Configuring a Cisco router or switch as a DHCP server

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Overview

Dynamic Host Configuration Protocol (DHCP) is a client/server protocol which allows you to allocate network configuration parameters like IP addresses, subnet masks, DNS servers, default gateways and many more to end devices. This protocol uses the UDP ports 67 and 68 as defined in RFC 2131. Nowadays allmost network devices support DHCP including workstations, printers, IP phones, handheld devices etc. Using dynamic IP address assigment you can minimize errors which can appear on manual configuration and also reduce the administrative overhead. Cisco embedded the DHCP server functionality beginning with IOS Version 12.0(1)T. By default, the Cisco IOS DHCP server and relay agent features are enabled but are not configured.

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Creating and configuring VLANs on Cisco Catalyst switches

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Overview

A VLAN (Virtual LAN)is a term derived from LAN (Local area network) and represents a logical grouping of hosts with the same set of requirements. All host in a VLAN communicate as if they were attached to the same broadcast domain, regardless of their physical location. By grouping several switch ports in different VLANs we are creating separate logical broadcast domains. All member ports of the same VLAN can communicate between them without using any Layer 3 routing. In order to allow communication between different VLANs we must use a layer 3 device like a router or we can use SVIs (Switched virtual interfaces). Using VLANs can provide the following benefits:

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How to configure logging on Cisco IOS

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Overview

Device logs often offer valuable information when troubleshooting a network issue. Interface status, security alerts, environmental conditions, CPU process hog, and many other events on the router or switch can be captured and analyzed later by studying the logs. By default, all log messages on a Cisco router or switch are sent to the console port. Only users that are physically connected to the console port may view these messages. If you are connected to a Cisco device via Telnet or SSH and want to see console messages, you can enter the command terminal monitor in privileged exec mode. Cisco devices support five types of logging:

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How to configure port security on Cisco switches

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Overview

In some environments, a network must be secured by controlling what stations can gain access to the network itself. Port security is a feature used on Cisco Catalyst switches which limits the MAC addresses allowed to appear on a specific port. In most cases network administrators use this to secure access to the physical network.

Using this feature only predefined static MAC addresses or limited number of dynamic MAC addresses can access the network. Suppose a user tries to connect to a port which has port security enabled and his MAC address does not appear on the list of allowed MAC addresses. In this case the port will be shut down or the packets arriving on that port will be dropped with a specific action. To resolve this issue the port must be re-enabled manually by the network administrator or automatically after a period of time if the errdisable cause is configured for automatic recovery (by default after 300 seconds).

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Understanding CDP (Cisco Discovery Protocol)

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Overview

Cisco Discovery Protocol (CDP) is a Cisco proprietary protocol which is used to collect information about neighboring routers and switches. It operates at Layer2 (data link layer) and comes in two versions CDPv1 (initial release) available since IOS version 10.3 and CDPv2 available from IOS Version 12.0(3)T. CDP is very useful when you need to gather information about the network topology like IP addresses, device capabilities, platform and also offers a quick way for troubleshooting and documenting the network. CDP is enabled by default on all available interfaces. CDP traffic between devices is not encrypted so this can be a real security issue. As best practice is recommended that CDP should be disabled mainly on devices that connects to external networks.

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