Functions of Switches, Routers and Hubs

Layer 2 switch is A network device that forwards traffic based on MAC layer (Ethernet or Token Ring) addresses.

Bridging technology has been around since the 1980s (and maybe even earlier). Bridging involves segmentation of local-area networks (LANs) at the Layer 2 level. A multiport bridge typically learns about the Media Access Control (MAC) addresses on each of its ports and transparently passes MAC frames destined to those ports. These bridges also ensure that frames destined for MAC addresses that lie on the same port as the originating station are not forwarded to the other ports. For the sake of this discussion, we consider only Ethernet LANs.

Layer 2 switches effectively provide the same functionality. They are similar to multiport bridges in that they learn and forward frames on each port. The major difference is the involvement of hardware that ensures that multiple switching paths inside the switch can be active at the same time.

There are three distinct functions of layer 2 switching

Address learning

Layer 2 switches and bridges remember the source hardware address of each frame received on an interface, and they enter this information into a MAC database called a forward/filter table.

Forward/filter decisions

When a frame is received on an interface, the switch looks at the destination hardware address and finds the exit interface in the MAC database. The frame is only forwarded out the specified destination port.

Loop avoidance

If multiple connections between switches are created for redundancy purposes, network loops can occur. Spanning Tree Protocol (STP) is used to stop network loops while still permitting redundancy.

Cisco Router Components & their functions:

Cisco routers (and switches) generally contain four types of memory:

• ROM (Read-Only Memory)
• Flash
• NVRAM (Non-Volatile RAM)
• RAM (Random-Access Memory)

CPU
The CPU executes operating system instructions, such as system initialization, routing functions, and switching functions.

RAM
RAM stores the instructions and data needed to be executed by the CPU.

RAM is used to store these components:

Operating System: The Cisco IOS (Internetwork Operating System) is copied into RAM during bootup.

Running Configuration File: This is the configuration file that stores the configuration commands that the router IOS is currently using. With few exceptions, all commands configured on the router are stored in the running configuration file, known as running-config.
IP Routing Table: This file stores information about directly connected and remote networks. It is used to determine the best path to forward the packet.
ARP Cache: This cache contains the IPv4 address to MAC address mappings, similar to the ARP cache on a PC. The ARP cache is used on routers that have LAN interfaces such as Ethernet interfaces.

Packet Buffer: Packets are temporarily stored in a buffer when received on an interface or before they exit an interface.

RAM is volatile memory and loses its content when the router is powered down or restarted. However, the router also contains permanent storage areas, such as ROM, flash and NVRAM.

ROM
ROM is a form of permanent storage. Cisco devices use ROM to store:

• The bootstrap instructions
• Basic diagnostic software
• Scaled-down version of IOS.

ROM uses firmware, which is software that is embedded inside the integrated circuit. Firmware includes the software that does not normally need to be modified or upgraded, such as the bootup instructions. Many of these features, including ROM monitor software, will be discussed in a later course. ROM does not lose its contents when the router loses power or is restarted.

Flash Memory
Flash memory is nonvolatile computer memory that can be electrically stored and erased. Flash is used as permanent storage for the operating system, Cisco IOS. In most models of Cisco routers, the IOS is permanently stored in flash memory and copied into RAM during the bootup process, where it is then executed by the CPU. Some older models of Cisco routers run the IOS directly from flash. Flash consists of SIMMs or PCMCIA cards, which can be upgraded to increase the amount of flash memory.Flash memory does not lose its contents when the router loses power or is restarted.

NVRAM
NVRAM (Nonvolatile RAM) does not lose its information when power is turned off. This is in contrast to the most common forms of RAM, such as DRAM, that requires continual power to maintain its information. NVRAM is used by the Cisco IOS as permanent storage for the startup configuration file (startup-config). All configuration changes are stored in the running-config file in RAM, and with few exceptions, are implemented immediately by the IOS. To save those changes in case the router is restarted or loses power, the running-config must be copied to NVRAM, where it is stored as the startup-config file. NVRAM retains its contents even when the router reloads or is powered off.

Hub:

·         It is a networking device working on a Physical Layer.

·         It is physically Star but Logically bus topology.

·         It is an unintelligent device.

·         It is also called Multiport Repeater.

·         It supports only half way communication.

·         Whenever the data sends to one port, the hub just forward it to all ports. So here flooding take place.