CCNA flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Broadcast domain

A network segment where all devices receive each other's broadcast frames; routers and VLANs do not forward broadcasts between them unless explicitly configured.

TCP three-way handshake

Connection setup using SYN, SYN-ACK, and ACK segments; TCP uses it to synchronize initial sequence numbers before any application data is sent.

APIPA address

IPv4 address a host self-assigns when DHCP fails; the host can communicate only on the local link and cannot reach remote networks.

Usable hosts per subnet

Subtract 2 from 2 raised to the number of host bits: a /26 has 6 host bits, so 64 minus 2 equals 62 usable addresses.

Address Resolution Protocol

Protocol that maps a known IPv4 address to a MAC address by broadcasting a request inside the local subnet, then caching the reply briefly.

IPv6 address format

128 bits written as eight groups of four hexadecimal digits; a double colon may compress exactly one run of consecutive all-zero groups.

802.1Q tag

Adds a 4-byte field to Ethernet frames carrying a 12-bit VLAN identifier; the native VLAN is the only traffic forwarded untagged across the trunk.

STP root bridge election

The switch with the lowest bridge ID wins, comparing the 2-byte priority first, then the MAC address; default priority is 32768 and is added to the VLAN ID.

PortFast and BPDU Guard

PortFast skips listening and learning so an access port forwards immediately; BPDU Guard err-disables the port if a BPDU arrives, preventing loops.

EtherChannel requirements

Bundled member links must match speed, duplex, and allowed VLANs; LACP, defined in 802.3ad, is the standards-based negotiation protocol; PAgP is Cisco proprietary.

CAPWAP tunnels

Access points and wireless controllers carry management and data traffic over UDP 5246 and 5247; the AP can find the controller by broadcast, DHCP option 43, or DNS.

Longest prefix match

A router first prefers the route with the most specific mask; administrative distance and metric break ties only between routes for the same prefix length.

Administrative distance values

Connected routes are 0, static routes 1, EIGRP 90, OSPF 110, and RIP 120; when the same prefix is learned from several sources the lowest value is installed.

OSPF timers on broadcast

OSPF sends hello packets every 10 seconds on broadcast and point-to-point networks and declares a neighbor dead after 40 seconds; both values must match.

OSPF DR election

The interface with the highest OSPF priority becomes designated router, default priority 1; ties are broken by highest router ID, and priority 0 can never be DR.

Floating static route

A static route given an administrative distance higher than the dynamic protocol so it stays out of the table until the primary route fails.

Router-on-a-stick

One physical router interface divided into 802.1Q subinterfaces, each assigned an IP address and acting as the default gateway for its VLAN.

DHCP DORA process

Four messages, Discover, Offer, Request, and Acknowledge, lease an IPv4 address; the client broadcasts from UDP 68 and the server replies from UDP 67.

Syslog severity levels

Eight levels numbered 0 through 7: emergency, alert, critical, error, warning, notice, informational, debug; lower numbers are more severe and debug is 7.

NAT overload

Translates many inside local addresses to a single inside global address, telling the flows apart by unique source port numbers; also called port address translation.

NTP stratum

Stratum 1 servers synchronize directly to an authoritative reference clock; each level away adds one, values run 0 to 15, and 16 means unsynchronized.

SNMP ports

Managers poll agents on UDP 161 and agents send unsolicited traps to UDP 162; SNMPv3 added authentication and encryption that earlier versions lack.

Standard versus extended ACL

Standard ACLs numbered 1-99 filter only the source address and are placed near the destination; extended ACLs 100-199 filter protocol, source, destination, and port near the source.

ACL wildcard mask

A zero bit means the corresponding address bit must match, 255 means ignore it; it is the inverse of a subnet mask, so /24 becomes 0.0.0.255.

Port security violation modes

Protect silently drops frames from unknown MAC addresses; restrict drops and logs them; shutdown places the port in err-disabled state until an administrator re-enables it.

TACACS+ versus RADIUS

TACACS+ runs over TCP 49 and encrypts the whole packet body; RADIUS runs over UDP 1812 and 1813 and encrypts only the password field.

DHCP snooping

Trusts uplink ports, treats access ports as untrusted, rate-limits DHCP messages there, and builds a bindings table of MAC, IP, VLAN, and port.

Northbound versus southbound APIs

Northbound APIs expose the controller to applications and automation scripts, while southbound APIs push configuration and read state from the network devices themselves.

REST HTTP verbs

GET reads, POST creates, PUT replaces, PATCH partially updates, and DELETE removes a resource; each REST request is stateless and carries all needed information.

Ansible

An agentless configuration management tool that connects to devices over SSH and applies declarative playbooks written in YAML, so no software runs on the managed node.

NETCONF

Uses XML-encoded remote procedure calls over an SSH session on TCP 830 to read and edit the configuration datastores of network devices.

SDN planes

Software-defined networking centralizes the control plane in a controller and leaves the data plane on the devices, which forward traffic using tables the controller builds.