Network Infrastructure
Interior gateway protocols
For OSPF, know the DR/BDR election precisely: highest interface priority wins, ties break on highest router ID, and there is no preemption - a newly arrived router with a better priority will not take the DR role until the existing DR fails or the interface/process is reset. Priority 0 makes an interface permanently ineligible. Cost defaults to reference bandwidth (100 Mbps) divided by interface bandwidth unless auto-cost reference-bandwidth is changed consistently on every router - the classic trap being that 10G, 40G and 100G links all show cost 1 under the default. Also be fluent in area types, LSA types and where summarisation is allowed (ABR for inter-area, ASBR for external).
BGP path selection
Memorise the order and, more importantly, where each attribute is applied: weight (Cisco-local, highest wins) → local preference (AS-wide, highest) → locally originated → shortest AS path → origin (IGP < EGP < incomplete) → MED (lowest; compared only among paths from the same neighbouring AS unless bgp always-compare-med) → eBGP over iBGP → lowest IGP metric to next hop → oldest route → lowest router ID → lowest neighbour address.
Switching
Root bridge election uses lowest priority then lowest MAC. Distinguish the guards: BPDU Guard err-disables a port that receives BPDUs, Root Guard blocks superior BPDUs on a designated port, Loop Guard stops a blocked port from moving to forwarding when BPDUs stop. EtherChannel members must agree on speed, duplex, trunk/access mode and allowed VLAN list, or the bundle will not form.
Worked rule of thumb
When a question asks which BGP path is best, list the attributes in order and stop at the first one that differs. If one path carries a weight that the others lack, weight has already decided the answer - do not continue to AS-path length.