100-490 RSTECH key terms

A 48-bit hardware address burned into a network interface, written as six hexadecimal pairs. Switches use it to forward frames within a local network segment.

The content-addressable memory table on a switch that maps learned MAC addresses to switch ports and VLANs. Entries age out after a period of inactivity.

A switch behavior that copies a frame out every port in the VLAN except the one it arrived on, used when the destination MAC address is unknown, broadcast, or multicast.

A logical Layer 2 broadcast domain created by configuring switch ports into groups. Traffic cannot cross between VLANs without a Layer 3 device such as a router or multilayer switch.

A switch port assigned to a single VLAN that carries untagged frames to an end device such as a PC, printer, or access point.

A switch port that carries traffic for multiple VLANs, tagging frames with 802.1Q headers so the receiving device can separate them into the correct broadcast domains.

The IEEE standard for VLAN tagging on Ethernet links. It inserts a 4-byte tag into the frame header containing a 12-bit VLAN identifier and priority bits.

The VLAN whose frames travel untagged across an 802.1Q trunk. Both ends of the link must agree on it, or traffic leaks between VLANs and a mismatch warning appears.

A Layer 2 protocol that prevents switching loops by electing a root bridge and blocking redundant paths, recalculating the topology when a link fails.

The switch elected as the logical center of a spanning-tree topology, chosen by lowest bridge ID. All other switches compute least-cost paths toward it.

A bridge protocol data unit, the spanning-tree message switches exchange every two seconds by default to advertise bridge IDs, root costs, and port roles.

A spanning-tree feature applied to edge ports that skips the listening and learning states so end devices can communicate immediately after link-up.

A bundle of physical links treated as one logical link to increase bandwidth and provide redundancy. All member ports must share speed, duplex, and VLAN settings.

Link Aggregation Control Protocol, an IEEE standard that negotiates EtherChannel formation by exchanging control frames between two devices.

The router address a host sends packets to when the destination is outside its own subnet. A wrong or missing gateway stops off-subnet communication.

A 32-bit value that separates the network portion of an IPv4 address from the host portion, letting a device decide whether a destination is local or remote.

A shorthand for expressing an IP network and its prefix length, such as 192.168.10.0/24, where the number after the slash counts network bits.

A device's list of known destinations, next hops, outgoing interfaces, and metrics. The router forwards each packet using the most specific matching entry.

A trust ranking from 0 to 255 that a router uses to choose between routes to the same prefix learned from different sources. Lower values win.

A route an administrator configures manually, specifying the destination prefix and next hop or exit interface. It does not adapt automatically to topology changes.

A route for 0.0.0.0/0 that matches any destination without a more specific entry, commonly pointing toward an ISP or upstream router.

An open-standard link-state interior gateway protocol that builds a shared topology database using link-state advertisements and computes shortest paths with the SPF algorithm.

A periodic OSPF message that discovers neighbors, verifies matching area, timers, and network type, and maintains adjacency by confirming the peer is still reachable.

The permanent MAC address assigned by the manufacturer to an interface. It can be overridden in software, but the hardware value remains unchanged.

The saved configuration file in NVRAM that a device loads at boot. Changes made in running configuration are lost on reload unless copied to startup.

The active configuration held in RAM that controls current device behavior. It is edited in configuration mode and disappears on reload if not saved.

Nonvolatile RAM that retains its contents without power and stores the startup configuration, so settings survive a reboot.

The command-line interface used to configure and monitor Cisco devices, organized into user EXEC, privileged EXEC, global configuration, and interface configuration modes.

A hashed password that protects entry to privileged EXEC mode. It takes precedence over the older enable password when both are configured.

A physical serial port on a device used for out-of-band local management, typically reached with a rollover cable and terminal emulation software.

A encrypted protocol for remote command-line management of network devices, preferred over Telnet because credentials and session data are protected in transit.

An older remote management protocol that sends credentials and commands in cleartext. It should be replaced by SSH wherever possible.

The CLI context entered with the interface command, where settings such as IP address, speed, duplex, and shutdown state are applied to a specific port.

An interface-level command that administratively disables a port. The no shutdown form brings it back into service and is required on new router interfaces.

Cisco Discovery Protocol, a Layer 2 neighbor discovery protocol that advertises device name, platform, and connected port information to directly attached Cisco devices.

Link Layer Discovery Protocol, an IEEE standard neighbor discovery protocol that shares device and port details between directly connected devices from multiple vendors.

A tool that sends ICMP echo requests to test reachability and measure round-trip time to a destination address.

A diagnostic tool that reveals the path packets take to a destination by using incrementing TTL values and collecting time-exceeded responses from each hop.

Time to live, an IP header field decremented by each router. When it reaches zero the packet is dropped and an ICMP time-exceeded message is returned.

Internet Control Message Protocol, used for error reporting and diagnostics such as unreachable destinations, time exceeded, and echo request and reply.

A condition where one end of a link runs full duplex and the other half duplex, causing late collisions, CRC errors, and poor throughput on the full-duplex side.

Per-port statistics such as input errors, CRC errors, runts, giants, and collisions that help identify physical layer or duplex problems on a link.

A condition where packets circulate endlessly between routers because of inconsistent routing information, quickly consuming bandwidth and CPU until TTL expires.

A logging service that collects event messages from network devices, typically rated by severity level from 0 (emergencies) to 7 (debugging).

Simple Network Management Protocol, used to poll and receive traps from managed devices. Community strings or SNMPv3 credentials control access to management data.

The reported line and protocol state of a port. Line protocol down with line up often points to a Layer 2 mismatch; line down usually means a cabling or shutdown issue.

An access control list, an ordered set of permit and deny statements applied to an interface or line to filter traffic by address, protocol, or port.

A 32-bit value used with ACL and routing statements where a 0 bit means match exactly and a 1 bit means ignore that position of the address.

The invisible final rule in every ACL that drops traffic not matched by any earlier statement, so an ACL with no permits blocks everything.

Network Address Translation, which rewrites private source addresses to a public address so internal hosts can reach external networks while conserving registered IP space.

Port Address Translation, a NAT variant that maps many internal hosts to one public address by tracking unique source port numbers for each session.

Dynamic Host Configuration Protocol, which automatically supplies hosts with an IP address, subnet mask, default gateway, and DNS servers from a configured pool.

A router feature that forwards broadcast DHCP requests from a local subnet to a remote server, using the helper address configured on the client-facing interface.

A switch feature that limits which MAC addresses may send traffic on a port and how many, with violation actions such as shutdown, restrict, or protect.

A spanning-tree protection feature that error-disables an edge port if it receives a BPDU, preventing an unauthorized switch from altering the topology.

The Domain Name System resolves human-readable host names to IP addresses, allowing clients to reach services without knowing numeric addresses.

Authentication, Authorization, and Accounting, a framework for controlling who may access a device, what they may do, and logging the actions they perform.