DNS (Domain Name System) is an internet service used to resolve or translate domain names like www.skillset.com into a dotted decimal IPv4 address or hexadecimal notation IPv6 address. ARP stands for Address Resolution Protocol. ARP resolves IP Addresses to MAC Addresses. HTTP stands for Hypertext Transfer Protocol. HTTP is used by World Wide Web to define how messages are formatted and transmitted. DHCP or Dynamic Host Configuration Protocol. DHCP is a protocol used to dynamically distribute Network Parameters such as IP addresses for interfaces and services.
A timing problem can cause intermittent connectivity on a WAN link. To solve the issue, check the CSU/DSU configuration and verify that clocking settings are fine. A timing problem cannot be caused by a layer 1 failure. A timing problem will not increase input errors it will put the interface status in an up/down state. For more information refer to: http://penta2.ufrgs.br/trouble/trwan.htm
The Window field or size in the TCP header is a number that specifies the amount of data a sender can transmit before needing an acknowledgment from the receiver. (http://www.networksorcery.com/enp/protocol/tcp.htm)
The "0.0.0.0" wildcard mask represents each octet of the address. The zero in each octet must match exactly, which means that the exact the host address of 192.168.1.2 must be checked. The equivalent of the command "permit host 192.168.1.2" is "permit 192.168.1.2 0.0.0.0".
The command debug frame-relay lmi can give a real-time "debug" detail about the LMI exchange of packets. "show frame-relay lmi" displays statistics of the lmi and it can help on troubleshooting but does not display events occurring on the LMI exchange of packets. For more information on the debug command refer to: http://www.cisco.com/c/en/us/support/docs/dial-access/integrated-services-digital-networks-isdn-channel-associated-signaling-cas/10374-debug.html For more information on the "debug frame-relay lmi" refer to: http://www.cisco.com/c/en/us/td/docs/ios/12_2/wan/configuration/guide/fwan_c/wcffrely.html
The command "access-list 100 deny tcp host 192.168.1.2 host 203.125.32.254 eq 25" is used to deny TCP service that uses port 25. This port number is used by SMTP or Simple Mail Transfer Protocol. FTP or File Transfer Protocol uses port numbers 20 (Data) and 21 (Command). DNS or Domain Name Service/System uses port 53. ICMP or Internet Control Message Protocol, however, is a messaging protocol at the network layer, not in the transport layer. ICMP use different types of messages. As an example, you can use "echo-reply" or "echo-request", etc., instead of using port numbers.
Frame Relay, PPP and HDLC are WAN technologies while Ethernet and Token Ring are common LAN technologies. For more information refer to: http://computernetworkingnotes.com/wan-protocols-hdlc-ppp-frame-really-nat-pat/wan-terms.html
WAN operates at two layers, Physical layer and Data Link Layer.
To allow or deny ping packets, use the IP control protocol ICMP or Internet Message Control Protocol. The IP option means all IP protocols are allowed or filtered. Use UDP if you want to block or allow routing updates or other UDP services such as DNS. Use the TCP option if you want to allow or deny TCP protocols such as FTP, Telnet, SSH, etc.
The "debug frame-relay lmi" command is used to display LMI messages exchanged between the router and Frame Relay switch over access link. The debug output can be used to determine if the router and the Frame Relay switch are sending and receiving LMI packets properly. The "no" form of this command disables the debug output. The "show frame-relay lmi" command displays LMI statistics but it does not show the contents of LMI messages.
http://www.techrepublic.com/article/troubleshoot-problems-with-frame-relay-circuits-on-your-cisco-router/#
https://prasadkeni.wordpress.com/2012/02/10/ospf-neighbor-list-is-empty/
A point-to-point link does not assign a DR or BDR, because is only two routers. The Non-Broadcast Multi-Access "NBMA" does not assign the DR/BDR as well because it uses the Hub and Spoke technology. An example of the NBMA is the Frame Relay. On a broadcast and non-broadcast the DR and BDR is normally elected.
https://networklessons.com/ospf/ospf-drbdr-election-explained/
Clock Rate & Bandwidth cmds (serial int)
http://www.anythingoverip.co.za/tutorials/course-content/ccna/clock-rate-and-bandwidth-commands/
Which of the following command is used to set Frame Relay to use Cisco’s own encapsulation?
Configure Frame Relay encapsulation to use
either Cisco or IETF encapsulations. If the encapsulation type is not specified,
by default Cisco encapsulation is used.
http://www.9tut.com/gateway-load-balancing-protocol-glbp-tutorial
GLBP or Gateway Load Balancing Protocol is also a Cisco-proprietary protocol that works like HSRP and VRRP but overcomes the limitation of existing FHRPs by adding a load balancing mechanism. VRRP or Virtual Router Redundancy Protocol is a form of FHRP or First Hop Redundancy Protocol. VRRP is an open standard FHRP that achieves availability and reliability of routing paths via automatic default gateway selections on an IP subnetwork by creating virtual routers. HSRP or Hot Standby Router Protocol is a Cisco-proprietary redundancy protocol that provides high network availability by providing first-hop redundancy for IP hosts on an IEEE 802 LAN configured with a default gateway IP address.
"Router ID
http://en.community.dell.com/support-forums/network-switches/f/866/t/19465205
Cisco Discovery Protocol (CDP) is a Layer 2 OSI model Cisco proprietary protocol used to find a Cisco neighbor device and learn about the type of hardware, software version, and active interfaces. CDP can be used by a L2 or L3 device.
Cisco Discovery Protocol (CDP) is a Layer 2 Cisco proprietary protocol used to find a Cisco neighbor device and learn about the type of hardware, software version, and active interfaces. CDP can be used by a L2 or L3 device. In case that a Cisco IP phone is connected to an access port on a Layer 2 device and CDP is enabled, the switch will send CDP messages and discover the phone's information. It is possible to see the Cisco VoIP phone via the _show cdp neighbors” command on a Cisco switch.
Use the command show running-config to look at the current configuration. This command will display the active configuration file in memory including saved configuration changes. The working memory and running configuration are stored in RAM. _Show startup-config shows the backup configuration in NVRAM. The command _show memory displays the summary statistics about processor memory and I/O memory. There is no such command as _show current-config.
The command “show ip route” displays a summary of all configured routes in the router. The line “Gateway of last resort is not set” can be seen on the top of the output, which means that there is no gateway configured on the router R1. If no gateway is set and a packet with a destination not known by the routing table, reaches the router, the router will discard the packet. For more information refer to: [http://www.freeccnastudyguide.com/study-guides/ccna/ch4/routing/]
The command “show ip route” displays a summary of all configured routes in the router. A default gateway can be seen in the line “Gateway of last resort is 192.168.100.1 to network 0.0.0.0”, which means that any packet with unknown destination will be forwarded to its connected interface under the network 192.168.100.0/24, which can be seen below in the routing table. The line “C 192.168.100.0/24 is directly connected, Serial0/2”, states that packets destined to 192.168.100.0/24 will be forwarded via Serial0/2. For more information refer to: [http://www.freeccnastudyguide.com/study-guides/ccna/ch4/routing/]
https://www.skillset.com/tests?testId=50896
https://skillset.s3.amazonaws.com/img/question/43339/54d0037f-5668-49d8-96dc-4db10a0009b0
Use the no form of command to disable a function or a feature in any Cisco Router or Switch. Use the "no" keyword to reenable a disabled feature or to enable a disabled feature disabled by default. Use the "disable" command to exit user mode. The keyword "default" can be used to return any command to its default value. Use the "end" keyword to finish the current configuration session and return the EXEC mode.
sco Discovery Protocol (CDP) is a Layer 2 Cisco proprietary protocol used to find Cisco neighbor devices and learn about their type of hardware, software version, and active interfaces. CDP can be used by a L2 or L3 device. Theshow cdp neighbors delivers information about directly connected devices. Since R2 has only two physically attached devices and all devices have CDP enabled. Only R1 and SW2 will be displayed in the output of the command. CDP packets are not forwarded by Cisco Switches, it is only possible to see what is directly connected.
Cisco Discovery Protocol (CDP) is a Layer 2 Cisco proprietary protocol used to find a Cisco neighbor device and learn about the type of hardware, software version, and active interfaces. For more information refer to: [Todd Lammle. - CCNA Routing and Switching Study Guide, Chapter 9]
efore configuring an IPv6 version of a routing protocol, you have to enable first the forwarding of IPv6 Unicast Datagrams. To enable, issue the ipv6 unicast-routing global configuration command. The command ipv6 local enables IPv6 Policy Routing, ipv6 host configures static hostnames and ipv6 route configures static routing.
he command _show protocols displays the status of layers 1 and 2 interfaces as well as the IP addresses used. The command _show ip protocols is different; it shows the routing protocols and timers associated with each routing protocol configured on a router. The command _show controllers displays information about the physical interface. The _show version command provides basic configuration for the system hardware as well as the software version. The command _show startup config displays the configuration that will be loaded the next time that the router is loaded.
<sh ip int bri - also possible>
Use the command show running-config to look at the current configuration. This command will display the active configuration file in memory including saved configuration changes. The working memory and running configuration are stored in RAM. _Show startup-config shows the backup configuration in NVRAM. For more information refer to: [http://www.cisco.com/c/en/us/td/docs/security/asa/asa82/command/reference/cmd_ref/s5.html#]
Dynamic Host Configuration Protocol (DHCP) provides network configuration information to hosts, such as, IP Address, Subnet Mask, Default Gateway and DNS. DHCP eliminates the need to perform the configuration manually. DHCP allows easy administration and configuration for small to large networks. Manual configuration would work for small and large networks; however managing a large network through static configuration would become an issue. IPv4 auto-configuration is not intended for large networks. CDP is a L2 protocol used to find Cisco neighbors. For more information refer to: [Todd Lammle. - CCNA Routing and Switching Study Guide 2013, Chapter 3, page 98]
The right sequence of messages for the DHCP client to initially find the IP information from the DHCP Server is as follows: DHCP Discover> DHCP Offer> DHCP Request> DHCP ACK. The message SYN does not exist in this DHCP negotiation.
There are four basic steps in DHCP client IP address requests from the server. A DHCP client sends a DHCPDISCOVER broadcast message to locate a Cisco IOS DHCP server. Then, a DHCP server will offer configuration parameters to the client in a DHCPOFFER unicast. The client then will return a request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message. After which the DHCP server confirms with a DHCPACK unicast message to the client.
There are four basic steps in DHCP client IP address requests from the server. A DHCP client sends a DHCPDISCOVER broadcast message to locate a Cisco IOS DHCP server. Then, a DHCP server will offer configuration parameters to the client in a DHCPOFFER unicast. The client then will return a request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message. After which the DHCP server confirms with a DHCPACK unicast message to the client. However, there would be a situation when a DHCP server is unable to satisfy a DHCPREQUEST message, thus, responds with a DHCPNACK message. After receiving such message, the client restarts the configuration process by going into the Requesting state.
To enable the default gateway for the DHCP server, use the command "default-router" (default-router 172.16.1.1). To exclude a particular IP address from being distributed, use the command "ip dhcp excluded-address" (ip dhcp excluded-address 172.16.1.1 172.16.1.2).
http://networkdiagram101.com/ - good tips!
The clockrate command is needed on a router (DCE) to provide clocking (speed) to the other end (DTE). If no clocking is configured a Layer 2 problem can occur. Another possible layer 2 problem is if the device is not recording the MAC addresses in the table. All other answers are typical layer 1 problems.
The command "nslookup" is used to do a look-up to the specific IP address associated with a domain name. Nslookup can be used for either Windows or Linux Operative sytems. The command "arp" is used to find the Layer 2 to Layer 3 address mappings in a host. The command "ping" is used to provide a basic connectivity test between the requesting host and a destination host. The diagnostic tool "nbstat" is used for NetBIOS over TCP/IP and is only for Windows computers.
If physical layer is not properly functioning then higher layers will not work. A straight-trough cable is used to connect a network adapter (Router, PC, Firewall, etc) to a Switch or hub. If a straight-trough cable is used between two switches, communications will not work. Therefore to solve the issue the straight-through cable must be changed to a crossover cable. Autonegotiation can be used by devices that are capable of transmitting information at different rates, different duplex modes or different standards at the same speed. Autonegotiation can detect different speeds and duplex modes and use a defined one, but if cables are not properly installed, the negotiation will never happen.
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