Infrastructure
The Infrastructure domain is the largest ENCOR area (about 30%). It tests configuration-level understanding of Layer 2 switching, Layer 3 routing, IP services, and wireless, with an emphasis on how features interact.
Layer 2
- An 802.1Q trunk adds a 4-byte tag; the native VLAN is untagged and must match on both ends. Allowed VLAN lists and VTP modes can silently prune traffic.
- STP elects one root per VLAN/instance by lowest bridge ID (priority, then MAC). RSTP adds alternate/backup roles and rapid convergence; MST maps VLANs to instances. PortFast should be paired with BPDU Guard on access ports.
- EtherChannel: LACP uses active/passive, PAgP uses desirable/auto, and static uses on. Active+passive works; on+active does not. Member interfaces must match speed, duplex, VLAN/trunk config, and MTU.
Layer 3
- OSPF neighbors progress through Down, Init, 2-Way, ExStart, Exchange, Loading, Full. Broadcast networks elect a DR/BDR; point-to-point links do not. Cost is reference bandwidth divided by interface bandwidth by default.
- EIGRP uses feasible successors for fast failover; BGP selects paths by weight, local preference, locally originated, AS-path length, origin, MED, eBGP over iBGP, then router ID.
- Administrative distance: static 1, eBGP 20, EIGRP 90 internal, OSPF 110, RIP 120.
Services and wireless
- NAT order matters: inside-to-outside translates source; outside-to-inside translates destination. DHCP relay uses ip helper-address. NTP uses stratum to indicate distance from a reference clock.
- CAPWAP uses UDP 5246 for control and 5247 for data. An AP must discover a WLC, join, and receive configuration; FlexConnect allows local switching at a branch.
Worked example
Two routers connected over a Gigabit link form an OSPF adjacency that sticks in EXSTART/EXCHANGE. The interface IPs and area match, and Hellos are exchanged. The likely cause is an MTU mismatch: OSPF includes MTU in database description packets. A 1500-byte side talking to a 1400-byte side will fail to progress. Verify with show ip ospf neighbor and show interfaces; fix the MTU or, as a lab workaround, use ip ospf mtu-ignore. Reasoning: the adjacency reached two-way and entered database exchange, so the failure is in DBD handling, not Hello/area authentication.
Traps
- Native VLAN mismatch causing CDP/STP inconsistencies.
- Assuming LACP passive+passive forms a bundle; it does not.
- Forgetting that OSPF network type changes DR election and Hello/dead timers.
- Configuring NAT inside/outside on the wrong interfaces.
- Treating CAPWAP control and data as the same port.