Architecture
The Architecture domain (about 15% of ENCOR) tests whether you can choose and explain designs rather than just configure interfaces: hierarchical and spine-leaf topologies, Cisco SD-Access and SD-WAN fabrics, first-hop redundancy, and management-plane protocols.
Design and fabric models
- Classic campus: access-distribution-core. A collapsed core merges distribution and core; Layer 2 access and Layer 3 distribution limit failure domains.
- Spine-leaf: every leaf connects to every spine; leaves do not connect to each other. It uses routed links and ECMP, so STP is not the recovery mechanism. Fixed hop count suits east-west data-center traffic.
- SD-Access separates underlay and overlay. The underlay is a routed IP network. The overlay carries VXLAN data traffic; LISP provides the control-plane mapping of endpoint EIDs to RLOCs. Fabric edge, border, and control-plane nodes have distinct roles. Cisco DNA Center provisions the fabric, and ISE supplies identity and group policy (TrustSec SGT).
- SD-WAN: vManage is management, vSmart is the controller running OMP, and vBond is the orchestrator that handles authentication and NAT traversal. A TLOC is identified by system IP, color, and encapsulation. OMP advertises routes, TLOCs, and policy; IPsec protects WAN transport.
Worked example
Two routers share a LAN: R1 has HSRP priority 110 and preempt; R2 has priority 120 and no preempt. R2 boots first and becomes active. R1 then comes up; R1 does not preempt R2 because R2's priority is higher, even though R1 has preempt. If R2 fails, R1 becomes active. When R2 returns, R2 does not reclaim the active role because it lacks preempt; R1 stays active. Reasoning: preempt only allows a router to take over when its priority is higher than the current active router's; the current active router's lack of preempt does not force a change.
Traps
- Mixing SD-Access planes: LISP is control plane, VXLAN is data plane.
- Assuming vSmart forwards user traffic; it does not.
- Designing spine-leaf with STP-based redundant links instead of routed ECMP.
- Forgetting that HSRP preempt must be configured on the router that should take over, and that interface tracking can lower priority.
- Treating SNMP or syslog as a configuration protocol; they are monitoring/notification, not NETCONF-style change mechanisms.