Ruankao Network Engineer (Intermediate) flashcards

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

OSI seven layers

Physical, data link, network, transport, session, presentation, application. Each layer provides services to the layer above and communicates logically with its peer layer on the remote host.

Shannon capacity formula

C = W log2(1 + S/N) bits per second, where W is bandwidth in Hz and S/N is the linear signal-to-noise ratio. It gives the maximum error-free rate over a noisy channel.

Bandwidth-delay product

Link rate multiplied by round-trip time, expressed in bits. It equals the amount of data that can be in flight on a path and bounds throughput for window-limited protocols such as TCP.

Manchester encoding

Each bit is represented by a mid-bit transition: high-to-low or low-to-high. It is self-clocking and used in 10BASE-T, but it requires twice the signaling rate of NRZ.

CRC error detection

The sender divides the data bits by a generator polynomial and appends the remainder. The receiver repeats the division; a nonzero remainder indicates an error. Ethernet uses a 32-bit CRC in its FCS.

Ethernet minimum frame size

64 bytes including the 14-byte header and 4-byte FCS, so the payload is at least 46 bytes. This guarantees collision detection on shared 10 Mbps segments within the slot time.

802.1Q VLAN tag

A 4-byte field inserted after the source MAC address, containing a 12-bit VLAN identifier (4094 usable values) and a 3-bit priority field. Trunk links carry tagged frames; access links are untagged.

STP root bridge election

Switches exchange BPDUs; the bridge with the lowest bridge ID wins, where the ID combines a configurable priority (default 32768) and the switch MAC address. All other switches select a root port toward it.

CSMA/CA in WLAN

Wireless stations sense the medium, wait a random backoff, and transmit; the receiver returns an ACK. Collisions are avoided rather than detected because a station cannot hear while transmitting.

Usable hosts in a subnet

For a prefix length /n, total addresses are 2^(32-n) and usable host addresses are 2^(32-n) minus 2, subtracting the network address and the directed broadcast address.

VLSM

Variable Length Subnet Masking assigns different prefix lengths within one address block, allocating larger subnets to larger requirements first. It conserves addresses and supports route summarization.

ARP operation

A host broadcasts an ARP request asking which MAC address owns a target IPv4 address; the owner replies unicast and both cache the mapping. IPv6 performs the equivalent with neighbor solicitation and advertisement.

OSPF cost metric

Cost equals the reference bandwidth (default 100 Mbps) divided by the interface bandwidth. Lower cost is preferred; OSPF is link-state and converges using Dijkstra's shortest-path-first algorithm.

NAT with PAT overload

Port Address Translation maps many private addresses to one public address by translating source port numbers. It conserves public IPv4 addresses but breaks end-to-end reachability for inbound connections.

TCP three-way handshake

The client sends SYN with an initial sequence number, the server replies SYN-ACK with its own sequence number, and the client sends ACK. This synchronizes sequence numbers and establishes the connection.

TCP congestion control phases

Slow start doubles the congestion window each round trip; congestion avoidance increases it by one MSS per round trip; fast retransmit triggers on three duplicate ACKs, followed by fast recovery.

DHCP DORA process

Discover (client broadcast), Offer (server proposes an address), Request (client accepts), Ack (server confirms the lease). Clients renew at 50 percent of lease time and rebind at 87.5 percent.

DNS record types

A maps a name to IPv4, AAAA to IPv6, CNAME creates an alias, MX names mail servers, NS delegates a zone, PTR supports reverse lookup, and SOA holds zone authority data.

HTTP status code classes

1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error. Common examples are 200 OK, 301 moved permanently, 404 not found, and 500 internal server error.

POP3 versus IMAP

POP3 (port 110) typically downloads messages to one client and may delete them from the server. IMAP (port 143) keeps mail on the server and synchronizes state across multiple clients.

Switch MAC learning

The switch records the source MAC and ingress port in its forwarding table, forwards known unicast only to the learned port, and floods unknown unicast, broadcast, and multicast frames out all other ports.

Administrative distance

A ranking of route sources on Cisco-style routers; lower is preferred. Connected is 0, static 1, EIGRP 90, OSPF 110, RIP 120. A floating static route uses a distance above the dynamic protocol.

CAPWAP tunnels

The protocol between a wireless controller and access points. Control traffic uses UDP port 5247 and data traffic uses UDP port 5246; the controller manages SSIDs, security, and roaming centrally.

Structured cabling horizontal limit

Permanent horizontal cable may not exceed 90 meters, and patch cords in the work area and telecommunications room add up to 10 meters, giving a 100-meter channel limit.

Three-tier network design

The core layer forwards at high speed, the distribution layer applies policy and aggregates, and the access layer connects end devices. Small sites often collapse core and distribution.

VRRP and HSRP

First-hop redundancy protocols that present a shared virtual IP and MAC address to hosts. If the master or active router fails, the backup takes over, preserving the default gateway.

IPsec ESP and AH

ESP encrypts and authenticates the payload; AH authenticates but does not encrypt. Both can run in transport mode (host-to-host) or tunnel mode (gateway-to-gateway VPN).

Symmetric versus asymmetric encryption

Symmetric algorithms such as AES use one shared key and are fast. Asymmetric algorithms such as RSA use a public and private key pair, are slower, and typically protect key exchange and signatures.

FCAPS management model

Five functional areas of network management: fault, configuration, accounting, performance, and security. It structures the tasks covered by management platforms and SNMP-based monitoring.

RTO and RPO

Recovery Time Objective is the maximum acceptable downtime after a failure; Recovery Point Objective is the maximum acceptable data loss measured in time. Both drive backup frequency and redundancy design.

Hypervisor type 1 versus type 2

A type 1 hypervisor runs directly on hardware (ESXi, Hyper-V) with better performance. A type 2 hypervisor runs on a host operating system (VirtualBox) and is used mainly for testing.

SNMP versions and security

SNMPv1 and v2c authenticate with clear-text community strings; v3 adds user authentication and encryption. Agents listen on UDP 161 and send traps to the manager on UDP 162.

MLPS grading

China's Multi-Level Protection Scheme classifies information systems into five protection grades based on harm to national, social, and individual interests, with corresponding filing, assessment, and control requirements.