Arista ACE Specialist - WAN Routing key terms

A network that connects sites across large geographic distances, often using service provider links. In SD-WAN designs it is the transport underlay over which overlays are built.

The physical or provider-provided transport network that carries packets between sites, such as MPLS, internet broadband, or LTE. It provides reachability but no overlay policy or tunneling by itself.

A logical network built on top of the underlay using tunnels between endpoints. It hides transport details and allows consistent policy, segmentation, and path selection across mixed links.

An encapsulation that carries original packets inside new outer headers between two endpoints. In SD-WAN it provides the virtual link over which overlay traffic is forwarded and monitored.

A suite of protocols that encrypts and authenticates IP packets between peers. SD-WAN commonly uses IPsec tunnels to protect overlay traffic crossing untrusted internet links.

An encapsulation that places Layer 2 frames inside UDP packets, extending segments across a routed network. It is often combined with tunnels in SD-WAN and data-center overlays.

The process of wrapping a packet in additional headers so it can traverse a tunnel or overlay. Overhead from encapsulation affects MTU and must be accounted for on WAN links.

The largest packet size a link can carry without fragmentation. Tunnel encapsulation adds overhead, so effective MTU shrinks and must be tuned to avoid drops or fragmentation.

A technique that determines the smallest MTU along a path by probing with progressively larger packets and relying on ICMP responses. It helps avoid fragmentation across tunnels.

Splitting a packet into smaller pieces so it fits a link's MTU. It adds overhead and can degrade performance; SD-WAN designs usually avoid it by sizing packets correctly.

A topology where branch sites connect to a central hub, and traffic between branches passes through that hub. It simplifies policy but can add latency and backhaul traffic.

A topology where every site has a direct path to every other site. It minimizes transit hops but increases tunnel and peer counts as sites grow.

A topology where some site pairs connect directly while others route through intermediate sites. It balances direct-path performance against tunnel scale and management complexity.

The process of choosing a WAN path per application flow based on live measurements such as latency, loss, and jitter, rather than static routing alone.

A synthetic test, often sent periodically between overlay endpoints, that measures latency, loss, and jitter. Results drive path selection and health decisions for application traffic.

Variation in packet delay over time, which harms real-time traffic such as voice and video. SD-WAN monitors jitter and can steer flows to a path that meets the SLA.

The time a packet takes to travel from source to destination, usually measured in milliseconds. It is a primary metric for path selection and SLA compliance.

The percentage of packets that fail to reach their destination. Even small loss rates can degrade throughput and voice quality, so it is a key SLA metric.

A path-vector routing protocol used to exchange routes between autonomous systems and within large networks. Its attributes and policies heavily influence WAN path selection.

A BGP attribute listing the autonomous systems a route has traversed. Shorter paths are generally preferred, and prepending can influence inbound traffic decisions.

A BGP attribute used inside an autonomous system to rank preferred exit points. Higher values are preferred, and it is shared among internal BGP peers.

The Multi-Exit Discriminator BGP attribute that suggests a preferred entry point to a neighboring autonomous system. Lower values are typically preferred.

A link-state interior gateway protocol that computes shortest paths using link costs. It is common inside enterprise WAN cores and as a routing source for overlays.

Importing routes learned by one routing protocol into another. It requires care with metrics, filtering, and loop prevention when mixing protocols across a WAN.

A value that ranks the trustworthiness of route sources on a router. When multiple protocols offer the same prefix, the lower distance is installed.

The forwarding rule that selects the most specific matching route for a destination. It takes precedence over administrative distance when prefixes differ in length.

Forwarding decisions made from criteria other than destination address, such as source, application, or DSCP. It enables steering traffic over specific WAN paths.

Directing selected flows onto chosen WAN paths or tunnels based on policy and link health. It is central to meeting application SLAs across mixed transports.

Recognizing applications in traffic using signatures, ports, or flow behavior. Accurate identification is a prerequisite for applying per-application steering and QoS.

Quality of Service: classifying and prioritizing traffic to control bandwidth, delay, and drop treatment. On WAN links it protects latency-sensitive applications during congestion.

The Differentiated Services Code Point, a 6-bit field in the IP header used to mark packets for per-hop treatment. SD-WAN devices often trust, rewrite, or map these markings.

A technique that sends redundant data so the receiver can reconstruct lost packets without retransmission. It improves quality on lossy WAN links for real-time traffic.

Sending copies of a flow over multiple WAN paths and discarding duplicates at the receiver. It reduces effective loss and jitter for critical real-time applications.

The part of the network that distributes reachability, policy, and topology information to devices. In SD-WAN it often centralizes decisions that were once configured per router.

The forwarding path that actually moves packets based on programmed decisions. In SD-WAN it applies tunnels, policy, and encryption at each site.

A management component that provisions devices, pushes templates, and coordinates configuration across the WAN. It separates intent from per-device CLI configuration.

Bringing a new device online with minimal manual steps by having it fetch configuration automatically. It speeds branch turn-up and reduces configuration errors.

A reusable configuration model applied to many devices, often with per-site variables. Templates enforce consistency and reduce drift across a large WAN.

Streamed operational data such as interface counters, flow records, and path metrics sent to a collector. It supports monitoring, alerting, and capacity planning.

Summaries of traffic conversations including endpoints, volumes, and timing. They help identify top talkers, verify steering, and troubleshoot path issues.

A diagnostic that reveals the hop-by-hop path to a destination using incrementing TTL values. It helps locate where traffic deviates from the expected WAN path.

A basic reachability test using ICMP echo requests that reports round-trip time and loss. It is a first check when validating WAN link and tunnel health.

The set of known destinations and next hops a device uses for forwarding. Inspecting it confirms whether expected routes are installed and chosen.