3Infrastructure

• 3.1Troubleshooting 802.1Q trunking

  Covers how 802.1Q tagging multiplexes VLANs, how the native VLAN (untagged) is handled, the difference between dynamic trunking via DTP (dynamic auto / dynamic desirable) and static trunking, and pruning with the allowed VLAN list, building a procedure to isolate causes from symptoms such as "the trunk will not come up," "only one VLAN fails," and "VLAN leaking."

• 3.2Troubleshooting EtherChannel

  Covers EtherChannel, which bundles multiple physical links into one logical link: the mode combinations of LACP (active/passive), PAgP (desirable/auto), and on (static); bundling failures caused by mismatched bundling conditions such as speed, duplex, trunk mode, and allowed VLANs; reading the flags in show etherchannel summary; and choosing the load-balancing hash—all framed as diagnosis.

• 3.3Spanning tree (RSTP, MST, and guard features)

  Covers RSTP port roles (root, designated, alternate, backup) and states with its convergence in seconds; MST, which maps many VLANs onto a few instances for scalability; and the topology-protecting features root guard (a superior BPDU puts the port in root-inconsistent) and BPDU guard (a PortFast port receiving a BPDU goes err-disabled), understood through real fault scenarios.

• 3.4Comparing EIGRP and OSPF routing concepts

  Compares EIGRP (advanced distance vector, DUAL, a bandwidth-and-delay metric, unequal-cost load balancing via variance, AD 90) with OSPF (link-state, SPF, cost = 10^8 / bandwidth, equal-cost-only load balancing, AD 110, and area types), framed as the judgment of "which fits this requirement" and "why was this path chosen."

• 3.5Configuring OSPFv2 and OSPFv3

  Covers designing multiple normal areas, summarization and filtering at the ABR, the conditions for adjacency (area ID, timers, authentication, MTU, subnet mask, network type), the difference in DR/BDR election between point-to-point and broadcast networks, and the true meaning of passive-interface, all as diagnosis of "the adjacency will not come up" and "the route is not advertised."

• 3.6eBGP and policy-based routing

  Covers eBGP between autonomous systems (AD 20, TCP 179, directly connected neighbors), the conditions for establishing a session, the order of the best-path selection algorithm (weight -> LOCAL_PREF -> locally originated -> shortest AS_PATH -> ORIGIN -> MED -> eBGP over iBGP -> IGP metric), and PBR (policy-based routing), which overrides routing on criteria other than destination—framed as "why was this path chosen" and "which attribute should I touch."

• 3.7Time synchronization and address translation

  Covers the NTP hierarchy expressed by stratum and how to read its synchronization state, when to use the higher-precision PTP (IEEE 1588) instead, and NAT/PAT with its four address types (inside local, inside global, outside global, outside local) plus multiplexing via overload—framed as diagnosis of faults such as "log timestamps are skewed so events cannot be correlated" and "no translation is happening."

• 3.8FHRP and multicast

  Compares the FHRP options for gateway redundancy—HSRP (Cisco, active/standby, virtual MAC 0000.0c07.acXX, priority default 100, preempt disabled by default) and VRRP (standard, master/backup, preempt enabled by default)—and covers the multicast building blocks RPF check, PIM SM (RP shared tree, then shortest-path tree), IGMP v2/v3 (v3 enables SSM), bidir, and MSDP, framed as diagnosis.