DHCP relay agent

When a device is configured as a DHCP client, it will send a a broadcast packet to discover DHCP servers on the network. Broadcast packets are not forwarded by routers by default, so if a DHCP server is in a different network than the DHCP client, it will not receive the request. Consider the following scenario:The workstation on the left is configured as a DHCP client. R2 on the right is configured as a DHCP server. The workstation sends a DHCP discover packet, but it receives no request, since Read more [...]

Configure timezone and Daylight Saving Time (DST)

It is recommended to set the correct timezone and adjust the DST setting before configuring a router as an NTP client. The syntax of the command used to set the timezone is:(config)clock timezone NAME HOURS [MINUTES]The name of the timezone can be anything you like. After the name parameter, you need to specify the difference in hours (and optionally minutes) from Coordinated Universal Time (UTC). For example, to specify the Atlantic Standard Time, which is 4 hours behind UTC, we would use the following Read more [...]

Default static route

A default route defines where packets will be sent if no specific route for the destination network is listed in the routing table. If no default route is set, the router will discard all packets with destination addresses not found its routing table.Consider the following example: We have a network of three routers. R1 is directly connected to two subnets – 192.168.0.0/24 and 10.0.0.0/24. R3 is connected to the Internet.Here is the routing table on R1:R1#show ip route Gateway of last resort Read more [...]

The ARP table on a Cisco router

Just like regular hosts, if a Cisco router wants to exchange frames with a host in the same subnet, it needs to know its MAC address. The IP-to-MAC address mapping are kept in the router’s ARP table. Consider the following example:R1 has two connected subnets – 10.0.0.0/24 and 172.16.0.0./16. Before exchanging frames with either host, R1 will need to know their MAC addresses. Here is the output of the R1’s ARP table:R1#show ip arp Protocol Address Age (min) Hardware Addr Read more [...]

Uniform Resource Locator (URL) structure

An URL is a string of characters that contains information about how to fetch a resource from its location. An URL specifies the primary access mechanism and the network location of a resource. A typical URL consists of the following components:the protocol used to connect to the server (e.g. HTTPS)the server hostname (e.g. geek-university.com)the document path (e.g. /course) Here is a graphical representation of these components:URLs can also be used to reference documents other than the web Read more [...]

Create a static host route

In one of the previous lessons we’ve defined a static route for a specific subnet by using the subnet mask of 255.255.255.0 in the ip route command. IOS also allows you to specify a static host route for a single host by specifying the 255.255.255.255 (/32) subnet mask in the ip route command.Static host routes are usually used when redundant paths exist. Consider the following example:In the example above you can see that we have a network of three routers and a switch connected to the 10.0.0.0/24 Read more [...]

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Prerequisites for 200-301 200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers. The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course Read more [...]

Syslog message format

Syslog messages that appear on a Cisco device consists of several parts. Consider the following message:*Jan 18 03:02:42: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to downThe message consists of the following parts:Jan 18 03:02:42 Рthe timestamp%LINEPROTO Рthe source that generated the message. It can be a hardware device (e,g. a router), a protocol, or a module of the system software.5 Рthe severity level, from 0 to 7, with lower numbers Read more [...]

Syslog explained

Syslog is a standard for message logging. Syslog messages are generated on Cisco devices whenever an event takes place – for example, when an interface goes down or a port security violation occurs.You’ve probably already encountered syslog messages when you were connected to a Cisco device through the console – Cisco devices show syslog messages by default to the console users:R1# %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to downThis is because Read more [...]

Cisco console rollover cable

A rollover cable is usually shipped with each Cisco device. This cable connects a serial port on your computer to the console port of the device and it is used for the device’s initial configuration. It is called rollover because the wires on one end are rolled at the other end – the wire at pin 1 connects to the pin 8 on the other side, the wire at pin 2 to the pin 7, etc.The cable is of light blue color with a DB-9 connector at one end and a RJ-45 connector at the other:You connect Read more [...]

Encrypt local usernames and passwords

We’ve learned it is possible to configure local usernames and passwords on a Cisco device and then use them to login to the device. To do this, we’ve used the username USER password PASSWORD command, like in the example below:R1(config)#username tuna password peyoHowever, there is one problem with this command – the password is stored in clear text in the configuration:R1#show running-config Building configuration... Current configuration : 635 bytes ! version 15.1 .... ! username Read more [...]

no ip domain-lookup command

By default, any single word entered on an IOS device that is not recognized as a valid command is treated as a hostname to which you want to telnet. The device will try to translate that word to an IP address in a process that can last about a minute.Consider the following example:R1#writte Translating \"writte\"...domain server (255.255.255.255) % Unknown command or computer name, or unable to find computer addressIn the output above you can see that I’ve mistyped the command write. The router Read more [...]

Auxiliary Port on a Cisco router

Most Cisco routers include an additional auxiliary (Aux) port as a backup async port. This port is commonly used as a dial-up port for remote router management. It is connected to a modem and enables an administrator to make a phone call to connect to the router’s CLI.The following figure shows how to connect a modem to an aux port on a router (image source: Cisco):The network administrator uses a terminal emulation program to connect to the router over the aux port. The configuration of the Read more [...]

Interface status codes

Interface status codes can tell you whether an interface on a Cisco router or a switch is working. Status codes of an interface can be displayed using the show interface TYPE command, e.g.:SW1#show interface fa0/1 FastEthernet0/1 is up, line protocol is up (connected) Hardware is Lance, address is 0060.70d8.0001 (bia 0060.70d8.0001) BW 100000 Kbit, DLY 1000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) The first line Read more [...]
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