CETTM MTNL

1Switching & VLAN

SWITCHING & VLAN

MODULE ID: ISWGVLN001

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2Switching & VLAN

Topics Covered

1. Introduction2. Switch Operation & Functions3. Types of Switching4. Flat Networks & VLAN5. Benefits of VLAN6. Inter VLAN Communication7. Types of VLANs8. Frame Tagging

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Before switching

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The first switched LAN

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The typical switched network design

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Layer 2 Switching

Layer 2 switching is the process of using the hardware address of devices on a LAN to segment a network

Switching breaks up large collision domains into smaller ones

A collision domain is a network segment with two or more devices sharing the same bandwidth.

Each port on a switch is its own collision domain

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Layer 2 Switching

Layer 2 switching provides the following: Hardware-based bridging (ASIC)

Wire speed

Low latency

Low cost

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Switch Features

Bridges use software to create and manage a filter table

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

Think of a layer 2 switch as a multi port bridge because their basic reason for being is the same: to break up collision domains.

Layer 2 switches and bridges are faster than routers because they dont take up time looking at the Network layer header information.

Switches create private dedicated collision domains and provide independent bandwidth on each port

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Switch Features

In layer 2 switching no modification to the data packet takes place.

Limitations of Layer 2 Switching Bridged networks break up collision domains,

but the network is one large broadcast domain.

Broadcasts and multicasts, along with the slow convergence time of spanning trees, can considerably slow down the network

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Layer 2 Switch Functions

Address learning,

Forward/filter decisions, and

Loop avoidance

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Address Learning

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How switches learn hosts locations

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A switched network with switching loops

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Multiple frame copies

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Broadcast storm

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Spanning Tree Protocol (STP)

Digital Equipment Corporation (DEC) created the original version of Spanning Tree Protocol (STP)

The IEEE later created its own version of STP called 802.1D.

All Cisco switches run the IEEE 802.1D version of STP, which isnt compatible with the DEC version.

STPs main task is to stop network loops from occurring on your layer 2 network

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Spanning Tree Protocol (STP)

STP 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.

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LAN Switch Types

LAN switch types decide how a frame is handled when its received on a switch port.

Latencythe time it takes for a frame to be sent out an exit port once the switch receives the framedepends on the chosen switching mode.

There are three switching modes: Cut-through (Fast-Forward) Fragment-Free (modified cut-through) Store-and-forward

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Different switching modes within a frame

6 bytes 1 byte 6 bytes 6bytes 2 bytes Upto 1500 bytes 4 bytes

Preamble SFDDestination hardware address

Source hardware address

Length DATA FCS

Cut-through:No error checking

Fragment Free:Checks for collisions

Store-and-Forward:Error are checked,Has highest latency

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Cut-Through (Real Time)

With the cut-through switching method, the LAN switch reads only the destination address (the first six bytes following the preamble) onto its onboard buffers.

it then looks up the hardware destination address in the MAC switching table, determines the outgoing interface, and proceeds to forward the frame toward its destination.

A cut-through switch really helps to reduce latency because it begins to forward the frame as soon as it reads the destination address and determines the outgoing interface.

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Fragment-Free (Modified Cut-Through)

Fragment-Free is a modified form of cut-through switching in which the switch waits for the collision window (64 bytes) to pass before forwarding.

If a packet has a collision error, it almost always occurs within the first 64 bytes.

It means each frame will be checked into the data field to make sure no fragmentation has occurred.

Fragment-Free mode provides better error checking than the cut-through mode with practically no increase in latency.

Its the default switching method for the 1900 switches.

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Store-and-Forward

Store-and-forward switching is Ciscos primary LAN switching method.

The store-and-forward method provides efficient, error-free transport instead of fast transport.

LAN switch copies the entire frame onto its onboard buffers and then computes the cyclic redundancy check (CRC).

Latency through the switch varies with frame length. The frame is discarded if it contains a CRC errorif its

too short (less than 64 bytes including the CRC) or if its too long (more than 1518 bytes including the CRC).

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Flat network structure

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The benefit of a switched network

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VLAN Definition

By default, switches break up collision domains and routers break up broadcast domains.

How to break up broadcast domains in a pure switched internetwork?

By creating a virtual local area network A VLAN is a logical grouping of network users and

resources connected to administratively defined ports on a switch.

A VLAN is treated like its own subnet or broadcast domain

Frames broadcast onto the network are only switched between the ports logically grouped within the same VLAN

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VLAN Definition

By default, no hosts in a specific VLAN can communicate with any other hosts that are members of another VLAN,

For inter-VLAN communication, the router is required layer 2 switched networks are typically designed as

flat networks. Every broadcast packet transmitted is seen by every

device on the network, regardless of whether the device needs to receive that data

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Benefits of VLAN

VLANs simplify network management: Network adds, moves, and changes are achieved

by configuring a port into the appropriate VLAN. A group of users needing high security can be put

into a VLAN so that no users outside of the VLAN can communicate with them.

VLANs can be considered independent from their physical or geographic locations.

VLANs can enhance network security. VLANs increase the number of broadcast domains

while decreasing their size

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Broadcast Control

Broadcasts occur in every protocol, How often Broadcasts occur depends upon three things:

Type of protocol The application(s) running on the internetwork How these services are used

Bandwidth abusers are multimedia applications, faulty equipment, inadequate segmentation, and poorly designed firewalls

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Security

Anyone connecting to the physical network could access the network resources located on that physical LAN.

Anyone can observe any and all traffic happening in that network by plugging a network analyzer into the hub.

Users could join a workgroup by just plugging their workstations into the existing hub.

With VLANs, administrators can have control over each port and user

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Flexibility and Scalability

Layer 2 switches only read frames for filteringthey dont look at the Network layer protocol.

By default, switches forward all broadcasts. If VLANs are created and implemented, then smaller

broadcast domains at layer 2 are created. Broadcasts sent out from a node in one VLAN wont

be forwarded to ports configured to be in a different VLAN.

VLANs offer flexibility to add only the desired users into one broadcast domain regardless of their physical location

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Physical LANs connected to a router

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Switches removing the physical boundary

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VLAN Types

An administrator creates VLANs and assigns switch ports to each VLAN.

Such a VLAN is called a static VLAN If the administrator assign all the host devices

hardware addresses into a database, the switches can be configured to assign VLANs dynamically whenever a host is plugged into a switch.

This is called a dynamic VLAN

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Static VLANs

Static VLANs are the usual way of creating VLANs, and theyre also the most secure.

Static VLAN configuration is comparatively easy to set up and monitor,

Static VLAN works well in a network where the movement of users within the network is controlled

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Dynamic VLANs

A dynamic VLAN determines a nodes VLAN assignment automatically.

VLAN assignments can be based on hardware (MAC) addresses, protocols, or applications to create dynamic VLANs.

A dynamic VLAN makes management and configuration easier because if a user moves, the switch will assign them to the correct VLAN automatically.

A lot more work is required initially setting up the database

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Identifying VLANs

As frames are switched throughout the network, switches must be able to keep track of all the different types, plus understand what to do with them depending on the hardware address.

Frames are handled differently according to the type of link they are traversing

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Access and trunk links in a switched network

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Frame Tagging

A switch fabric is basically a group of switches sharing the same VLAN information.

There needs to be a way for each switch to keep track of all the users and frames as they travel the switch fabric and VLANs.

Frame tagging uniquely assigns a user-defined ID to each frame. people refer to it as a VLAN ID or color.

Each switch that the frame reaches first identifies the VLAN ID from the frame tag, then it finds out what to do with the frame by looking at the information in the filter table.

If the frame reaches a switch that has another trunked link, the frame will be forwarded out the trunk-link port

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Frame Tagging

The frame reaches an exit to an access link matching the frames VLAN ID, the switch removes the VLAN identifier.

The destination device can receive the frames without having to understand their VLAN identification

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VLAN Identification Methods

Switches use VLAN identification to keep track of all those frames as theyre traversing a switch fabric. Switches identify which frames belong to which VLANs. Trunking methods:

Inter-Switch Link (ISL) IEEE 802.1Q

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Inter-Switch Link (ISL)

This is proprietary to Cisco switches, and its used for Fast Ethernet and Gigabit Ethernet links only

Inter-Switch Link (ISL) is a way of explicitly tagging VLAN information onto an Ethernet frame

ISL is an external tagging process, the original frame isnt alteredits only encapsulated with a new 26-byte ISL header

It also adds a second 4-byte Frame Check Sequence (FCS) field at the end of the frame

Because the frame has been encapsulated by ISL with information, only ISL-aware devices can read it

These frames can be up to 1522 bytes long

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ISL Frame Structure

ISL Header 26 bytes Encapsulated Ethernet Frame

CRC4 bytes

DA Type User SA LEN AAAA03 HSA VLAN BPDU INDEX RES

VLAN BPDU

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IEEE 802.1q

Created by the IEEE as a standard method of frame tagging,

It inserts a field into the frame to identify the VLAN. If trunking between a Cisco switched link and a

different brand of switch, use 802.1Q for the trunk to work.

The ports that populate the same trunk create a group thats known as a native VLAN, and each port gets tagged with an identification number that reflects its native VLAN, the default being VLAN 1

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IEEE 802.1q Frame

DA SA Tag Type/Length Data FCS

6 6 4 2 Upto 1500 4 bytes

TPID Priority CFI VID16 3 1 12 bits

Tag Protocol Identifier

Priority levels(0 to 7)

Canonical Format Indicator

Unique VLAN Identifier

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VLAN Trunking Protocol (VTP)

VLAN Trunking Protocol (VTP) manages all configured VLANs across a switched internetwork and maintains consistency throughout that network

VTP allows an administrator to add, delete, and rename VLANs and this information is then propagated to all other switches in the VTP domain

Accurate tracking and monitoring of VLANs Dynamic reporting of added VLANs to all switches in

the VTP domain

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Routing between VLANs

Hosts in a VLAN live in their own broadcast domain and can communicate freely.

VLANs create network partitioning and traffic separation at layer 2 of the OSI

If hosts or any other IP-addressable device is to communicate between VLANs, a layer 3 device is absolutely necessary.

Use a router that has an interface for each VLAN or a router that supports ISL routing.

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Router with individual VLAN associations

Termination VLANs in separate physical interfaces

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Router on a stick

Termination of individual VLANs using sub-interfaces

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Summary Switching reduces collisions Every port of Switch is a separate collision domain Switch uses forward/filter table to handle frames STP removes switching loops and creates tree like

structure VLAN is a logical grouping of network users VLAN is treated like its own subnet or broadcast domain VLANs simplify network management, controls

broadcasts, enhances security There are 2 types of VLAN trunking/tagging

ISL IEEE 802.q

VLAN Trunking Protocol (VTP) is used to manage all configured VLANs

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References

Books Hand book on Internetworking by CISCO CCNA Study guide by Todd Lammle

URLs http://www.cisco.com http://en.wikipedia.org

Slide 1Slide 2Before switchingThe first switched LANThe typical switched network designSlide 6Slide 7Switching ServicesSlide 9Layer 2 Switch FunctionsAddress LearningHow switches learn hosts locationsA switched network with switching loopsMultiple frame copiesBroadcast stormSpanning Tree Protocol (STP)Slide 17LAN Switch TypesDifferent switching modes within a frameCut-Through (Real Time)Fragment-Free (Modified Cut-Through)Store-and-ForwardFlat network structureThe benefit of a switched networkSlide 25Slide 26Slide 27Broadcast ControlSecurityFlexibility and ScalabilityPhysical LANs connected to a routerSwitches removing the physical boundaryVLAN TypesStatic VLANsDynamic VLANsIdentifying VLANsAccess and trunk links in a switched networkFrame TaggingSlide 39VLAN Identification MethodsInter-Switch Link (ISL)Slide 42IEEE 802.1qIEEE 802.1q FrameVLAN Trunking Protocol (VTP)Routing between VLANsRouter with individual VLAN associationsRouter on a stickSlide 49Slide 50