd-network
Step-by-step GRE tunnel troubleshooting on Cisco IOS XE. Five-minute triage, recursive routing, MTU drops, keepalive flap, debug commands, and EPC cap
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
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.
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.
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.
