network-routing

name	network-routing
description	Routing & connectivity discipline — BGP, OSPF, hybrid links, the tunnel classics. Protocol-level (IETF), vendor syntax from memory-long §stack.

Skill: Network Routing & Connectivity

Protocols are standards; only the CLI is vendor. Complements network-design (topology) — this is how traffic actually finds its way, on-prem, cloud and hybrid.

BGP discipline

1. Never accept or announce routes unfiltered — explicit prefix lists/route maps both directions; a missing filter is how you become a transit AS for the internet (or blackhole production).
2. eBGP between organizations/domains, iBGP (or an IGP) inside · private ASN ranges for internal use, documented in the codemap · prepend/communities/local-pref are the tuning knobs — document intent next to every policy, path selection debugging at 3am is real.
3. Advertise summaries, not host routes — route table size is a shared resource · dampening/max-prefix limits on external sessions (protect yourself from the other side's mistakes).

OSPF discipline

Area 0 is the backbone, everything attaches to it · use stub/NSSA areas to shrink LSA flooding at the edges · cost follows bandwidth policy, set explicitly — default auto-cost on mixed link speeds produces surprise paths · adjacency checklist when it won't form: MTU mismatch, area type mismatch, timers, authentication (in that order).

Hybrid connectivity (cloud ↔ on-prem)

Dedicated link (Direct Connect/ExpressRoute-class) for steady bandwidth + predictable latency; VPN as backup on a different failure domain — both running BGP so failover is routing, not runbooks · test the failover path on schedule (an untested backup link is decoration — same law as backups) · asymmetric routing kills stateful firewalls: keep both directions on the same path or design for it.

The tunnel classics (check before debugging anything else)

MTU/fragmentation — tunnels eat bytes; clamp MSS, test with DF-bit pings · DNS split-horizon: internal names resolve internally on both sides · overlapping CIDRs between sites = NAT hell — prevent via the network-design CIDR plan, not mitigation.

Verification

Routing changes are architectural tier (never fast-path): peer review + maintenance window + rollback plan · before/after route table diff in the report · monitoring on session state and prefix counts (see observability principles).