If you only learn one Cisco ASA troubleshooting command, make it packet-tracer. It simulates a single hypothetical packet through every ASA decision point - existing-connection lookup, security check, NAT untranslate, ACL, NAT translate, route lookup, egress checks - and tells
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 in EXSTART. The PingLabz OSPF Field Reference puts all of
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 stuck in OpenSent. The PingLabz BGP Field Reference
Configure GRE tunnels on Linux with ip tunnel add. Cisco-to-Linux interop, MTU, MSS clamping, IPsec, gretap (L2), and persistence with systemd-network
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.
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.