NCoE Selection Test key terms

A seven-layer reference framework describing how data moves from an application on one host to an application on another, separating physical transmission, addressing, routing, and application concerns into distinct layers.

The process of wrapping data with protocol headers at each layer as it moves down the stack, so each layer adds the information its peer layer needs to process the payload.

The SYN, SYN-ACK, ACK exchange that establishes a TCP connection and synchronizes initial sequence numbers before any application data is sent between the two endpoints.

The largest TCP payload a sender will place in a single segment, normally derived from the path MTU so that segments fit inside a single IP packet without fragmentation.

The maximum size of an IP packet, headers included, that a link can carry. Packets larger than the path MTU must be fragmented or dropped, which commonly causes silent application failures.

Resending of unacknowledged segments after a timeout or duplicate acknowledgements. High retransmission rates indicate loss, congestion, or a broken path rather than a slow application.

Address Resolution Protocol maps a known IPv4 address to a link-layer MAC address on the local segment by broadcasting a request and caching the reply for later reuse.

An unsolicited ARP announcement a host sends to advertise or refresh its own IP-to-MAC binding, used for duplicate address detection and for speeding failover of a shared address.

A logical Layer 2 broadcast domain created by tagging or port assignment on a switch, letting one physical infrastructure carry isolated segments that behave as separate networks.

A link that carries traffic for multiple VLANs by inserting a four-byte tag containing the VLAN identifier into each frame, with one untagged native VLAN typically allowed.

A condition where the two ends of a trunk disagree on which VLAN is untagged, causing frames to land in the wrong broadcast domain and often producing silent connectivity or security failures.

A Layer 2 loop-prevention protocol that elects a root bridge and blocks redundant ports so that exactly one active path exists between any pair of segments in a bridged domain.

Bundling several physical links into one logical interface to increase bandwidth and provide redundancy, requiring matching configuration and hashing behavior on both ends of the bundle.

The algorithm that selects which member link of an aggregated bundle carries a given flow, typically hashing addresses and ports so a single conversation stays on one link.

A vendor-assigned preference value that a router uses to rank routes learned from different sources to the same prefix, with lower values preferred before any metric comparison occurs.

A value computed by a routing protocol to compare paths to the same destination learned from the same source, such as hop count, cost, or bandwidth-based composite values.

The forwarding rule that selects the most specific route containing the destination address, so a /24 entry wins over a /16 entry regardless of metric or administrative distance.

Injecting routes learned by one routing protocol into another so both domains can reach each other, requiring careful control of metrics, filtering, and loop prevention at the boundary.

A group of protocols that present a shared virtual gateway address to hosts so that if the active router fails, a standby router assumes the address and forwarding continues.

An address not bound to a single physical interface but owned by whichever node is currently active, used for gateway redundancy and for load balancer front ends.

An ordered set of permit and deny rules applied to traffic at an interface or service, evaluated top-down with an implicit deny at the end unless a permit statement matches.

A filter that tracks connection state so return traffic for an approved session is allowed automatically, rather than requiring an explicit rule for each direction of the flow.

A port-based access control standard in which a supplicant authenticates through an authenticator to a central server before the switch port grants normal network access.

A central authentication, authorization, and accounting protocol used by network devices to validate user or device credentials against a shared policy server and log the results.

The principle of granting an identity only the permissions required for its task, minimizing the blast radius if credentials are compromised or a process misbehaves.

An authorization model that assigns permissions to roles rather than individuals, so users gain rights by being placed in a role and changes are managed in one place.

Rewriting source or destination addresses and ports as packets cross a boundary, allowing private hosts to share public addresses and hiding internal addressing from outside observers.

A function on a router that forwards broadcast DHCP requests from a client subnet to a central server and returns the reply, enabling one server to serve many segments.

The defined pool of addresses, plus options such as gateway, DNS servers, and lease duration, that a DHCP server can hand out to clients on a particular subnet.

Two devices using the same address on one segment, producing intermittent connectivity, ARP flapping, and log warnings that are often mistaken for application faults.

A server that accepts a client query and performs the full lookup chain on its behalf, caching answers according to TTL so repeated queries are answered quickly.

Time to live, a value limiting how long a cached record may be reused or how many hops a packet may traverse, depending on whether it appears in DNS or IP context.

Serving different answers for the same name depending on whether the query arrives from inside or outside the network, commonly used to return private addresses internally.

A periodic probe that tests whether a backend member can serve traffic, removing failing members from rotation and restoring them once probes succeed again.

A load balancing behavior that routes all requests from one client to the same backend member, usually by cookie or source address, so local session state stays valid.

The path-vector protocol that exchanges routes between autonomous systems, selecting best paths using attributes such as AS path length, local preference, and multi-exit discriminator.

A group of IP prefixes and routers administered under one routing policy and identified by a unique number, forming the unit of interdomain routing on the internet.

Recording live traffic on an interface for later analysis, ideally at a point that sees both directions of a flow so loss, retransmission, and latency can be measured accurately.

A diagnostic that reveals the forwarding path to a destination by sending probes with increasing hop limits and collecting the responses from each router along the way.

An unsolicited notification a managed device sends to a monitoring station when a defined event occurs, complementing polling by delivering fault signals immediately.

A standard for shipping event messages from network devices to a central collector, with severity levels that let operators filter noise from actionable faults.

A documented record of normal performance and configuration captured when the network is healthy, used as the reference against which later anomalies are judged.

A property of an operation that produces the same end state whether run once or many times, which makes automation safe to retry after partial or interrupted execution.

The gradual divergence between a device's live configuration and its intended, documented state, usually caused by manual changes made outside the automation pipeline.

Managing network and system configuration through version-controlled definition files that are applied by tooling, rather than through ad hoc manual changes on individual devices.

A controlled process for requesting, reviewing, approving, scheduling, and recording changes, including a tested rollback plan and a defined window for implementation.

A pre-prepared, tested procedure for restoring the previous known-good state if a change fails or degrades service, including the trigger conditions for invoking it.