Router

• LAN switches make use of  packet switching. The switch establishes a connection between two segments and keeps the connection just long enough to send the current packet.
• Incoming packets, which are part of an Ethernet frame, save to a temporary memory area. The temporary memory area is a buffer.
• The switch reads the MAC address that is in the frame header and compares the address to a list of addresses in the switch lookup table. In a LAN with an Ethernet basis, an Ethernet frame contains a normal packet as the payload of the frame. The frame has a special header that includes the MAC address information for the source and destination of the packet.
• Switches  work at Layer 2 (Data or Datalink) of the Open System Interconnection (OSI) reference model
• Use  MAC addresses to switch the traffic.
• Switches use one of three methods for routing traffic  Cut-through,Store and forward, Fragment-free
• Most Ethernet LAN switches use transparent bridging to create the address lookup tables.Transparent bridging has five parts:
  • Learning
  • Flooding
  • Filtering
  • Forwarding
  • Aging

Switches use one of three methods for routing traffic:

• Cut-through
• Store and forward
• Fragment-free

Cut-through:

• Switches only read the MAC address as soon as a packet is detected by the switch.
• Address consists of   six bytes therefore switch  store the six bytes.
• After this the switches immediately begin to send the packet to the destination node, even though the rest of the packet is coming into the switch.
• Reduces latency  but decreases reliability.
• Only be used where the speed of the outgoing interface is less than or equal to the incoming interface speed.

Store and forward:

• In a store and forward method switch saves the entire packet to the buffer
• Then checks the packet for Cyclic Redundancy Check (CRC) errors or other problems.
• If the packet has an error, the packet is discarded.
• Otherwise, the switch looks up the MAC address and sends the packet on to the destination node.
• Many switches combine the two methods by using cut-through until a certain error level is reached, then changing over to store and forward.
• Very few switches are strictly cut-through because this provides no error correction.

Fragment free:

• A less common method is fragment-free.
• It works like cut-through, but stores the first 64 bytes of the packet before sending the packet on.
• Because the most errors and all collisions occur during the initial 64 bytes of a packet.
• Only useful if there is a chance of a collision on the source port

There are four major phases to the bootup process:

1. Performing the POST
2. Loading the bootstrap program
3. Locating and loading the Cisco IOS software
4. Locating and loading the startup configuration file or entering setup mode

1. Performing the POST
The Power-On Self Test (POST) is a common process that occurs on almost every computer during bootup. The POST process is used to test the router hardware. When the router is powered on, software on the ROM chip conducts the POST. During this self-test, the router executes diagnostics from ROM on several hardware components including the CPU, RAM, and NVRAM. After the POST has been completed, the router executes the bootstrap program.

2. Loading the Bootstrap Program
After the POST, the bootstrap program is copied from ROM into RAM. Once in RAM, the CPU executes the instructions in the bootstrap program. The main task of the bootstrap program is to locate the Cisco IOS and load it into RAM.
Note: At this point, if you have a console connection to the router, you will begin to see output on the screen.

3. Locating and Loading Cisco IOS
Locating the Cisco IOS software. The IOS is typically stored in flash memory, but can also be stored in other places such as a TFTP (Trivial File Transfer Protocol) server.
If a full IOS image can not be located, a scaled-down version of the IOS is copied from ROM into RAM. This version of IOS is used to help diagnose any problems and can be used to load a complete version of the IOS into RAM.
Note: A TFTP server is usually used as a backup server for IOS but it can also be used as a central point for storing and loading the IOS. IOS management and using the TFTP server is discussed in a later course.

4. Locating and Loading the Configuration File
Locating the Startup Configuration File. After the IOS is loaded, the bootstrap program searches for the startup configuration file, known as startup-config, in NVRAM. This file has the previously saved configuration commands and parameters including:

• interface addresses
• routing information
• passwords
• any other configurations saved by the network administrator

If the startup configuration file, startup-config, is located in NVRAM, it is copied into RAM as the running configuration file, running-config.
Executing the Configuration File. If a startup configuration file is found in NVRAM, the IOS loads it into RAM as the running-config and executes the commands in the file, one line at a time. The running-config file contains interface addresses, starts routing processes, configures router passwords and defines other characteristics of the router.

Enter Setup Mode (Optional). If the startup configuration file can not be located, the router prompts the user to enter setup mode. Setup mode is a series of questions prompting the user for basic configuration information. Setup mode is not intended to be used to enter complex router configurations, and it is not commonly used by network administrators.
When booting a router that does not contain a startup configuration file, you will see the following question after the IOS has been loaded:

Command Line Interface
Depending on the platform and IOS, the router may ask the following question before displaying the prompt:
Would you like to terminate autoinstall? [yes]:
Press the Enter key to accept the default answer.
Router>
Note: If a startup configuration file was found, the running-config may contain a hostname and the prompt will display the hostname of the router.
Once the prompt displays, the router is now running the IOS with the current running configuration file. The network administrator can now begin using IOS commands on this router