Routing and Switching
Core concepts
Candidates must be fluent in switching fundamentals (VLANs, trunking, Spanning Tree variants, port security, EtherChannel) and in routing fundamentals for OSPF, EIGRP, and BGP. Know the OSPF neighbor state machine, area types, LSA types, and the order of preference for route selection. For BGP, understand eBGP versus iBGP, next-hop behavior, the best-path algorithm order (weight, local preference, origin, AS path, MED, eBGP over iBGP, IGP metric), and route reflectors. Understand administrative distance and longest-prefix match.
Traps in questions
A frequent trap is a question that mixes redistribution with route selection: redistribution does not override longest-prefix match. Another is assuming that a lower MED wins across autonomous systems without confirming it was received from the same neighbor AS. Watch for STP questions where the root bridge is not the intended device because priority was left at default. Also be careful with OSPF network types, which change DR election and timer behavior.
Worked rule of thumb
When comparing routes, apply the order strictly: longest prefix first, then administrative distance, then protocol-specific metrics. Example: a /24 learned by OSPF (AD 110) and a /25 learned by eBGP (AD 20) both match a destination; the /25 wins because prefix length precedes administrative distance, even though eBGP also has the lower AD. State the order explicitly in the rationale.
Practical guidance
Draw the control plane and data plane separately when troubleshooting, and verify that any filtering (distribute lists, route maps, prefix lists) is applied in the intended direction on the intended neighbor.