Cisco CCNA Exam Prep
120 flashcards by MentisLibrary
The CCNA 200-301 objectives as study items: switching and VLANs, IPv4 and IPv6 addressing, OSPF, spanning tree, wireless, security fundamentals, IP services and network automation, with the command for each.
- Collision domain vs broadcast domain
- Every switch port is its own collision domain; a broadcast domain is bounded by a router (or a VLAN). A hub is one collision domain for all its ports.
- Ethernet frame size limits
- A standard Ethernet frame is 64 to 1518 bytes; an 802.1Q tag adds 4 bytes for 1522. The default IP MTU is 1500 bytes of payload.
- Which cable connects a PC to a router directly?
- A crossover cable, because both are DTE devices with the same pinout. PC to switch and router to switch use straight-through.
- Single-mode vs multimode fiber
- Single-mode uses a laser through a tiny core (about 9 microns) for tens of kilometers; multimode uses an LED or VCSEL through a larger core (50 or 62.5 microns) for a few hundred meters.
- TCP vs UDP
- TCP is connection-oriented with sequencing, acknowledgments, flow control and retransmission; UDP is connectionless with an 8-byte header and no delivery guarantee.
- TCP three-way handshake
- SYN from the client, SYN-ACK from the server, ACK from the client. The connection is then established and sequence numbers are synchronized.
- Well-known ports every CCNA candidate must know
- FTP 20/21, SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67/68, TFTP 69, HTTP 80, POP3 110, NTP 123, SNMP 161/162, HTTPS 443, syslog 514.
- IPv4 address classes
- Class A 1 to 126 (/8), Class B 128 to 191 (/16), Class C 192 to 223 (/24), Class D 224 to 239 multicast, Class E 240 to 255 experimental.
- RFC 1918 private address ranges
- 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0 to 172.31.255.255) and 192.168.0.0/16. These are not routed on the internet and need NAT to reach it.
- How many usable hosts does a /27 provide?
- 30. A /27 leaves 5 host bits, so 2 to the 5th is 32 addresses minus the network and broadcast addresses.
- Find the network and broadcast for 192.168.1.77/27
- Network 192.168.1.64, broadcast 192.168.1.95, usable range .65 to .94. The block size for a /27 is 32, so subnets start at 0, 32, 64, 96.
- Variable-length subnet masking (VLSM)
- Using different subnet masks within one major network so each subnet is sized to its host count, saving addresses.
- APIPA address range
- 169.254.0.0/16. A Windows host self-assigns one when it cannot reach a DHCP server; it can talk only to the local link.
- What does ARP do?
- Resolves a known IPv4 address to a MAC address on the local segment by broadcasting a request that the owner answers with a unicast reply.
- How a switch builds its MAC address table
- It records the source MAC of every incoming frame with the port it arrived on, then forwards frames to the port matching the destination MAC. Unknown destinations are flooded.
- Cisco IOS command modes
- User EXEC (Router>) for basic show commands, privileged EXEC (Router#) for all show and debug, global configuration (Router(config)#) for changes, plus sub-modes such as interface and line.
- enable secret vs enable password
- enable secret stores a hashed password and takes precedence; enable password stores clear text unless service password-encryption is set, and is legacy.
- Running config vs startup config
- The running config lives in RAM and takes effect immediately; the startup config lives in NVRAM and loads at boot. Changes are lost on reload unless saved.
- show ip interface brief
- Lists every interface with its IP address, whether it was learned manually or by DHCP, and its status and protocol columns.
- What do late collisions on an interface indicate?
- A duplex mismatch: one side runs full duplex and the other half duplex. The half-duplex side logs late collisions and the full-duplex side logs CRC and runt errors.
- CDP vs LLDP
- CDP is Cisco proprietary, on by default, advertising every 60 seconds with a 180-second holdtime; LLDP is the IEEE 802.1AB standard, off by default on Cisco, advertising every 30 seconds with a 120-second holdtime.
- Steps to enable SSH on a Cisco device
- Set a hostname and ip domain-name, generate an RSA key with crypto key generate rsa, create a local user with username name secret pass, then on the VTY lines set login local and transport input ssh.
- What does line vty 0 4 configure?
- The first five virtual terminal lines used for Telnet and SSH sessions. Settings there control remote login, timeouts and which transport is allowed.
- What is a VLAN?
- A logical broadcast domain created on a switch, so ports in different VLANs cannot talk without a router or layer 3 switch even on the same physical hardware.
- Configure an access port in VLAN 10
- switchport mode access followed by switchport access vlan 10 on the interface. If the VLAN does not exist the switch creates it.
- How does an 802.1Q trunk identify VLANs?
- It inserts a 4-byte tag after the source MAC carrying a 12-bit VLAN ID, except for the native VLAN, which is sent untagged.
- Configure a trunk and restrict its VLANs
- switchport mode trunk, then switchport trunk allowed vlan 10,20,30 to limit what crosses and switchport trunk native vlan 99 to move the native VLAN.
- DTP modes and what they negotiate
- Dynamic desirable actively tries to form a trunk; dynamic auto only trunks if asked; trunk and access are fixed. Auto plus auto stays access; desirable plus auto becomes a trunk.
- Native VLAN mismatch
- When the two ends of a trunk have different native VLANs, untagged frames leak between VLANs and CDP reports a native VLAN mismatch error.
- Voice VLAN
- A second VLAN allowed on an access port so an IP phone tags its voice traffic while the PC behind it stays untagged in the data VLAN.