JNCIS-ENT key terms

A logical Layer 2 broadcast domain built by grouping switch ports or by tagging frames, so devices can share a segment across switches while staying isolated from other VLANs.

A switch port assigned to exactly one VLAN that sends untagged frames to an attached end device such as a PC or printer, which never sees VLAN tags.

A switch port that carries traffic for several VLANs at once, adding a tag to each frame so the far end can place it back into the right VLAN.

A four-byte field inserted into an Ethernet header holding a VLAN identifier and priority bits, letting one physical link transport traffic for many VLANs.

The one VLAN whose frames cross a trunk without a tag; both ends must agree on it, or untagged traffic lands in the wrong broadcast domain.

A bridge protocol data unit is the Layer 2 message switches exchange to elect a root bridge, advertise path costs, and announce topology changes under spanning tree.

The switch elected as the reference point of a spanning tree; every other bridge builds a loop-free path toward it, and inter-switch traffic tends to cross it.

Rapid Spanning Tree Protocol (802.1w) converges in seconds using explicit port roles and a proposal-agreement handshake instead of the long timers of classic STP.

Multiple Spanning Tree Protocol assigns groups of VLANs to separate spanning tree instances, so blocking a link for one group does not affect the others.

A spanning tree port facing an end host that skips the listening and learning delays and goes straight to forwarding, because one attached device cannot create a loop.

A value derived from link bandwidth or set manually that spanning tree sums along a path; the lowest total decides which ports forward and which block.

A Layer 2 security feature that blocks or disables a port as soon as it receives a BPDU, stopping unauthorized switches and loops on user-facing access ports.

A protection that refuses superior BPDUs on a port, so a downstream switch cannot take over as root bridge and reshape the whole spanning tree topology.

A per-interface threshold on broadcast, multicast, or unknown unicast traffic; frames above the limit are dropped or the port is shut down to protect the switch.

A port setting that caps the number of source MAC addresses learned, optionally restricting them to a fixed list, so unknown devices cannot send traffic.

A switch feature that marks ports trusted or untrusted and discards server-sourced DHCP messages received on untrusted ports, defeating rogue DHCP servers and lease exhaustion attacks.

A Layer 2 check that compares ARP packets with the DHCP snooping binding table and drops replies claiming an IP address the sender was never leased.

The collection of known destinations with their next hops and preferences that a router consults to decide where each packet should go.

The rule that when several routes cover a destination address, the router uses the one with the most specific, longest network mask.

A per-protocol ranking, also called administrative distance, that decides which source of routing information wins when two protocols offer the same prefix; lower values are preferred.

A route an administrator enters manually, mapping a destination prefix to a next hop or interface; it changes only when edited, removed, or withdrawn by a tracking mechanism.

A logical group of OSPF routers and links; detailed topology stays inside the area, while summarized information crosses between areas through the backbone, area 0.

The relationship two OSPF routers form after hello parameters match and they exchange database descriptions; only adjacent routers swap link-state information directly.

On a multi-access OSPF segment, the elected designated router and its backup represent the segment in the topology database, cutting the number of required adjacencies.

A link-state advertisement is the OSPF record describing a router, network, or prefix, flooded through an area and collected into the database that SPF runs on.

A network entity title is the OSI address given to an IS-IS router; it carries the system ID identifying the node and the area address the node belongs to.

The two-tier IS-IS hierarchy: level 1 routers exchange routes inside an area, while level 2 routers form the backbone carrying traffic between areas.

The designated intermediate system elected on a broadcast IS-IS link to generate the pseudonode LSP, giving the shared segment one representation in the topology.

A link-state PDU is the IS-IS packet carrying a router's neighbors and prefixes; all LSPs in an area together form the database used for SPF.

A group of routers and networks under one administration with a single routing policy, identified by a number that BGP uses to compare paths between organizations.

A BGP session between peers in different autonomous systems; it exchanges routes between organizations and normally needs directly connected peers unless multihop is configured.

A BGP session between peers inside one autonomous system; routes learned from one internal peer are not passed to another, so a full mesh or route reflectors are required.

A field attached to a BGP route, such as AS path, local preference, or MED, that describes the route; comparing these in order determines the best path.

The address a router uses to reach a BGP-learned prefix; the route stays unusable until that address is resolvable through the IGP or a connected interface.

An internal BGP router permitted to re-advertise routes learned from one iBGP peer to other iBGP peers, removing the need for a full mesh of sessions.

Generic Routing Encapsulation wraps an original packet inside a new IP header so it can cross a network that would not otherwise carry it, creating a point-to-point logical link.

A simple tunnel that places an IPv4 packet directly inside another IPv4 header, connecting two sites without the extra protocol field that GRE adds.

Virtual Router Redundancy Protocol lets several routers share one virtual IP and MAC address; a master forwards traffic while backups monitor it and take over on failure.

Bidirectional Forwarding Detection is a lightweight hello protocol that detects path failures in milliseconds, letting routing protocols and redundancy features react faster than their own timers.

A link aggregation group bundles several physical links into one logical interface for more bandwidth and resiliency; LACP negotiates which member links belong to the bundle.