Infrastructure: Routing, Switching, and Core Services
Routing
OSPF questions test neighbor states (down, init, 2-way, exstart, exchange, loading, full), LSA types, area types, and cost. Cost derives from the reference bandwidth divided by interface bandwidth, so a reference bandwidth of 100 Mbps makes Gigabit and faster links tie at cost 1 unless changed. EIGRP uses bandwidth and delay by default; a feasible successor must have a reported distance lower than the successor's feasible distance. BGP path selection order starts with weight, then local preference, then locally originated, then shortest AS path, origin, MED, eBGP over iBGP, oldest route, and lowest router ID.
Switching
Know 802.1D versus RSTP port roles and states, MST region parameters, and the guard features: BPDU guard err-disables a PortFast port that receives a BPDU, root guard blocks superior BPDUs, and loop guard blocks a port that stops receiving BPDUs. EtherChannel requires matching speed, duplex, VLAN, and trunk configuration; LACP uses active/passive and PAgP uses desirable/auto, and on/on skips negotiation entirely.
Services
First-hop redundancy covers HSRP, VRRP, and GLBP. NAT covers static, dynamic, PAT, and static PAT. NTP, DHCP relay, SNMP, syslog, and QoS marking and queuing round out the list, along with wireless fundamentals such as CAPWAP, roaming, and 802.11ax.
Traps
BGP weight is Cisco-proprietary and local only. EIGRP named mode behaves differently from classic mode in configuration, not convergence. A device with a lower router ID does not necessarily win in OSPF DR election when priority differs.
Worked example
If a successor has a feasible distance of 1000 and a neighbor advertises a reported distance of 900, that neighbor becomes a feasible successor because 900 is less than 1000. At 1000 it would not qualify.