3.0 Infrastructure

Infrastructure is where ENCOR stops being a design conversation and starts asking whether you can reason through the network that is actually forwarding traffic.

The exam expects three broad skills:

1. Layer 2 — trunks, EtherChannels, spanning tree, and edge protections.
2. Layer 3 — routing protocol behavior, OSPF configuration, eBGP adjacency and best path basics, and policy-based routing.
3. IP Services — time, NAT/PAT, first-hop redundancy, and multicast fundamentals.

Do not study these as isolated commands. Study the packet path. A good answer usually comes from knowing which device makes the next decision, what table it uses, and what would make that decision fail.

How to use these articles

For each topic:

• Read the mental model first.
• Build the lab once.
• Break one thing on purpose.
• Verify with show commands before you fix it.
• Write down the one symptom that would point you back to that topic.

Topic map

Pass-first mindset

Infrastructure questions reward calm troubleshooting. Start with the layer where the symptom appears, then walk backward:

• Missing VLAN across a switch link? Check trunk mode, allowed VLANs, native VLAN, and DTP.
• Port-channel down? Check both member links for identical speed, duplex, VLANs, trunking, and LACP/PAgP mode.
• OSPF neighbor stuck? Check area, network type, timers, MTU, authentication, passive-interface, and reachability.
• BGP neighbor idle? Check IP reachability, AS numbers, update-source, TTL, and ACLs.
• NAT works for one host but not another? Check the ACL, inside/outside markings, route, translation table, and overload pool.

Learn the show commands until they feel boring. Boring is good. Boring means you can find the problem under exam pressure.