Layer 3 Fabrics

JNCIP-DC · 50 questions

• A city replaces STP access pods with a leaf-spine Clos for the new civic cloud. Which trait best describes that IP fabric architecture?
• County planners ask why every leaf should connect to every spine in the municipal Clos. What is the primary architectural intent of that full leaf-to-spine wiring?
• A township proposes adding leaf-to-leaf underlay links as a backup path between racks. Why does a classic Clos IP fabric avoid that pattern?
• Municipal architects compare a simple 3-stage leaf-spine Clos with a 5-stage Clos for a larger regional data center. When does a multi-stage Clos typically enter the design conversation?
• Ops asks what role the spines play in the civic fabric if overlays typically terminate on the leaves. Which statement is most accurate?
• A city diagram of the new IP fabric still shows STP blocked links between switches. What correction should the fabric team make to the diagram’s mental model?
• The public-safety data center wants predictable hop counts between any two leaves on the Clos. Which architecture property delivers that regularity?
• A regional design dual-homes each server to two leaves on the IP fabric. Where does that multihoming sit in the architecture?
• Council slides describe the municipal fabric as “Layer 2 everywhere.” What is the accurate IP-fabric correction?
• A metro pod wants separate IP fabrics for storage versus general compute. What architecture choice does that request illustrate at concept level?
• Engineers debate oversubscribed versus non-oversubscribed spine capacity for the civic Clos. What does fabric oversubscription measure in this architecture discussion?
• A smart-city edge DC proposes running the Clos with only a single spine to save cost. What is the main architectural risk?
• The city underlay will use EBGP between leaves and spines. Which characteristic best matches that IP fabric routing approach?
• County fabric designers prefer IBGP with route reflectors placed on the spines. What underlay routing pattern does that describe?
• A leaf loses all spine underlay peers and its server subnets become unreachable from the rest of the civic fabric. What routing principle explains that black hole?
• Ops enables OSPF as the underlay for a small municipal pod instead of BGP. How should that choice be understood at concept level?
• A township sees unequal ECMP utilization because underlay metrics were set inconsistently across leaf-spine paths. What does that symptom highlight?
• BGP underlay sessions are up, but one leaf’s loopback used as a VTEP address is missing from remote routing tables. What underlay routing gap should be investigated first?
• City security wants aggressive underlay ACLs on fabric links and immediately breaks BGP between leaves and spines. What lesson applies to IP fabric routing?
• A metro EBGP Clos design assigns a private ASN per leaf. What concept does that ASN allocation illustrate?
• The NOC asks whether enabling VXLAN means the civic fabric can drop underlay routing between leaves and spines. What is the correct separation of roles?
• After the city adds a new spine, traffic still avoids it. What underlay routing condition must be true for the new spine to join ECMP?
• Public works dual-stacks the IP fabric underlay with both IPv4 and IPv6. What operational consideration matters most for overlay VTEP reachability?
• A leaf redistributes connected routes into BGP with a loose policy and leaks tenant networks into the underlay. What routing-hygiene principle did the fabric violate?
• The city IBGP fabric has grown past a comfortable full mesh among leaves. Which scaling approach fits IP fabric underlay practice?
• A county IP fabric’s leaf RIBs keep filling with host and external prefixes that servers never need for east-west reachability. What scaling practice should the fabric team apply first?
• A regional municipal data center plans to grow from dozens of leaves to hundreds while keeping non-blocking east-west capacity. How should the architects scale the Clos fabric?
• A civic EBGP-per-leaf fabric is drowning operators in unique ASNs and per-leaf policy objects as pods expand. What operational scaling tradeoff should the design review highlight versus an IBGP-plus-route-reflector underlay?
• Overlay /32 host routes from EVPN are exploding on county leaves and stressing RIB and FIB, even though the underlay still has only a modest number of loopbacks and fabric links. What distinction should engineers make when diagnosing the scale problem?
• County architects want each leaf to maintain fewer BGP sessions as spine count grows, without abandoning multipath. Which control-plane approach best targets session-count scaling?
• A municipal pod is approaching hardware TCAM limits for distinct ECMP groups as more spines and prefixes are added. What IP fabric scaling constraint should the team treat as first-class in the design?
• A metro interconnect starts injecting large volumes of external prefixes into the city’s IP fabric underlay. What boundary practice protects fabric scale?
• A municipal GPU cluster needs near-lossless behavior for RoCE storage traffic across the IP fabric. Which CoS approach belongs in the fabric design?
• A city enables Priority Flow Control on every interface, including the Internet edge, after reading about RoCE. What scoping correction should the design review demand?
• After a township marks ordinary backup flows with the RoCE codepoint, operators see PFC storms across the pod. What should they fix first?
• A township engineer asks why modern RoCE-capable IP fabrics bother with Explicit Congestion Notification. What role does ECN play?
• Spine buffer profiles for lossless queues differ sharply from leaf profiles on a civic RoCE path, and storage latency spikes under load. What CoS consistency concern should operators address?
• The NOC schedules civic VoIP in the same strict lossless class as RoCE storage on the IP fabric. What class-design correction is appropriate?
• After enabling lossless queues for RoCE, a microburst still drops UDP telemetry on county leaves. What CoS tradeoff should the team recognize?
• Architects ask whether enabling IP fabric CoS for RoCE means application and stack retry logic can be removed. What is the correct stance?
• A city runbook sets underlay interface MTU higher than the overlay payload size requires, specifically to leave headroom for encapsulation. Which IP fabric best practice does that reflect?
• A leaf reuses one undocumented loopback for underlay router-id, VTEP source, and several overlapping services. What addressing hygiene best practice should the fabric standard require?
• Change windows routinely edit underlay BGP and overlay EVPN together across the whole civic fabric with no canary leaves. What best practice should replace that pattern?
• A township still ships fabric switches with vendor default passwords on the management plane. Which IP fabric operational best practice is being violated?
• Design review for a new municipal Clos insists on symmetric leaf-to-spine cabling and clearly labeled ports. Why is that an IP fabric best practice?
• The civic NOC must confirm a leaf installs multipath next hops toward all spines for its fabric prefixes. What monitoring check best verifies underlay ECMP?
• A city leaf shows BGP Established to a spine but receives zero prefixes from that peer. Where should troubleshooting focus first?
• After a cabling change, one municipal spine carries no production traffic while peers are busy. What should engineers verify first on the leaf-spine fabric links?
• An engineer brings up underlay BGP on a new leaf but forgets to advertise the VTEP loopback into the fabric. What failure class does this misconfiguration create?
• Telemetry flags rising CRC errors on one civic fabric optic between leaf and spine. How should that finding rank in IP fabric troubleshooting?