LAYER 2 SWITCHING By

Anil Kumar Vishwakarma

MCA,MCTS,CCNA

OVERVIEW

Introduction

Spanning Tree Protocol

Spanning Tree Terms

Spanning Tree Operations

LAN Switch Types

Configuring Switches

INTRODUCTION

Switching Services Unlike bridges that use software to create and manage a

filter table, switches use application specific integrated circuits (ASICs) to build and maintain their filter tables.

A switch can be viewed as a multiport bridge.

Three Switch Functions at Layer 2

1. 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.

2. 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.

3. 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.

ADDRESS LEARNING When a switch is first powered on, the MAC forward/filter

table is empty,

FORWARD/FILTER DECISIONS

When a frame arrives at a switch interface, the

destination hardware address is compared to the

forward/filter MAC database.

If the destination hardware address is known

and listed in the database, the frame is sent out

only the correct exit interface.

The switch doesn’t transmit the frame out any

interface except for the destination interface.

This preserves bandwidth on the other network

segments and is called frame filtering.

LOOP AVOIDANCE Redundant links between switches are a good idea because they help

prevent complete network failures in the event one link stops working.

the switches will flood broadcasts endlessly throughout the nternetwork.

This is sometimes referred to as a broadcast storm

SPANNING TREE PROTOCOL

STP’s main task is to stop network loops from occurring on your layer 2 network switches.

It monitors the network to find all links, making sure that no loops occur by shutting down any redundant links.

STP uses the spanning-tree algorithm (STA) to first create a topology database, then search out and destroy redundant links.

With STP running, frames will be forwarded only on the premium, STP-picked links.

SPANNING TREE TERMS Bridge ID: It is determined by a combination of the bridge

priority (32,768 by default on all Cisco switches) and the base MAC address. The bridge with the lowest bridge ID becomes the root bridge in the network.

Root bridge: is the bridge with the best bridge ID (the lowest bridge ID).

Nonroot bridge: These are all bridges that are not the root bridge. Nonroot bridges exchange BPDUs with all bridges and update the STP topology database on all switches.

BPDU: Bridge Protocol Data Unit (BPDU) the name of packet that they send to one neighbor with the one that they receive from another neighbor.

Root port: The root port is always the link directly connected to the root bridge, or the shortest path to the root bridge.

SPANNING TREE TERMS Designated port A designated port is one that has been

determined as having the best (lowest) cost. A designated

port will be marked as a forwarding port.

Port cost Port cost determines when multiple links are

used between two switches and none are root ports. The

cost of a link is determined by the bandwidth of a link.

Nondesignated port is one with a higher cost than the

designated port. Nondesignated ports are put in blocking

mode—they are not forwarding ports.

Blocked port A blocked port is the port that will not

forward frames, in order to prevent loops. However, a

blocked port will always listen to frames.

SPANNING TREE OPERATIONS

1. Selecting the Root Bridge

2. Selecting the Root Port

3. Selecting the designated (Forward) port.

4. Spanning-Tree Port States

Blocking A blocked port won’t forward frames; it

just listens to BPDUs.

Forwarding The port sends and receives all data

frames on the bridged port.

LAN SWITCH TYPES

LAN switch types decide how a frame is handled

when it’s received on a switch port.

There are three switching modes:

1. Cut-through (FastForward)

2. FragmentFree (modified cut-through) the switch

checks the first 64 bytes (caused by collision) of a

frame before forwarding it for fragmentation.

3. Store-and-forward: In this mode, the complete data

frame is received on the switch’s buffer, a CRC is

run, and, if the CRC passes, the switch looks up the

destination address in the MAC filter table.

CONFIGURING SWITCHES

Setting the Passwords

1900 Switch

(config)#enable password level 1 kkkk -----> User mode

(config)#enable password level 15 kkkk1 -----> Enable Mode

(config)#enable secret todd2 -----> Encrypted

2950 Switch

Switch(config)#enable password todd -----> non Encrypted

Switch(config)#enable secret todd -----> Encrypted

Setting the Hostname

1900 Switch

(config)#host S1900

2950 Switch

Switch(config)#host S2950

CONFIGURING SWITCHES

Setting IP Information

1900 Switch

S1900#config t

S1900(config)#ip address 172.16.10.16 255.255.255.0

S1900(config)#ip default-gateway 172.16.10.1

2950 Switch

S2950#config t

S2950(config)#int vlan1

S2950(config-if)#ip address 172.16.10.17 255.255.255.0

S2950(config-if)#no shut

S2950(config-if)#exit

S2950(config)#ip default-gateway 172.16.10.1

CONFIGURING SWITCHES

Configuring Interface Descriptions

1900 Switch

S1900#config t

S1900(config)#int e0/1

S1900(config-if)#desc Finance_VLAN

2950 Switch

S2950#config t

S2950(config)#int f 0/1

S2950(config-if)#desc Sales Printer

CONFIGURING SWITCHES

Erasing the Switch Configuration

1900 Switch

Todd1900#delete nvram

2950 Switch

Todd2950#erase startup-config

Thank You