Network Services and Applications
Core concepts
Know how DHCP works (DORA, relay behavior, option 82), DNS resolution paths and record types (A, AAAA, CNAME, MX, TXT, PTR), and how NAT and PAT affect application reachability. Understand QoS building blocks: classification, marking, queuing, shaping, policing, and the behavior of priority queues. Be able to explain load balancing modes, health checks, and session persistence, plus the role of NTP, syslog, and AAA services. Content delivery, proxy versus reverse proxy, and API gateway concepts may appear.
Traps in questions
Questions often confuse DNS caching with authoritative data, or ask why a change propagated slowly when TTLs were long. Another trap is QoS: marking alone does not guarantee treatment, and policing drops excess while shaping buffers it. Watch for NAT questions where an application embeds IP addresses in payloads, breaking without an application-layer gateway. Also distinguish a forward proxy (client-side) from a reverse proxy (server-side).
Worked rule of thumb
Locate the failure by asking which service must answer first. Example: a client receives an address but cannot reach an internal site by name while IP connectivity works. Test DNS directly: if resolution fails, check the configured DNS server and relay path before touching routing. If resolution succeeds but the connection fails, move to transport and firewall checks. This ordering avoids chasing routing for a naming problem.
Practical guidance
Document service dependencies (DHCP before DNS before application), set TTLs deliberately before migrations, and verify QoS end to end because a single unmarked hop can erase the intended treatment.