CCNA Basics

The layer in OSI reference model communicates with each other using protocol data units ( PDUs).

Hub is a physical layer device and it is concerned only with propagation of the physical signaling without any regard for upper -layer functions.

Logical link control (LLC) sublayer (802.2) is responsible for logically identifying different protocol types and then encapsulating them in order to be transmitted across the network.

A Bridge learns all the station destinations by listening to source addresses so it never learns the broadcast address.

A network address is protocol-specific. If a router supports more than one protocol, it will have a unique table for each protocol.

Metric in routers refers to the cost or distances to the target network. This is a value that helps the router choose the best path to a given network. This metric changes depending on how the router chooses paths.

The term Ethernet refers to a family of LAN implementations. Physical layer implementations vary and all support various media types.

A cross over cable crosses the critical pairs in order to properly align, transmit and receive signals on devices with like connectors.

For connection between similar devices such as DCE to DCE, DTE to DTE, crossover cable is used. Whereas, for connecting dissimilar devices such as DCE to DTE, straight cable is used.

BRI-S/T port is used when NT1 is provided separately. BRI-U port is used when built in NT1 is used.

The AUX port provided on some devices can be used to provide out-of-band management via a modem. The AUX port must be configured using the console port before it can be used.

When Cisco router is started for the first time, it does not have any initial configuration.

During LED/POST Test, if LED 4x (console port) fails, then you cannot access the management console through the console port, but you can still Telnet to the management console.

A question mark (?) during the EXEC session will always provide help.

The configuration changes to the switch are immediate. As soon as you press `Return` in any configuration mode, the parameter has been changed and the action executes in running memory.

Commands in the CISCO IOS can be abbreviated by entering enough of the command to be unique.

By default, command history is enabled and the system records ten commands line in the history buffer.

The commands `write term` and `show config` used with CISCO IOS release 10.3 and earlier have been replaced by new commands, but continue to perform their normal functions.

If you type `Exit` in the router mode, the router will back out one level, eventually allowing you to log out.

Logging synchronous console line command line is useful whenever console messages are being displayed at the same time that you are attempting to input EXEC or configuration command.

Serial links have two sides. One side of the link is responsible for clocking and is called a DCE.

CDP functionality is enabled by default on all interfaces.

Some CISCO devices such as the catalyst 1900 switches don't support the telnet command.

The ping command (packet internet gropper) verifies connectivity.

When a configuration is copied into RAM from any source, the configuration merges with, or overlays, any existing configuration in RAM rather then overwriting it.

Layer 2 switches have three major functions: address learning, packet forwarding / filtering and loop avoidance.

The purpose of the spanning-Tree protocol is to maintain a loop-fee network. Spanning Tree protocol is enabled by default in catalyst switches.

To make a bridge as Root Bridge, its priority should be lowered.

On the catalyst 1900 switch, the running configuration is automatically saved to NVRAM whenever a change is made to the running configuration.

Though VLANs are a layer 2 implementation in the switch fabric but they are protocol independent.

ARP is used to resolve or map a known destination IP address to a MAC sublayer address to allow communication on a multi-access medium such as Ethernet.

The static route is configured for connectivity to a data link not directly connected to your routes. For end-to-end connectivity a route must be configured in both directions.

Administrative distance value is used to rate the trustworthiness of each routing source.

The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original update came.

The 'show ip protocols' command displays values associated with routing timers and network information associated with the entire router.

By defaults only bandwidth and delay are used by IGRP metric.

All routers within an autonomous system must use the same autonomous number, or they will not exchange routing information.

With 'ip classless' configured, if a packet is received with a destination address within an unknown subnet of a directly attached network, the router will match it to the default route and forward it to the next hop specified by the default route.

Access lists identify traffic to be filtered in the transit through the router, but they do not filter traffic that originated from the router.

Because of the implicit deny any, an access list should have at least one permit statement in it, otherwise the access test will block all traffic.

An access list can be applied to multiple interfaces. However there can be only one access list per protocol, per direction, per interface.

Access lists are processed from the top down. If you place more specific tests and tests that will test true frequently in the beginning of the access list, you can reduce processing overhead.

A Novell IPX address has 80 bits - 32 bits for the network number and 48 bits for the node number. It is expressed as a hexadecimal number.

Cisco IPX echoes are used to ping between Cisco routers to verify connectivity and are not compatible with Novell IPX pings, which can be used to ping between servers.

Customer premises equipment is always the data terminal equipment (DTE). DCE provides the clock and DTE needs external clock.

Cisco's HDLC is a point to point protocol that can be used on leased lines between two devices supporting Cisco proprietary HDLC encapsulation. If communicating with a non-Cisco device, synchronous PPP is a more viable option.

While configuring PPP authentication, configure interface first and then router for password and username.

While configuring PAP or CHAP on Cisco router the password must be the same for both routers.

If both PAP and CHAP are configured, the one that is listed first in the PPP authentication interface configuration command is tried first.

The D channel between the router and the ISDN switch is always up. When the call is initiated the called number is sent to the local ISDN switch. The D channel is used for call setup, signaling and call termination.