3.0 Infrastructure

What this domain tests

Domain 3 is 30% of the ENCOR exam. It is the largest domain. It has three sections: Layer 2, Layer 3, and IP Services.

The exam wants proof, not theory. For each topic, learn to name three things:

• Control point: the device or table that makes the forwarding decision.
• Failure point: the mismatch or state that stops traffic.
• Proof: the command that shows the correct state.

The three sections

• 3.1 Layer 2: trunks, EtherChannels, and spanning tree. Control point: the switch port state. Proof: show interfaces trunk, show etherchannel summary, show spanning-tree.
• 3.2 Layer 3: routing concepts, OSPF, eBGP, and policy routing. Control point: the routing table and the routing protocol database. Proof: show ip route, show ip ospf neighbor, show bgp ipv4 unicast summary.
• 3.3 IP Services: time, NAT, first-hop redundancy, and multicast. Control point: the service state on the router. Proof: show ntp associations, show ip nat translations, show standby brief, show ip mroute.

Topic map

How to study this domain

For each objective, do these steps:

1. Read the mental model first.
2. Build the lab once.
3. Break one thing on purpose.
4. Verify with show commands before you fix it.
5. Write down the symptom that points back to that topic.

Pass check

You are ready for Domain 3 when you can do these things:

• Read a trunk, port-channel, or spanning-tree problem from show output alone.
• Explain why an OSPF neighbor or eBGP peer is down and prove it with one command.
• Configure NAT, HSRP, and NTP from memory and verify each one.