Routing & Switching terms

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• 6PECarrying IPv6 traffic over an IPv4 MPLS core using MP-BGP with labeled IPv6 routes, without VRFs.Advanced
• 6VPEIPv6 VPN service over an IPv4 MPLS core, using VPNv6 MP-BGP and VRF-aware IPv6 forwarding (RFC 4659).Advanced
• Access PortA switch port that carries traffic for a single data VLAN (plus optionally a voice VLAN) without tagging.Foundational
• Address FamilyA BGP (or other protocol) configuration context identifying a type of network layer information, such as IPv4 unicast or L2VPN EVPN.Advanced
• Adj-RIB-InThe conceptual BGP table holding unprocessed routes received from a specific peer before inbound policy.Advanced
• Adjacency SIDA local segment that forces a packet across a specific IGP adjacency (link), regardless of shortest path.Advanced
• Adjacency TableThe CEF table holding Layer 2 rewrite information for each next hop, built from ARP and other resolution protocols.Advanced
• Administrative DistanceA Cisco value (0-255) ranking the trustworthiness of route sources; the lowest AD wins when protocols offer the same prefix.Foundational
• Aggregate RouteA Junos summary route defined under routing-options aggregate that is active when at least one contributing more-specific route exists.Intermediate
• Aggregated EthernetJuniper Junos link aggregation interface (aeX) that bundles physical links with or without LACP.Intermediate
• Allowas-InA BGP option permitting a router to accept routes containing its own ASN in the AS_PATH.Advanced
• Allowed VLAN ListThe set of VLANs permitted to traverse a trunk, all VLANs (1-4094) by default on Cisco.Foundational
• Alternate PortAn RSTP port role for a discarding port that provides a backup path to the root bridge via another switch.Intermediate
• Anycast RPConfiguring the same RP address on multiple routers for redundancy and load sharing, synchronized by MSDP or PIM.Advanced
• Application-Aware Routing (AAR)A Cisco SD-WAN data policy that steers applications over tunnels meeting SLA thresholds for loss, latency and jitter measured by BFD.Advanced
• Area Border Router (ABR)An OSPF router with interfaces in area 0 and at least one other area, which summarizes routes between them.Intermediate
• ARP SuppressionAn EVPN feature where VTEPs answer ARP requests locally using IP-MAC bindings learned from Type 2 routes, reducing flooding.Advanced
• AS Path PrependingAdding extra copies of an ASN to the AS_PATH of advertised routes to make a path less attractive to other ASes.Intermediate
• AS-OverrideA PE-side BGP option that replaces the customer's ASN in the AS_PATH with the provider's ASN, letting sites with the same ASN learn each other's routes.Advanced
• AS_PATH AttributeA well-known mandatory BGP attribute listing the ASNs a route has traversed; used for loop prevention and path length comparison.Intermediate
• Assured Forwarding (AF)A DiffServ PHB group of four classes (AF1-AF4) each with three drop precedences (AFx1-AFx3).Intermediate
• Asymmetric IRBEVPN inter-subnet routing where the ingress VTEP routes and bridges into the destination VNI, and the egress VTEP only bridges.Advanced
• Attached BitA flag set in an L1/L2 router's Level 1 LSP signalling it has Level 2 connectivity, prompting L1 routers to install a default route.Advanced
• Auto-RPCisco-proprietary method for distributing RP information using candidate RP announcements to 224.0.1.39 and mapping agent discovery to 224.0.1.40.Advanced
• Auto-SummaryAutomatic summarization of routes to classful network boundaries when advertised across a different major network.Intermediate
• Autonomous System (AS)A set of networks under one administrative control with a single routing policy, identified in BGP by an AS number.Foundational
• Autonomous System Boundary Router (ASBR)An OSPF router that redistributes external routes into OSPF, originating Type 5 or Type 7 LSAs.Intermediate
• Autonomous System Number (ASN)A number identifying a BGP autonomous system: 2-byte (1-65535) or 4-byte (up to 4,294,967,295).Intermediate
• AutoQoSCisco feature that automatically generates recommended QoS configuration for voice, video and data on switches and routers.Intermediate
• Backup Designated Router (BDR)The OSPF router that monitors the DR on a multi-access segment and takes over if the DR fails.Intermediate
• Best EffortThe default QoS model where all packets are treated equally with no guarantees, typically first in first out.Foundational
• BGP Add-PathA BGP capability that allows advertising multiple paths for the same prefix to a peer.Advanced
• BGP Aggregate AddressA BGP command that creates a summary route when at least one component exists in the BGP table.Intermediate
• BGP Best Path SelectionBGP's ordered decision process for choosing one best path per prefix.Intermediate
• BGP CommunityAn optional transitive 32-bit BGP attribute (often written AS:value) used to tag routes for policy decisions.Intermediate
• BGP Conditional AdvertisementAdvertising a prefix to a peer only when another prefix is present (exist-map) or absent (non-exist-map) in the BGP table.Advanced
• BGP ConfederationSplitting one AS into multiple internal sub-ASes that run eBGP-like sessions among themselves, reducing iBGP mesh size.Advanced
• BGP Default-OriginateA BGP neighbor option that sends a default route to a specific peer, even if none exists in the BGP table.Intermediate
• BGP EVPNThe MP-BGP address family (L2VPN/EVPN, AFI 25/SAFI 70) that carries EVPN routes between VTEPs or PEs.Advanced
• BGP FlowspecA BGP address family (RFC 8955) that distributes traffic filtering rules matching on packet fields, with actions such as drop or rate-limit.Advanced
• BGP Full MeshThe requirement that every iBGP speaker peer with every other iBGP speaker unless route reflectors or confederations are used.Intermediate
• BGP Labeled Unicast (BGP-LU)BGP distribution of labels for IPv4 or IPv6 prefixes (RFC 8277), used in Inter-AS Option C, Seamless MPLS and CsC.Advanced
• BGP Link-State (BGP-LS)A BGP address family that exports IGP topology and TE information to controllers such as PCEs.Advanced
• BGP Message TypesBGP's message types: OPEN, UPDATE, KEEPALIVE, NOTIFICATION and ROUTE-REFRESH.Intermediate
• BGP MultihomingConnecting an AS to two or more upstream links or providers using BGP for redundancy and traffic control.Intermediate
• BGP MultipathAllowing BGP to install multiple equal paths for a prefix for load sharing.Advanced
• BGP Neighbor StatesThe BGP finite state machine: Idle, Connect, Active, OpenSent, OpenConfirm and Established.Intermediate
• BGP Network StatementThe BGP command that advertises a prefix only if an exact match (prefix and mask) exists in the routing table.Intermediate
• BGP Path AttributesProperties attached to BGP routes, categorized as well-known mandatory, well-known discretionary, optional transitive and optional non-transitive.Intermediate
• BGP Peer GroupA Cisco IOS construct that applies the same outbound policy to many neighbors and generates updates once per group.Intermediate
• BGP Prefix Independent Convergence (BGP PIC)A Cisco feature that precomputes backup BGP paths in the FIB so failover time does not depend on the number of prefixes.Advanced
• BGP Regular ExpressionPattern matching used in AS-path access lists to filter or select routes by AS_PATH contents.Intermediate
• BGP Route Reflector (RR)An iBGP router allowed to re-advertise iBGP-learned routes to its clients, removing the full-mesh requirement.Intermediate
• Bidirectional Forwarding Detection (BFD)A lightweight hello protocol that detects forwarding path failures in milliseconds and notifies routing protocols.Intermediate
• Bidirectional PIM (Bidir-PIM)A PIM variant with only shared trees where traffic flows up and down the tree to the RP, suited to many-to-many applications.Advanced
• Bootstrap Router (BSR)The standards-based PIMv2 mechanism for distributing RP-to-group mappings hop by hop.Advanced
• Border Gateway Protocol (BGP)The path-vector EGP (BGP-4) that exchanges reachability between autonomous systems and is the routing protocol of the Internet.Foundational
• Border LeafA leaf switch that connects the fabric to external networks such as WAN routers, firewalls or other data centers.Advanced
• BPDU FilterAn STP feature that stops sending and, depending on configuration, receiving BPDUs on a port.Advanced
• BPDU GuardAn STP feature that errdisables a port if it receives any BPDU, protecting PortFast edge ports.Intermediate
• Bridge ID (BID)An 8-byte STP identifier made of a 2-byte priority (with extended system ID) and the switch MAC address.Foundational
• Bridge PriorityThe configurable portion of the STP bridge ID, 0-61440 in steps of 4096, default 32768.Foundational
• Bridge Protocol Data Unit (BPDU)The STP message switches exchange to elect the root bridge and compute the loop-free topology.Intermediate
• Candidate ConfigurationThe Junos working copy of the configuration edited in configuration mode, which becomes active only after commit.Foundational
• Centralized Control PolicyA Cisco SD-WAN policy applied on the SD-WAN Controller that manipulates OMP routes and TLOCs to shape the overlay topology.Advanced
• Cisco Catalyst SD-WANCisco's software-defined WAN solution (formerly Viptela) with separate management, control, orchestration and data planes.Intermediate
• Cisco Discovery Protocol (CDP)Cisco-proprietary Layer 2 protocol that advertises device identity, platform, addresses, VLAN and capabilities to directly connected neighbors.Foundational
• Cisco Express Forwarding (CEF)Cisco's default topology-based switching mechanism using a precomputed FIB and adjacency table for fast forwarding.Intermediate
• Cisco IOS XECisco's Linux-based modular operating system for enterprise routers and switches such as Catalyst 8000 and 9000, running IOS as a daemon.Foundational
• Cisco IOS XRCisco's service provider operating system for platforms such as ASR 9000, NCS and 8000 series, with a commit-based configuration model.Intermediate
• Cisco SD-Access (SDA)Cisco's campus fabric using a LISP control plane, VXLAN data plane and TrustSec group policy, managed by Catalyst Center.Advanced
• Class of Service (CoS)The 3-bit Priority Code Point in the 802.1Q tag used for Layer 2 QoS marking (IEEE 802.1p).Foundational
• Class Selector (CS)DSCP values CS0-CS7 (multiples of 8) that preserve backward compatibility with 3-bit IP Precedence.Intermediate
• Class-Based Weighted Fair Queuing (CBWFQ)A queuing method that guarantees minimum bandwidth to user-defined traffic classes during congestion.Intermediate
• Classification and MarkingIdentifying traffic (by ACL, NBAR, DSCP, interface) and writing a QoS value into the packet so later devices can act on it.Foundational
• Clos NetworkA multistage non-blocking switching topology, the theoretical basis for spine-leaf and multi-tier data center fabrics.Advanced
• Cloud OnRampCisco Catalyst SD-WAN features that optimize access to SaaS, IaaS and colocation by probing paths and extending the fabric into cloud providers.Advanced
• Cluster ListAn optional non-transitive BGP attribute listing the cluster IDs of route reflectors a route has passed through.Advanced
• Committed Information Rate (CIR)The average rate a provider guarantees or a policer/shaper enforces, in bits per second.Foundational
• Complete Sequence Number PDU (CSNP)An IS-IS PDU listing all LSPs in a router's database, used to synchronize databases.Advanced
• Configuration TemplateCisco SD-WAN Manager construct (feature and device templates, now configuration groups) that standardizes device configuration with variables.Advanced
• Connected RouteA route automatically installed for the subnet of an interface that is up/up and has an IP address.Foundational
• Constrained Shortest Path First (CSPF)A modified SPF that prunes links not meeting constraints (bandwidth, affinity, SRLG) before computing a path.Advanced
• ConvergenceThe state in which all routers have consistent, accurate routing information after a topology change.Foundational
• Count to InfinityA distance-vector routing loop problem where routers keep incrementing the metric of an unreachable route.Intermediate
• Customer Edge Router (CE)The customer's router connecting to a provider PE; it runs normal IP routing and is unaware of MPLS.Foundational
• Default-Information OriginateA command that makes OSPF (or other protocols) advertise a default route into the routing domain.Intermediate
• Designated Forwarder (DF)The single EVPN PE on a multihomed Ethernet segment responsible for forwarding BUM traffic toward the segment for a given VLAN.Advanced
• Designated Intermediate System (DIS)The IS-IS router elected on a broadcast segment to create the pseudonode and send periodic CSNPs.Advanced
• Designated Port (DP)The port on each segment that offers the lowest cost path to the root and forwards toward that segment.Foundational
• Designated Router (DR)The OSPF router elected on a multi-access segment to form full adjacencies with all others and originate the Type 2 LSA.Intermediate
• Differentiated Services (DiffServ)A scalable QoS model that classifies and marks packets with DSCP values and applies per-hop behaviors at each router.Foundational
• Diffusing Update Algorithm (DUAL)The EIGRP algorithm that guarantees loop-free paths and selects successors and feasible successors.Intermediate
• Direct Internet Access (DIA)Sending branch Internet traffic directly out a local Internet link instead of backhauling it to a data center.Intermediate
• Discard RouteA route that silently drops matching packets, used to anchor aggregates and prevent loops (Junos discard next hop).Intermediate
• Distance-Vector Routing ProtocolA routing protocol class where routers share their routing tables with neighbors and know only distance and direction to destinations.Foundational
• Distribute ListA Cisco command that filters routing updates in or out of a routing protocol using an ACL, prefix list or route map.Intermediate
• DMVPN Phase 3The DMVPN mode where the hub sends NHRP redirects and spokes install NHRP shortcut routes, allowing hub summarization.Advanced
• Dynamic Multipoint VPN (DMVPN)A Cisco hub-and-spoke VPN design using mGRE, NHRP and usually IPsec that can build dynamic spoke-to-spoke tunnels.Advanced
• Dynamic Trunking Protocol (DTP)A Cisco protocol that negotiates whether a link becomes a trunk, using modes dynamic auto, dynamic desirable, trunk and access.Intermediate
• eBGP MultihopA BGP option that raises the TTL of eBGP packets so peers need not be directly connected, such as loopback peering.Intermediate
• Edge PortAn RSTP port connected to an end host that transitions directly to forwarding and generates no topology changes.Intermediate
• EIGRP AuthenticationMD5 or HMAC-SHA-256 authentication of EIGRP packets using key chains (MD5) or named-mode passwords.Intermediate
• EIGRP Composite MetricEIGRP's metric calculated from bandwidth and delay by default, with optional load and reliability via K values.Intermediate
• EIGRP Hello and Hold TimersEIGRP sends hellos every 5 seconds (60 on slow NBMA) and declares a neighbor down after the hold time (15 or 180 seconds).Intermediate
• EIGRP Named ModeHierarchical EIGRP configuration that groups all IPv4 and IPv6 settings under address families in one process.Intermediate
• EIGRP Packet TypesThe five EIGRP packet types: Hello, Update, Query, Reply and Acknowledgment (plus SIA-Query and SIA-Reply).Intermediate
• EIGRP StubAn EIGRP router configured to advertise only selected routes and to be excluded from query propagation.Intermediate
• EIGRP Summary RouteA manually configured EIGRP summary advertised out an interface, which installs a Null0 discard route with AD 5.Intermediate
• EIGRP Topology TableThe EIGRP table storing all learned routes with their FD and RD, including successors and feasible successors.Intermediate
• EIGRP VarianceAn EIGRP multiplier that allows feasible successors with an FD up to variance times the best FD to be used for load sharing.Intermediate
• EIGRP Wide Metrics64-bit EIGRP metrics used in named mode to differentiate high-speed links, with a RIB scaling factor.Advanced
• Encapsulated Remote SPAN (ERSPAN)SPAN that encapsulates mirrored traffic in GRE so it can be routed across Layer 3 to a remote collector.Advanced
• Endpoint Identifier (EID)The LISP address that identifies an endpoint (host IP or MAC), independent of its network location.Advanced
• Enhanced Interior Gateway Routing Protocol (EIGRP)Cisco-developed advanced distance-vector IGP using DUAL for loop-free fast convergence; published as informational RFC 7868.Foundational
• Equal-Cost Multipath (ECMP)Installing and load sharing across multiple routes to the same prefix that have identical cost.Intermediate
• ErrdisableA Cisco port state where the switch shuts an interface due to a detected error such as BPDU guard, port security or UDLD.Intermediate
• EtherChannelCisco term for bundling multiple physical Ethernet links into one logical link for bandwidth and redundancy.Foundational
• Ethernet VPN (EVPN)A BGP-based control plane (RFC 7432) for Layer 2 and Layer 3 VPNs that advertises MAC and IP reachability with multihoming support.Advanced
• EVPN Route TypesEVPN NLRI types: 1 Ethernet A-D, 2 MAC/IP Advertisement, 3 Inclusive Multicast Ethernet Tag, 4 Ethernet Segment and 5 IP Prefix.Advanced
• EVPN-VPWSPoint-to-point Ethernet service signalled with BGP EVPN (RFC 8214) instead of LDP, supporting all-active multihoming.Advanced
• Expedited Forwarding (EF)A DiffServ PHB (DSCP 46) for low-loss, low-latency, low-jitter traffic such as voice, usually served by a priority queue.Foundational
• Explicit Congestion Notification (ECN)A mechanism using the two low-order bits of the DS byte to signal congestion without dropping packets.Advanced
• Extended CommunityA 64-bit BGP community with type and value fields, used for route targets, site of origin, EVPN and other functions.Advanced
• Extended System IDThe 12-bit field in the STP bridge priority that carries the VLAN ID, allowing a unique bridge ID per VLAN with one MAC.Intermediate
• External BGP (eBGP)A BGP session between routers in different autonomous systems.Foundational
• Fabric Border NodeThe SD-Access role connecting the fabric to external networks, translating VNs and SGTs at the boundary.Advanced
• Fabric Control Plane NodeThe SD-Access role running the LISP map server and map resolver, tracking where every endpoint is attached.Advanced
• Fabric Edge NodeThe SD-Access access switch that connects endpoints, authenticates them, registers them with the control plane and provides the anycast gateway.Advanced
• Fast Reroute (FRR)Precomputed local protection that switches traffic onto a backup path within about 50 ms of a failure.Advanced
• Feasibility Condition (FC)EIGRP's loop-free test: a neighbor qualifies as a feasible successor only if its reported distance is less than the current feasible distance.Intermediate
• Feasible Distance (FD)The lowest total EIGRP metric from the local router to a destination, through the successor.Intermediate
• Feasible Successor (FS)An EIGRP backup neighbor whose reported distance is lower than the current feasible distance, guaranteeing a loop-free path.Intermediate
• Filter-Based Forwarding (FBF)Junos policy routing that uses a firewall filter to direct matching traffic to a specific routing instance.Advanced
• Firewall FilterA Junos stateless packet filter, like an ACL, whose terms match packets (from) and apply actions such as accept, discard, count or policer.Intermediate
• First Hop Redundancy Protocol (FHRP)A class of protocols that provide a redundant default gateway through a shared virtual IP and MAC address.Foundational
• First In First Out (FIFO)The simplest queuing method where packets are transmitted in arrival order with no prioritization.Foundational
• Flexible Algorithm (Flex-Algo)An IGP extension that lets operators define additional SPF algorithms (128-255) with custom metrics and constraints, each with its own prefix SIDs.Advanced
• Floating Static RouteA static route given a higher administrative distance than the primary route so it installs only when the primary fails.Foundational
• Flood and LearnThe original VXLAN data-plane learning model in which BUM traffic is flooded through multicast and remote MACs are learned from received packets.Intermediate
• Forwarding Equivalence Class (FEC)A group of packets forwarded the same way through an MPLS network, mapped to the same label.Intermediate
• Forwarding Information Base (FIB)The data-plane table, derived from the RIB, used to forward packets at high speed.Intermediate
• Fully Specified Static RouteA static route that names both the exit interface and the next-hop IP address.Intermediate
• Fusion RouterA router or firewall outside an SD-Access fabric that leaks routes between virtual networks and shared services.Advanced
• Gateway Load Balancing Protocol (GLBP)Cisco-proprietary FHRP that provides gateway redundancy and load balancing using one virtual IP and multiple virtual MACs.Intermediate
• Generalized TTL Security Mechanism (GTSM)Protection that accepts BGP packets only if their TTL is at or near 255, blocking spoofed sessions from distant attackers.Advanced
• Graceful RestartA mechanism letting neighbors keep forwarding through a restarting router's control plane using preserved forwarding state.Advanced
• Hello PacketA periodic routing protocol message used to discover neighbors, verify parameters and maintain adjacencies.Foundational
• Hierarchical QoS (HQoS)A nested QoS policy where a parent policy shapes aggregate traffic and a child policy queues classes within the shaped rate.Advanced
• Hold-Down TimerA timer during which a router ignores updates with worse metrics for a route recently marked unreachable.Intermediate
• Hot Standby Router Protocol (HSRP)Cisco-proprietary FHRP with one active and one standby router sharing a virtual IP and MAC address.Foundational
• HSRP Interface TrackingReducing an HSRP router's priority when a tracked interface or object fails so the standby router can take over.Intermediate
• HSRP PreemptionAn HSRP option that lets a router with higher priority take over as active when it comes online.Foundational
• IEEE 802.1ad (Q-in-Q)A standard for stacking two VLAN tags (S-tag and C-tag) so providers can carry customer VLANs transparently.Advanced
• IEEE 802.1QThe IEEE standard for VLAN tagging that inserts a 4-byte tag with a 12-bit VLAN ID into Ethernet frames.Foundational
• IGMP SnoopingA switch feature that listens to IGMP messages to forward multicast frames only to ports with interested receivers.Intermediate
• Inclusive Multicast Ethernet Tag Route (IMET)EVPN Type 3 route that a VTEP or PE advertises to signal interest in BUM traffic for a VNI or EVI.Advanced
• Incremental SPF (iSPF)An optimization where only the affected portion of the SPF tree is recalculated after a change.Advanced
• Ingress Replication (IR)A BUM handling method where the ingress VTEP sends a unicast copy of each BUM packet to every remote VTEP in the VNI, avoiding underlay multicast.Advanced
• Integrated Routing and Bridging (IRB)A logical Layer 3 interface bound to a bridge domain or VLAN, the Junos and EVPN equivalent of an SVI.Intermediate
• Integrated Services (IntServ)A QoS model that reserves resources per flow end to end using RSVP signaling.Intermediate
• Inter-AS MPLS VPNOptions for extending MPLS L3VPNs across autonomous systems: Option A (back-to-back VRF), B (VPNv4 eBGP between ASBRs) and C (multihop VPNv4 between RRs).Advanced
• Inter-VLAN RoutingRouting packets between VLANs using a router or Layer 3 switch.Foundational
• Interior Gateway Protocol (IGP)A routing protocol used to exchange routes within a single autonomous system, such as OSPF, IS-IS, EIGRP or RIP.Foundational
• Intermediate System to Intermediate System (IS-IS)A link-state IGP originally for OSI CLNS that runs directly over Layer 2 and is widely used in service provider cores.Intermediate
• Internal BGP (iBGP)A BGP session between routers in the same autonomous system.Intermediate
• Internal Spanning Tree (IST)MST instance 0, which carries BPDUs for the region and connects it to the Common and Internal Spanning Tree (CIST).Advanced
• IP MulticastOne-to-many or many-to-many delivery where a source sends a single stream to a group address and routers replicate it only where receivers exist.Foundational
• IP PrecedenceThe legacy 3-bit priority field in the IPv4 ToS byte, values 0-7, superseded by DSCP.Foundational
• IP Prefix RouteEVPN Type 5 route that advertises an IP prefix decoupled from any MAC address, used for external and inter-subnet routes.Advanced
• IP SLACisco IOS feature that generates synthetic probes (ICMP, UDP jitter, HTTP) to measure reachability and performance.Intermediate
• IS-IS Level 1Intra-area IS-IS routing; L1 routers know only their own area and use the closest L1/L2 router for other destinations.Intermediate
• IS-IS Level 1/2 RouterAn IS-IS router participating in both levels, holding separate L1 and L2 databases, similar to an OSPF ABR.Intermediate
• IS-IS Level 2Inter-area (backbone) IS-IS routing between L2 and L1/L2 routers, which may be in different areas.Intermediate
• IS-IS Route LeakingAdvertising selected Level 2 prefixes into Level 1 so L1 routers choose optimal exits instead of the nearest default.Advanced
• IS-IS Wide MetricsIS-IS extended 24-bit interface and 32-bit path metrics carried in TLVs 22 and 135, replacing the 6-bit narrow metric.Advanced
• Junos Active Route SelectionThe Junos process of choosing one active route per prefix: lowest preference, then protocol-specific tie-breakers.Intermediate
• Junos Class of ServiceJuniper's QoS framework using forwarding classes, loss priorities, behavior aggregate classifiers, schedulers and rewrite rules.Advanced
• Junos Commit ModelThe Junos process of validating and activating the candidate configuration with commit, with options such as commit check, commit confirmed and commit at.Foundational
• Junos Default Routing PoliciesBuilt-in Junos policies applied at the end of each protocol's policy chain when no explicit policy accepts or rejects a route.Intermediate
• Junos OSJuniper Networks' network operating system, built on a FreeBSD-derived kernel (or Linux in Junos OS Evolved) with a single code train across platforms.Foundational
• Junos Policy ChainThe ordered list of policies applied to a protocol, evaluated until a terminating action (accept or reject) is reached, ending with the default policy.Intermediate
• Junos RollbackThe Junos command that loads a previously committed configuration (rollback 0-49) into the candidate.Foundational
• Junos Routing PolicyJunos import/export policy-statements whose terms use from (match) and then (action) clauses to control routes in and out of routing tables.Intermediate
• Junos Routing TablesDefault Junos tables such as inet.0 (IPv4 unicast), inet.3 (MPLS next hops), inet6.0 (IPv6), mpls.0 (labels) and bgp.l3vpn.0 (VPN routes).Advanced
• K ValuesThe EIGRP metric weights K1-K5 (plus K6 in wide metrics) that determine which components are used in the metric.Intermediate
• Key ChainA Cisco construct holding one or more numbered keys with optional lifetimes, used for routing protocol authentication.Intermediate
• L3VNIA VXLAN VNI associated with a tenant VRF, used to carry routed traffic between VTEPs in symmetric IRB.Advanced
• Label Distribution Protocol (LDP)A protocol that distributes label bindings for IGP prefixes between MPLS neighbors.Intermediate
• Label Edge Router (LER)An MPLS router at the edge of the label-switched domain that imposes or removes labels, typically a PE router.Intermediate
• Label Forwarding Information Base (LFIB)The data-plane table mapping incoming labels to outgoing labels, operations and next hops.Advanced
• Label Information Base (LIB)The table of all label bindings a router has learned from LDP neighbors, plus its own local bindings.Advanced
• Label StackThe ordered set of MPLS labels on a packet; routers act only on the top label.Intermediate
• Label Switch Router (LSR)Any router that forwards MPLS packets based on labels; core LSRs are called P routers.Intermediate
• Label Switched Path (LSP)The unidirectional path a labeled packet follows from ingress to egress LER through an MPLS network.Intermediate
• LDP-IGP SynchronizationA feature that keeps the IGP from using a link until LDP is operational on it, preventing traffic blackholing.Advanced
• Link Aggregation Control Protocol (LACP)IEEE 802.3ad/802.1AX protocol that negotiates link aggregation bundles; modes active and passive.Foundational
• Link Layer Discovery Protocol (LLDP)IEEE 802.1AB vendor-neutral Layer 2 neighbor discovery protocol using TLVs.Foundational
• Link-State Advertisement (LSA)An OSPF data unit describing a router's links, a network, or external and summary prefixes, flooded to build the LSDB.Intermediate
• Link-State Database (LSDB)The collection of LSAs a link-state router holds, identical among all routers in the same area.Intermediate
• Link-State PDU (LSP)The IS-IS packet that carries a router's link-state information, analogous to an OSPF LSA.Intermediate
• Link-State Routing ProtocolA routing protocol class where every router floods topology information and independently computes shortest paths with SPF.Foundational
• Load Balancing HashThe algorithm that selects which member link of an EtherChannel or ECMP group carries a flow, based on header fields.Intermediate
• Local Preference (LOCAL_PREF)A well-known discretionary BGP attribute shared among iBGP peers; the highest value is preferred for outbound traffic from the AS.Intermediate
• Localized PolicyA Cisco SD-WAN policy configured on WAN Edge devices for QoS, ACLs and route policy affecting local behavior.Advanced
• Locator/ID Separation Protocol (LISP)A protocol that separates endpoint identity (EID) from location (RLOC) using a mapping system, used as the SD-Access control plane.Advanced
• Longest Prefix Match (LPM)The forwarding rule that the route with the most specific (longest) matching prefix is used, regardless of AD or metric.Foundational
• Loop GuardAn STP feature that stops a non-designated port from becoming designated when BPDUs stop arriving, preventing loops from unidirectional links.Advanced
• Loop-Free Alternate (LFA)A precomputed backup next hop that is guaranteed loop-free, enabling sub-50 ms IGP fast reroute.Advanced
• Loopback InterfaceA logical, always-up router interface used for management, router IDs and stable peering addresses.Foundational
• Low Latency Queuing (LLQ)CBWFQ with a strict priority queue for real-time traffic, policed during congestion to prevent starvation of other classes.Intermediate
• MAC Address TableThe switch table mapping learned source MAC addresses to ports and VLANs, used for frame forwarding (CAM table).Foundational
• MAC MobilityAn EVPN mechanism using a sequence-number extended community to track hosts moving between VTEPs and to detect duplicate MACs.Advanced
• MAC/IP Advertisement RouteEVPN Type 2 route that advertises a host's MAC address and optionally its IP address, with L2 and L3 VNIs or labels.Advanced
• Maximum-PrefixA BGP neighbor limit on the number of prefixes accepted, protecting against route leaks and table overflows.Advanced
• MetricA protocol-specific value expressing the cost of a path; within one routing protocol the lowest metric route is preferred.Foundational
• Modular QoS CLI (MQC)Cisco's three-step QoS configuration framework: class-map, policy-map and service-policy.Intermediate
• MPLS LabelA 32-bit shim header containing a 20-bit label value, 3-bit Traffic Class (EXP), 1-bit bottom-of-stack flag and 8-bit TTL.Intermediate
• MPLS Layer 3 VPN (L3VPN)A provider service (RFC 4364) that carries customer IP routes in separate VRFs across an MPLS core using MP-BGP.Advanced
• MPLS Traffic Class (TC)The 3-bit field in an MPLS label (formerly EXP) used to carry QoS markings across the core.Advanced
• MPLS Traffic Engineering (MPLS TE)Steering traffic along explicit, constraint-based paths using RSVP-TE tunnels to use network capacity efficiently.Advanced
• MST RegionA group of interconnected switches with identical MST name, revision number and VLAN-to-instance mapping.Advanced
• Multi-Exit Discriminator (MED)An optional non-transitive BGP attribute that suggests to a neighboring AS which entry point to use; lower is preferred.Intermediate
• Multicast Address RangesKey IPv4 multicast blocks: 224.0.0.0/24 link-local, 232.0.0.0/8 SSM, 233.0.0.0/8 GLOP and 239.0.0.0/8 administratively scoped.Foundational
• Multicast Listener Discovery (MLD)The IPv6 equivalent of IGMP, carried in ICMPv6, for hosts to signal multicast group membership.Intermediate
• Multicast Source Discovery Protocol (MSDP)A TCP-based protocol (port 639) that shares active source information between RPs in different PIM domains or anycast RP sets.Advanced
• Multicast VPN (mVPN)Technology for carrying customer multicast across an MPLS L3VPN, using GRE/PIM (draft-Rosen) or BGP and mLDP/P2MP RSVP-TE.Advanced
• Multiple Spanning Tree Protocol (MST)IEEE 802.1s, which maps many VLANs to a small number of RSTP-based spanning tree instances.Advanced
• Multipoint GRE (mGRE)A GRE tunnel interface with no fixed destination that can reach many endpoints, used as the DMVPN overlay.Advanced
• Multipoint LDP (mLDP)LDP extensions for building point-to-multipoint and multipoint-to-multipoint LSPs, used for multicast VPN.Advanced
• Multiprotocol BGP (MP-BGP)Extensions to BGP (RFC 4760) that carry routing information for multiple address families such as IPv6, VPNv4, EVPN and multicast.Advanced
• Multiprotocol Label Switching (MPLS)A forwarding technology that switches packets using short labels instead of IP lookups, enabling VPNs, traffic engineering and fast reroute.Intermediate
• Mutual RedistributionRedistributing routes in both directions between two routing protocols, a common source of loops when done at multiple points.Advanced
• Native VLANThe VLAN whose frames are sent untagged across an 802.1Q trunk, VLAN 1 by default on Cisco.Foundational
• Network Entity TitleThe IS-IS router address in NSAP format, containing the area ID, system ID and NSEL of 00.Intermediate
• Network-Based Application Recognition (NBAR2)Cisco deep packet inspection engine that identifies applications (over 1,500 protocols) for classification and visibility.Intermediate
• Next Hop Resolution Protocol (NHRP)A protocol that maps overlay tunnel addresses to underlay NBMA addresses, acting like ARP for DMVPN.Advanced
• Next-Hop-SelfA BGP neighbor option that rewrites the NEXT_HOP to the router's own address when advertising to iBGP peers.Intermediate
• NEXT_HOP AttributeA well-known mandatory BGP attribute giving the IP address to forward toward for a prefix.Intermediate
• Nonstop Routing (NSR)A high-availability feature where the standby route processor maintains full routing protocol state so neighbors never notice a switchover.Advanced
• Not-So-Stubby Area (NSSA)An OSPF stub-like area that blocks Type 5 LSAs but allows a local ASBR to inject externals as Type 7 LSAs.Advanced
• Null0 InterfaceA virtual Cisco interface that discards traffic routed to it, used for summary and blackhole routes.Intermediate
• Object TrackingCisco feature that monitors the state of an interface, route or IP SLA and lets other features react to it.Intermediate
• On-Demand Next Hop (ODN)A Cisco SR-TE feature that automatically instantiates an SR policy when a BGP route arrives with a matching color community.Advanced
• Open Shortest Path First (OSPF)An open-standard link-state IGP that uses areas, LSAs and Dijkstra's SPF algorithm with a cost metric.Foundational
• Origin AttributeA well-known mandatory BGP attribute indicating how a route entered BGP: IGP (i), EGP (e) or incomplete (?).Intermediate
• Originator IDAn optional non-transitive BGP attribute set by a route reflector to the router ID of the route's original iBGP source.Advanced
• OSPF AreaA logical grouping of OSPF routers and links that share an LSDB, limiting flooding and SPF scope.Foundational
• OSPF Area 0The OSPF backbone area to which all other areas must connect and through which all inter-area traffic passes.Foundational
• OSPF Area RangeInter-area summarization configured on an ABR to advertise one Type 3 LSA for a block of intra-area routes.Intermediate
• OSPF AuthenticationMethods for authenticating OSPF packets: null, plaintext, MD5 and HMAC-SHA with key chains.Intermediate
• OSPF CostThe OSPF interface metric, calculated as reference bandwidth divided by interface bandwidth, summed along a path.Foundational
• OSPF Down BitA flag a PE sets in Type 3/5/7 LSAs sent to a CE so other PEs will not redistribute them back into MP-BGP.Advanced
• OSPF External Route TypeOSPF E1 and E2 routes: E2 (default) keeps only the redistributed metric, while E1 adds internal cost to reach the ASBR.Intermediate
• OSPF Forwarding AddressA field in Type 5 and Type 7 LSAs that tells routers which next-hop address to use for an external route.Advanced
• OSPF Hello and Dead IntervalsOSPF timers: how often hellos are sent and how long to wait without hellos before declaring a neighbor down.Foundational
• OSPF LSA FilteringTechniques that block specific LSAs from entering an area, such as filter-list on an ABR or database-filter on an interface.Advanced
• OSPF MTU MismatchA condition where OSPF neighbors have different interface MTUs, leaving the adjacency stuck in ExStart or Exchange.Intermediate
• OSPF Neighbor StatesThe progression an OSPF adjacency follows: Down, Attempt, Init, 2-Way, ExStart, Exchange, Loading, Full.Intermediate
• OSPF Network StatementAn OSPF configuration command that uses an address and wildcard mask to select interfaces and assign them to an area.Foundational
• OSPF Network TypesOSPF interface modes - broadcast, non-broadcast, point-to-point, point-to-multipoint and loopback - controlling DR election, timers and neighbor discovery.Intermediate
• OSPF Packet TypesThe five OSPF packet types: Hello, Database Description (DBD), Link-State Request (LSR), Link-State Update (LSU) and Link-State Acknowledgment.Intermediate
• OSPF Path Selection OrderOSPF prefers intra-area over inter-area over E1/N1 over E2/N2 routes for the same prefix, before comparing cost.Intermediate
• OSPF PriorityAn interface value from 0 to 255 (default 1) used in DR/BDR election; 0 makes the router ineligible.Foundational
• OSPF Sham LinkA logical intra-area link between PEs across the MPLS backbone so VPN routes are preferred over an OSPF backdoor link.Advanced
• OSPF Stub AreaAn OSPF area that blocks Type 4 and Type 5 external LSAs and receives a default route from the ABR instead.Intermediate
• OSPF Stub Router AdvertisementOSPF max-metric router-lsa: advertising maximum link cost so other routers avoid transiting this router.Advanced
• OSPF Summary AddressAn ASBR command that summarizes external (Type 5 or Type 7) routes redistributed into OSPF.Intermediate
• OSPF Totally Stubby AreaA Cisco OSPF area type that blocks Type 3, 4 and 5 LSAs, leaving only a default route from the ABR.Intermediate
• OSPF Type 1 Router LSAThe LSA every OSPF router originates in each area, describing its links, costs and neighbors; flooded within the area only.Intermediate
• OSPF Type 2 Network LSAThe LSA originated by the DR on a multi-access segment, listing all routers attached to that transit network.Intermediate
• OSPF Type 3 Summary LSAThe LSA an ABR originates to advertise prefixes from one area into another.Intermediate
• OSPF Type 4 ASBR Summary LSAAn LSA originated by an ABR to tell routers in another area how to reach an ASBR.Advanced
• OSPF Type 5 External LSAAn LSA originated by an ASBR to advertise routes redistributed from outside OSPF, flooded throughout all normal areas.Intermediate
• OSPF Type 7 NSSA LSAAn external LSA originated by an ASBR inside an NSSA, converted to Type 5 by the NSSA ABR.Advanced
• OSPF Virtual LinkA logical link through a transit area that connects a remote area or partitioned backbone to area 0.Advanced
• OSPFv3The version of OSPF for IPv6 (and, with address families, IPv4) defined in RFC 5340.Intermediate
• Overlay Management Protocol (OMP)The TCP-based Cisco SD-WAN control protocol, running inside DTLS/TLS tunnels, that distributes routes, TLOCs, keys and policy.Advanced
• Overload BitAn IS-IS LSP flag telling other routers not to use this router for transit traffic.Advanced
• Packet Forwarding Engine (PFE)The Junos data plane component, usually ASIC based, that forwards transit traffic using the forwarding table and applies filters, policers and CoS.Foundational
• Partial Sequence Number PDU (PSNP)An IS-IS PDU used to acknowledge received LSPs on point-to-point links or request missing LSPs on broadcast links.Advanced
• Passive InterfaceAn interface on which a routing protocol advertises the connected network but does not send hellos or form adjacencies.Foundational
• Path Computation Element (PCE)A controller or router that computes paths for headends, communicating via PCEP.Advanced
• Path Computation Element Communication Protocol (PCEP)The TCP-based protocol (port 4189) between a PCE and path computation clients for path requests, updates and delegation.Advanced
• Path-Vector Routing ProtocolA routing protocol class that advertises the full sequence of autonomous systems a route has traversed, used for loop prevention.Intermediate
• PE-CE RoutingThe routing protocol between a provider PE (in a VRF) and a customer CE: static, eBGP, OSPF, EIGRP, IS-IS or RIP.Advanced
• Penultimate Hop Popping (PHP)The MPLS behavior where the next-to-last router removes the top label so the egress router does only one lookup.Advanced
• Per-Hop Behavior (PHB)The forwarding treatment a DiffServ router applies to packets based on their DSCP marking.Intermediate
• Per-VLAN Spanning Tree Plus (PVST+)Cisco's STP variant that runs a separate 802.1D instance per VLAN, allowing per-VLAN root placement and load sharing.Foundational
• PIM Dense Mode (PIM-DM)A push-model multicast protocol that floods traffic everywhere and prunes branches without receivers.Intermediate
• PIM Designated RouterThe PIM router elected on a multiaccess segment (highest DR priority, then highest IP) to send joins and registers for local hosts.Advanced
• PIM Sparse Mode (PIM-SM)A pull-model multicast protocol where routers explicitly join a shared tree rooted at a rendezvous point and may switch to a source tree.Intermediate
• Poison ReverseA rule where a router advertises a route back toward its source with an infinite metric, overriding split horizon.Intermediate
• PolicingA QoS action that enforces a rate limit by dropping or remarking traffic that exceeds it, without buffering.Foundational
• Policy-Based Routing (PBR)Forwarding packets based on criteria other than destination, such as source address or application, using a route map.Intermediate
• Port Aggregation Protocol (PAgP)Cisco-proprietary EtherChannel negotiation protocol with modes desirable and auto.Foundational
• Port-Channel InterfaceThe logical interface representing an EtherChannel bundle on Cisco devices, where Layer 2 or Layer 3 settings are applied.Foundational
• PortFastA Cisco feature that moves an access port directly to forwarding, skipping listening and learning.Foundational
• Prefix ListA Cisco filter that matches routes by prefix and prefix length range, preferred over ACLs for route filtering.Intermediate
• Prefix SIDA global segment associated with an IGP prefix that steers traffic along the ECMP-aware shortest path to that prefix.Advanced
• Private AS NumberASN ranges 64512-65534 (2-byte) and 4200000000-4294967294 (4-byte) reserved for private use and not routed on the Internet.Intermediate
• Private VLAN (PVLAN)A feature that subdivides a primary VLAN into isolated and community secondary VLANs that share one subnet.Advanced
• Process SwitchingThe slowest Cisco forwarding method, where the CPU performs a full routing lookup for every packet.Intermediate
• Proposal and AgreementThe RSTP handshake on point-to-point links that lets a designated port move to forwarding immediately.Advanced
• Protocol Independent Multicast (PIM)The family of multicast routing protocols that build distribution trees using the existing unicast routing table for RPF checks.Intermediate
• Provider Edge Router (PE)A service provider router that connects to customer edge devices and holds customer VRFs in MPLS VPNs.Intermediate
• Provider RouterA service provider core router that only switches labeled packets and has no customer-facing interfaces or VPN routes.Intermediate
• PseudonodeA virtual node representing a broadcast segment in a link-state topology, created by the IS-IS DIS or OSPF DR.Advanced
• Pseudowire (PW)An emulated point-to-point Layer 2 circuit across an MPLS or IP network, carrying Ethernet, Frame Relay or ATM frames.Advanced
• QoS Trust BoundaryThe point in the network where QoS markings from endpoints start being trusted rather than rewritten.Intermediate
• Quality of Service (QoS)Tools that classify, mark, queue, shape and drop traffic to give applications appropriate bandwidth, delay, jitter and loss.Foundational
• Random Early Detection (RED)A congestion avoidance mechanism that randomly drops packets as average queue depth rises between minimum and maximum thresholds.Intermediate
• Rapid PVST+Cisco's per-VLAN implementation of RSTP (802.1w), the default STP mode on current Catalyst switches.Intermediate
• Rapid Spanning Tree Protocol (RSTP)IEEE 802.1w, a faster STP that uses proposal/agreement handshakes, edge ports and alternate ports to converge in seconds or less.Intermediate
• Recursive Route LookupResolving a route's next-hop address by performing another routing table lookup to find the outgoing interface.Intermediate
• Reference BandwidthThe bandwidth value OSPF divides by interface bandwidth to calculate cost, 100 Mbps by default.Foundational
• Remote SPAN (RSPAN)SPAN that transports mirrored traffic across a Layer 2 network in a dedicated RSPAN VLAN to a remote switch.Advanced
• Remote-Triggered Black Hole (RTBH)A DDoS mitigation technique where a BGP-signaled route sends attack traffic to a discard (Null0) next hop at the network edge.Advanced
• Rendezvous Point (RP)The PIM-SM router where sources and receivers meet; it roots the shared tree for a group.Intermediate
• Reported Distance (RD)The EIGRP metric a neighbor advertises for its own path to a destination; also called advertised distance.Intermediate
• Rescue ConfigurationA known-good Junos configuration saved by the administrator for recovery, loadable with rollback rescue.Foundational
• Resource Public Key Infrastructure (RPKI)A cryptographic system that lets networks validate that an AS is authorized to originate a prefix (route origin validation).Advanced
• Resource Reservation Protocol (RSVP)A signaling protocol (IP protocol 46) that reserves resources along a path, used by IntServ and MPLS TE.Advanced
• Reverse Path Forwarding Check (RPF)The multicast loop-prevention check that accepts a packet only if it arrives on the interface the router would use to reach the source (or RP).Intermediate
• RIB GroupA Junos mechanism to install routes from a protocol or interface into multiple routing tables simultaneously.Advanced
• RIP Version 2 (RIPv2)Classless RIP that carries subnet masks, supports authentication and multicasts updates to 224.0.0.9.Foundational
• RIPngRIP next generation: the IPv6 version of RIP, using UDP port 521 and multicast FF02::9.Intermediate
• Root BridgeThe switch with the lowest bridge ID, which acts as the reference point of the STP topology; all its ports are designated.Foundational
• Root GuardAn STP feature that prevents a port from becoming a root port; superior BPDUs place it in root-inconsistent state.Intermediate
• Root Port (RP)The single port on each non-root switch with the lowest cost path to the root bridge.Foundational
• Route Distinguisher (RD)A 64-bit value prepended to an IPv4 prefix to make it a unique VPNv4 route, allowing overlapping customer addresses.Advanced
• Route FilterA Junos policy match condition that selects routes by prefix and match type: exact, orlonger, longer, upto, through and prefix-length-range.Intermediate
• Route Flap DampeningA BGP mechanism that suppresses routes that repeatedly flap by assigning penalties that decay over time.Advanced
• Route LeakingSelectively copying routes between routing tables such as VRFs or IS-IS levels to allow controlled reachability.Advanced
• Route MapAn ordered Cisco IOS policy construct of permit/deny sequences with match and set clauses, used for filtering, redistribution, PBR and BGP policy.Intermediate
• Route PoisoningAdvertising a failed route with an infinite metric so neighbors immediately remove it instead of waiting for it to age out.Intermediate
• Route Policy Language (RPL)The IOS XR policy language using route-policy blocks with if/then/else logic and named sets instead of route maps.Advanced
• Route PreferenceJunos term for administrative distance: a 0-4294967295 value where the lower preference wins between route sources.Intermediate
• Route RedistributionInjecting routes learned from one routing source into another routing protocol.Intermediate
• Route RefreshA BGP capability that lets a router ask a peer to resend its routes so new inbound policy can be applied without resetting the session.Intermediate
• Route SummarizationAdvertising a single aggregate prefix that covers multiple more specific routes to reduce table size and contain instability.Intermediate
• Route TagA numeric label attached to a route that carries through redistribution and can be matched by policy.Intermediate
• Route Target (RT)A BGP extended community that controls which VRFs export and import VPN routes, enabling topologies like hub-and-spoke and extranet.Advanced
• Routed PortA physical switch port converted to Layer 3 with no switchport, behaving like a router interface.Intermediate
• Router ID (RID)A 32-bit identifier, written as an IPv4 address, that uniquely identifies a router within OSPF, EIGRP or BGP.Foundational
• Router on a Stick (ROAS)Inter-VLAN routing using one router interface trunked to a switch with an 802.1Q subinterface per VLAN.Foundational
• Routing Engine (RE)The Junos control plane component that runs routing protocols, the CLI and management, and builds the forwarding table.Foundational
• Routing Information Base (RIB)The control-plane table holding the best routes chosen from all routing sources, used to build the forwarding table.Intermediate
• Routing Information Protocol (RIP)A distance-vector IGP that uses hop count as its metric, with a 15-hop maximum and 30-second periodic updates.Foundational
• Routing InstanceA Junos construct that creates a separate routing table and set of interfaces, with types including virtual-router, vrf, forwarding and evpn/mac-vrf.Intermediate
• Routing Locator (RLOC)The LISP address of the tunnel router behind which an EID resides, typically a loopback in the underlay.Advanced
• Routing TableThe router's table of best known paths to destination prefixes, built from connected, static and dynamic routing sources.Foundational
• RSVP-TERSVP extensions used to signal MPLS TE LSPs with explicit routes, bandwidth reservations and label distribution.Advanced
• Running ConfigurationThe active configuration held in RAM on Cisco IOS devices, distinct from the startup configuration stored in NVRAM.Foundational
• Scalable Group Tag (SGT)A 16-bit Cisco TrustSec tag that identifies a group of users or devices for role-based policy regardless of IP address.Advanced
• SD-WAN ColorA Cisco SD-WAN label on a transport interface (such as mpls, private1, biz-internet, lte) that identifies the transport type for tunnel building and policy.Advanced
• SD-WAN ControllerThe Cisco Catalyst SD-WAN control plane element (formerly vSmart) that distributes routes, TLOCs and policies to WAN Edges via OMP.Intermediate
• SD-WAN ManagerThe Cisco Catalyst SD-WAN management plane (formerly vManage): GUI and API for configuration, templates, monitoring and policy.Intermediate
• SD-WAN Service VPNA Cisco SD-WAN segment (VRF) on the LAN side carrying user traffic, numbered 1-511 except the reserved VPN 0 and VPN 512.Intermediate
• SD-WAN ValidatorThe Cisco Catalyst SD-WAN orchestration element (formerly vBond) that authenticates devices and helps them discover controllers and traverse NAT.Intermediate
• Seed MetricThe initial metric assigned to routes redistributed into a routing protocol.Intermediate
• Segment Identifier (SID)An identifier for a segment: an MPLS label in SR-MPLS or an IPv6 address in SRv6.Advanced
• Segment Routing (SR)A source-routing architecture where the ingress node encodes a path as an ordered list of segments, with no per-flow state in the core.Advanced
• Segment Routing Global Block (SRGB)The label range reserved on each router for global SR-MPLS prefix SIDs, default 16000-23999 on Cisco.Advanced
• Segment Routing Header (SRH)An IPv6 routing extension header (routing type 4) that lists SRv6 segments and a Segments Left pointer.Advanced
• Segment Routing Traffic Engineering (SR-TE)Traffic engineering using SR policies identified by headend, color and endpoint, steering traffic onto explicit or dynamic segment lists.Advanced
• ShapingA QoS action that buffers excess traffic to send it at a configured rate, smoothing bursts.Foundational
• Shared Risk Link Group (SRLG)A group of links that share a common physical resource, such as a conduit, and could fail together.Advanced
• Shared TreeA multicast distribution tree rooted at the RP and used by all sources for a group, written (*,G).Intermediate
• Shortest Path First Algorithm (SPF)Dijkstra's algorithm, used by OSPF and IS-IS to compute a loop-free shortest path tree from the LSDB.Intermediate
• Shortest Path Tree (SPT)A multicast source tree rooted at the source, written (S,G), giving the optimal path from source to receivers.Intermediate
• Site of Origin (SoO)A BGP extended community identifying the customer site a route came from, preventing loops at multihomed sites.Advanced
• SLA ClassA Cisco SD-WAN policy object defining maximum loss, latency and jitter for application-aware routing.Advanced
• Soft ReconfigurationA BGP option that stores unmodified inbound routes from a neighbor so policy can be re-applied locally.Advanced
• Source-Specific Multicast (SSM)A PIM mode in which receivers join a specific (S,G) channel directly, eliminating the RP and shared tree.Intermediate
• Spanning Tree Protocol (STP)IEEE 802.1D protocol that prevents Layer 2 loops by electing a root bridge and blocking redundant paths.Foundational
• Split HorizonA loop-prevention rule that stops a router from advertising a route out the interface on which it was learned.Intermediate
• SR-MPLSSegment routing using the MPLS data plane, where each segment is an MPLS label.Advanced
• SRv6Segment routing over the IPv6 data plane, where segments are IPv6 addresses carried in the destination address and optional Segment Routing Header.Advanced
• StackWise VirtualCisco Catalyst 9000 technology that combines two switches into one logical switch with a single control plane.Advanced
• Stateful Switchover (SSO)A Cisco high-availability feature where the standby route processor is kept synchronized so it can take over without resetting interfaces.Advanced
• Static RouteA manually configured route to a destination prefix through a next-hop address or exit interface.Foundational
• Storm ControlA switch feature that limits broadcast, multicast or unknown unicast traffic on a port to a threshold.Intermediate
• STP Path CostThe cost STP assigns to a port based on its speed, summed toward the root to choose root and designated ports.Intermediate
• STP Port States802.1D states: blocking, listening, learning, forwarding and disabled; RSTP collapses these to discarding, learning and forwarding.Foundational
• STP TimersThe 802.1D timers: hello time 2 s, forward delay 15 s and max age 20 s, all dictated by the root bridge.Intermediate
• Stuck in Active (SIA)An EIGRP condition where a router waits too long for replies to queries, eventually resetting neighbor adjacencies.Advanced
• SuccessorThe EIGRP neighbor offering the lowest-metric loop-free path to a destination; its route is installed in the routing table.Intermediate
• Switch StackingConnecting multiple switches with dedicated stack cables so they operate as one logical switch with one management plane.Intermediate
• Switched Virtual Interface (SVI)A virtual Layer 3 interface on a switch representing a VLAN, used for inter-VLAN routing or management.Foundational
• Symmetric IRBEVPN inter-subnet routing where both ingress and egress VTEPs route, using a shared L3VNI per tenant VRF.Advanced
• System IDThe 6-byte field in an IS-IS NET that uniquely identifies a router within the routing domain.Advanced
• System IPA persistent IPv4 address identifying a Cisco SD-WAN device, similar to a router ID, independent of interface addresses.Intermediate
• Tail DropThe default congestion behavior where packets arriving at a full queue are discarded.Foundational
• Targeted LDP (T-LDP)An LDP session between non-adjacent routers using unicast hellos, used for pseudowires and remote LFA.Advanced
• TCP Global SynchronizationA condition where many TCP flows experience drops simultaneously, all reduce their windows, then ramp up together, causing oscillating link utilization.Intermediate
• Ternary Content-Addressable Memory (TCAM)Hardware memory that matches on 0, 1 or don't-care bits, used for ACLs, QoS and longest-prefix route lookups.Advanced
• Token BucketThe algorithm policers and shapers use: tokens accumulate at the CIR and each transmitted byte consumes one token.Intermediate
• Topology Change Notification (TCN)An 802.1D BPDU sent toward the root when a port changes state, causing switches to shorten MAC aging to forward delay.Advanced
• Topology-Independent Loop-Free Alternate (TI-LFA)A segment routing fast reroute method that provides 100 percent link, node and SRLG protection using the post-convergence path.Advanced
• Transit ASAn autonomous system that carries traffic between two other autonomous systems.Intermediate
• Transport Locator (TLOC)A Cisco SD-WAN identifier of a WAN Edge transport attachment point: system IP, color and encapsulation.Advanced
• Trunk PortA switch port that carries multiple VLANs over one link, identifying them with 802.1Q tags.Foundational
• Two-Rate Three-Color Marker (trTCM)A policer (RFC 2698) with CIR and PIR that colors traffic conform (green), exceed (yellow) or violate (red).Advanced
• Type-Length-Value (TLV)An extensible encoding of a type code, length and value used by IS-IS, LLDP, BGP and other protocols.Advanced
• Unequal-Cost Load BalancingDistributing traffic across paths with different metrics, natively supported by EIGRP through the variance command.Intermediate
• Unicast Reverse Path Forwarding (uRPF)A feature that drops packets whose source address is not reachable via the routing table, mitigating spoofing.Advanced
• Unidirectional Link Detection (UDLD)A Cisco Layer 2 protocol that detects links where traffic flows in only one direction, common with fiber faults.Advanced
• Unknown Unicast FloodingForwarding a frame out all ports in its VLAN (except the source) when the destination MAC is not in the MAC table.Intermediate
• Update-SourceA BGP neighbor option that sets the source interface (usually a loopback) for the TCP session.Intermediate
• Virtual ChassisJuniper technology that interconnects multiple EX or QFX switches to be managed as a single logical device.Intermediate
• Virtual Extensible LAN (VXLAN)An overlay encapsulation (RFC 7348) that carries Layer 2 frames inside UDP/IP packets, identified by a 24-bit VNI.Intermediate
• Virtual Local Area Network (VLAN)A logical Layer 2 broadcast domain created on switches, independent of physical location.Foundational
• Virtual Network (VN)An SD-Access macro-segment implemented as a VRF across the fabric, isolating groups such as employees, guests and IoT.Advanced
• Virtual Private LAN Service (VPLS)A multipoint Layer 2 VPN that emulates an Ethernet LAN across an MPLS network using a mesh of pseudowires.Advanced
• Virtual Private Wire Service (VPWS)A point-to-point Layer 2 VPN service delivered over pseudowires or EVPN-VPWS.Advanced
• Virtual Router Redundancy Protocol (VRRP)IETF-standard FHRP (RFC 5798 for VRRPv3) with one master and backups sharing a virtual IP that may be a real interface address.Foundational
• Virtual Routing and Forwarding (VRF)A technology that creates multiple independent routing tables on one router so overlapping or separated networks coexist.Intermediate
• VLAN Trunking Protocol (VTP)A Cisco protocol that propagates VLAN definitions from a server to other switches in the same VTP domain.Intermediate
• VLAN Trunking Protocol PruningA VTP feature that stops flooding of broadcast and unknown traffic for a VLAN across trunks to switches with no ports in that VLAN.Intermediate
• Voice QoS RequirementsGuidelines for acceptable voice quality: one-way delay under 150 ms, jitter under 30 ms and packet loss under 1 percent.Foundational
• Voice VLANA separate VLAN on an access port for IP phone traffic, which the phone tags while passing PC traffic untagged.Foundational
• VPNv4 Address FamilyThe MP-BGP address family that carries 12-byte VPN-IPv4 routes (RD + IPv4 prefix) and VPN labels between PEs.Advanced
• VRF-LiteUsing VRFs for Layer 3 segmentation without MPLS or MP-BGP, by carrying each VRF on separate interfaces or subinterfaces hop by hop.Intermediate
• VXLAN Network Identifier (VNI)The 24-bit segment ID in the VXLAN header that identifies a Layer 2 (L2VNI) or Layer 3 (L3VNI) virtual network.Intermediate
• WeightA Cisco-proprietary BGP value local to one router; the highest weight is preferred first in best-path selection.Intermediate
• Weighted Random Early Detection (WRED)RED with different drop profiles per IP Precedence or DSCP, so lower-priority packets are dropped earlier.Intermediate
• Well-Known BGP CommunitiesReserved communities with standard meaning: NO_EXPORT, NO_ADVERTISE, NO_EXPORT_SUBCONFED (local-AS) and others.Intermediate
• Wildcard MaskAn inverse mask where 0 bits must match and 1 bits are ignored, used in ACLs and OSPF/EIGRP network statements.Foundational
• Zero-Touch Provisioning (ZTP)Automated onboarding where a new device obtains configuration and software from a server without manual console work.Intermediate