Skip to content

ENCOR 03 - Infrastructure

Layer 2 switching, routing protocols, wireless, and IP services. The largest section at roughly 30% of the exam.

Layer 2

  • VLAN - a logical broadcast domain.
  • Trunking (802.1Q) - carries multiple VLANs, tagging all but the native VLAN.
  • Native VLAN - untagged on a trunk. Mismatched native VLANs between switches cause traffic to cross VLAN boundaries and generate CDP errors.
  • DTP (Dynamic Trunking Protocol) - negotiates trunking. Should be disabled (switchport nonegotiate) on ports facing end devices, because auto-negotiation enables VLAN hopping.
  • VTP (VLAN Trunking Protocol) - propagates VLAN databases. Modes: server, client, transparent, and off. A higher revision number overwrites the domain, so inserting a switch with a high revision number can wipe the VLAN database. VTP transparent or version 3 mitigates this.

Spanning Tree

  • STP (802.1D) - prevents loops by blocking redundant paths.
  • RSTP (802.1w) - rapid convergence, port roles root/designated/alternate/backup.
  • MST (802.1s) - maps many VLANs onto a few spanning-tree instances, reducing CPU load.
  • PVST+ / Rapid PVST+ - Cisco per-VLAN instances, allowing per-VLAN load balancing.
  • Root bridge election - lowest bridge ID, which is priority plus MAC address. Priority is set in increments of 4096.
  • Path cost - lower is better, based on link speed.
  • Port states (RSTP) - discarding, learning, forwarding.
  • PortFast - immediate forwarding for access ports connected to end devices.
  • BPDU Guard - err-disables a PortFast port that receives a BPDU.
  • Root Guard - prevents a neighbor from becoming root on that port.
  • Loop Guard - protects against unidirectional link failures causing a blocked port to start forwarding.
  • UDLD - detects unidirectional fiber links.

Root bridge election and the guard features are staple exam content. Set the root bridge deliberately with priority rather than letting MAC address decide.

EtherChannel

  • LACP (802.3ad) - standard negotiation; modes active and passive. At least one side must be active.
  • PAgP - Cisco proprietary; modes desirable and auto. At least one side must be desirable.
  • On - no negotiation; both sides must be On, and a mismatch causes a loop.
  • Requirements - matching speed, duplex, VLAN allowed list, and trunk mode across all member ports.
  • Load balancing - by source/destination MAC, IP, or port. Choose a method that varies across your traffic, or one link carries everything.

Routing protocols

EIGRP

  • Advanced distance vector, Cisco-originated, AD 90 internal and 170 external.
  • Metric - bandwidth and delay by default (K1 and K3).
  • Feasible distance (FD) - the best metric to a destination.
  • Reported/advertised distance (RD) - the neighbor's metric to the destination.
  • Feasibility condition - RD < FD, which guarantees loop freedom.
  • Successor - the best path. Feasible successor - a backup meeting the feasibility condition, allowing instant failover without going active.
  • Active versus passive - a route goes active when no feasible successor exists and queries must be sent. Stuck-in-active is a scaling problem.

OSPF

  • Link state, AD 110, metric is cost derived from bandwidth.
  • Areas - area 0 is the backbone; all other areas must connect to it, directly or via a virtual link.
  • Router types - internal, backbone, ABR (area border), ASBR (autonomous system boundary).
  • LSA types - Type 1 router, Type 2 network, Type 3 summary, Type 4 ASBR summary, Type 5 external, Type 7 NSSA external.
  • Area types - standard, stub (no Type 5), totally stubby (no Type 3 or 5), NSSA (allows Type 7), totally NSSA.
  • Network types - broadcast (DR/BDR elected), point-to-point (no DR), non-broadcast, point-to-multipoint.
  • DR/BDR election - highest priority, then highest router ID. Priority 0 means never DR.
  • Adjacency requirements - matching area, hello and dead timers, subnet, authentication, MTU, and area type. Mismatches are the standard troubleshooting scenario.

BGP

  • Path vector, AD 20 external and 200 internal, uses TCP 179.
  • eBGP - between autonomous systems; AD 20; TTL 1 by default so peers must be directly connected unless multihop is configured.
  • iBGP - within an AS; AD 200; iBGP routes are not advertised to other iBGP peers, requiring a full mesh, route reflectors, or confederations.
  • Path selection order - weight (highest, Cisco-only, local), local preference (highest, AS-wide), locally originated, AS path (shortest), origin (IGP < EGP < incomplete), MED (lowest), eBGP over iBGP, lowest IGP metric to next hop, oldest route, lowest router ID.
  • Attributes - well-known mandatory (origin, AS path, next hop), well-known discretionary (local preference, atomic aggregate), optional transitive (community, aggregator), optional non-transitive (MED).
  • Route reflector - relaxes the iBGP full-mesh requirement.

Memorize the BGP path selection order. It is examined directly, and weight before local preference is the pair most often confused.

Wireless

  • RF fundamentals - 2.4 GHz has three non-overlapping channels (1, 6, 11); 5 GHz has many more.
  • Signal metrics - RSSI (received signal strength), SNR (signal-to-noise ratio), and noise floor. SNR is the better predictor of usable throughput.
  • 802.11 standards - 802.11n (Wi-Fi 4), ac (Wi-Fi 5), ax (Wi-Fi 6/6E), be (Wi-Fi 7).
  • AP-to-WLC association - the AP discovers a WLC by DHCP option 43, DNS, broadcast, or a primed controller, then builds a CAPWAP tunnel.
  • Roaming - intra-controller, inter-controller (Layer 2), and Layer 3 roaming with anchor and foreign controllers.
  • WLAN security - WPA2 with AES, WPA3 with SAE, 802.1X with EAP, and PSK.

IP services

  • NTP - stratum hierarchy; authenticated NTP prevents spoofed time.
  • PTP - sub-microsecond precision where NTP is insufficient.
  • NAT and PAT - inside local, inside global, outside local, outside global. Learn the four terms; questions use them precisely.
  • First-hop redundancy:
  • HSRP - Cisco, active and standby, virtual MAC 0000.0c07.acXX, default priority 100, preemption off by default.
  • VRRP - standard, master and backup, can use a real interface address as the virtual IP, preemption on by default.
  • GLBP - Cisco, load balances across multiple forwarders (AVG and AVFs).
  • SNMP - v2c uses community strings in clear text; v3 adds authentication and encryption.
  • Syslog severities - 0 emergency through 7 debug. Lower is more severe.

Exam pointers

  • BGP path selection order, memorized, in sequence.
  • OSPF adjacency failures: check area, timers, subnet, MTU, and authentication.
  • EIGRP feasible successor requires RD < FD.
  • A VTP server with a higher revision number can overwrite the VLAN database.
  • LACP active/passive and PAgP desirable/auto; at least one side must actively negotiate.
  • HSRP preemption is off by default; VRRP preemption is on.

Official documentation

📖 ENCOR 350-401 exam topics - authoritative blueprint 📖 BGP best path selection algorithm - the ordered criteria 📖 OSPF design guide - areas and LSA types