12 lan switching

7/29/2019 12 Lan Switching

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2009 Cisco Systems, Inc. All rights reserved. Cisco Confidential 1

LAN Switching

Novan Aryandi

[email protected]

Cisco Systems Indonesia
mailto:[email protected]:[email protected]


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Hierarchical Network Model

DistributionLayer

Core Layer

AccessLayer


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Catalyst Switching Portfolio

Catalyst 2900

Catalyst 3750

Catalyst 3560

Catalyst 4500/E

Catalyst 6500

Catalyst Express 500

Catalyst 4500/E

Catalyst 6500

Small Medium-sized Large

Feat

ures,

Scalabilit

y,

Longevity

BladeSwitches

Catalyst 4900

Catalyst 6500


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Physical Layer Functions

Defines:

Media type

Connector type

Signaling type

Voltage levels, pulse width,pulse intervals etc.

802.3

Physical


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Physical Layer: Ethernet

Hub

Hosts

Host

10Base2Thick Ethernet10Base5Thick Ethernet

10BaseTTwisted Pair


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Hubs Operate at Physical layer

A B C D

Physical

All devices in the same collision domain

All devices in the same broadcast domain

Devices share the same bandwidth


7/46 2009 Cisco Systems, Inc. All rights reserved. Cisco Confidential 7

Hubs: One Collision Domain

More end stations means

more collisions

CSMA/CD is used



Data Link Layer Functions

Identification of encapsulated

data (framing)

Arbitration

Addressing

Error detection

Error recovery

Flow control

DataLink

Physica

l

EIA/TIA-232v.35

802.2

802.3



MAC Addresses

Ethernet layer-2 address is referred to as MAC address

MAC address is 6 bytes long:

first 3 bytes identifies the Organization (OUI)

last 3 bytes identifies a particular device on the network.

Basic MAC address types:

Broadcast MAC: FF-FF-FF-FF-FF-FF

Every device should process the frame

Multicast MAC: 01-00-5E-xx-xx-xx

Only a subset of all devices process the frame Unicast MAC: 00-08-02-8E-50-FD

Only the intended recipient process the frame



Each segment has its own collision domain

All segments are in the same broadcast domain

Data Link

Switches and Bridges Operate at Data Link Layer

OR1 2 3 1 24



Switches

Each segment has its own

collision domain

Broadcasts are forwardedto all segments

Memory

Switch



Address learning

Forward/filter decision

Loop avoidance

Three Switch Functions



How Switches Learn Host Locations

Initial MAC address table is empty

MAC address table

0260.8c01.1111

0260.8c01.2222

0260.8c01.3333

0260.8c01.4444

E0 E1

E2 E3

A B

C D



How Switches Learn Hosts Locations

Station A sends a frame to Station C Switch caches station A MAC address to port E0 by learning

the source address of data frames

The frame from station A to station C is flooded out to allports except port E0 (unknown unicasts are flooded)

MAC address table

0260.8c01.1111

0260.8c01.2222

0260.8c01.3333

0260.8c01.4444

E0: 0260.8c01.1111

E0 E1

E2 E3DC

BA



How Switches Learn Host Locations

Station D sends a frame to station C

Switch caches station D MAC address to port E3 by learning thesource Address of data frames

The frame from station D to station C is flooded out to all portsexcept port E3 (unknown unicasts are flooded)

MAC address table

0260.8c01.1111

0260.8c01.2222

0260.8c01.3333

0260.8c01.4444

E0: 0260.8c01.1111

E3: 0260.8c01.4444

E0 E1

E2 E3 DC

A B



How Switches Filter Frames

Station A sends a frame to station C

Destination is known, frame is not flooded

E0: 0260.8c01.1111

E2: 0260.8c01.2222

E1: 0260.8c01.3333E3: 0260.8c01.4444

0260.8c01.1111

0260.8c01.2222

0260.8c01.3333

0260.8c01.4444

E0 E1

E2 E3

XX DC

A B

MAC address table



Broadcast and Multicast Frames

Station D sends a broadcast or multicast frame

Broadcast and multicast frames are flooded toall ports other than the originating port

0260.8c01.1111

0260.8c01.2222

0260.8c01.3333

0260.8c01.4444

E0 E1

E2 E3 DC

A B

E0: 0260.8c01.1111

E2: 0260.8c01.2222

E1: 0260.8c01.3333E3: 0260.8c01.4444

MAC address table



Redundant Topology

Redundant topology eliminates single points of failure

Redundant topology causes broadcast storms, multiple frame copies, andMAC address table instability problems

Segment 1

Segment 2

Server/host X Router Y



Broadcast Storms

Segment 1

Segment 2


Broadcast

Switch A Switch B

Host X sends a broadcast



Broadcast Storms (cont.)

Segment 1

Segment 2


Broadcast

Switch A Switch B

Broadcast is flooded by Switch A and B



Broadcast Storms (cont.)

Segment 1

Segment 2


Broadcast

Switches continue to propagate broadcast trafficover and over

Switch A Switch B



Multiple Frame Copies

Segment 1

Segment 2

Server/host X Router YUnicast

Switch A Switch B

Host X sends an unicast frame to router Y

Router Y MAC address has not been learned by eitherswitch yet



Multiple Frame Copies (cont.)

Segment 1

Segment 2


Unicast

Switch A Switch B

Host X sends an unicast frame to Router Y Router Y MAC Address has not been learned by either

Switch yet

Router Y will receive two copies of the same frame

Unicast

Unicast



MAC Database Instability

Segment 1

Segment 2


Unicast Unicast

Switch A Switch B

Host X sends an unicast frame to Router Y Router Y MAC Address has not been learned by either

Switch yet

Switch A and B learn Host X MAC address on port 0

Port 0

Port 1

Port 0

Port 1


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MAC Database Instability (cont.)

Segment 1

Segment 2


Unicast Unicast

Switch A Switch B

Host X sends an unicast frame to Router Y Router Y MAC Address has not been learned by either Switch yet Switch A and B learn Host X MAC address on port 0 Frame to Router Y is flooded

Switch A and B incorrectly learn Host X MAC address on port 1

Port 0

Port 1

Port 0

Port 1


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Complex topology can cause multiple loops to occur

Layer 2 has no mechanism to stop the loop

Server/host

Workstations

Loop

Loop

Loop

Multiple Loop Problems

Broadcast


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Solution: Spanning-Tree Protocol

Allows switches to communicate with each other fordiscovering physical loops in the network

Places certain ports in blocking state to arrive at aredundant loop-free network topology

Blockx


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Spanning-Tree Operations

One root bridge per network

One root port per non-root bridge

One designated port per segment

x

Designated port (F) Root port (F)

Designated port (F) Non-designated port (B)

Root bridge Non-root bridge

SW X SW Y

100baseT

10baseT


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Switch YDefault priority =32768 (hex 8000)

MAC = 0c0022222222

Switch XDefault priority =32768 (hex 8000)

MAC = 0c0011111111

Spanning-Tree Operations (cont.)

BPDU

Bridge ID = Bridge priority + Bridge MAC address Root Bridge = Bridge with the lowest bridge ID

In the example, which switch has the lowest Bridge ID ?

Port ID = Port priority + Port index


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Structure of Configuration BPDU

Root BID

Root Path Cost

Sender BID

Port ID

Who is the Root Bridge ?

How far away is the Root Bridge ?

What is the BID of the bridge that

sent this BPDU ?What port on the sending bridgedid this BPDU come from ?

BPDU = Bridge Protocol Data Unit(sent every 2 seconds by default)


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Spanning-Tree ProtocolPath Cost

Link Speed Cost per re-ratify IEEEspec (non-linear

scale)

Cost per older IEEEspec (linear scale)

10 Gbps 2 1

1 Gbps 4 1

100 Mbps 19 10

10 Mbps 100 100


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Switch Y

Default priority = 32768MAC = 0c0022222222

Switch X

Default priority = 32768MAC = 0c0011111111

Spanning-Tree ProtocolPort States

Root bridge

x

Port 0

Port 1

Port 0

Port 1

100baseT

10baseT

Designated port (F) Root port (F)

Non-designated port (B)Designated port (F)


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Spanning-Tree Election Criteria

Lowest Root BID

Lowest Path cost to the Root Bridge

Lowest Sender BID

Lowest Port ID


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Spanning-Tree Port States

Spanning-tree transitions each port

through several different states

Init

Blocking

Listening Disabled

Forwarding

Learning


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Spanning-Tree Port States(cont.)

State Function

Forwarding Sends and receives user data

Learning (15 secs) Builds bridging table

Listening (15 secs) Builds active topology

Blocking (20 secs) Only receives BPDUs

Disabled Non-operational state


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Spanning-Tree Recalculation

Switch YMAC = 0c0022222222Default priority = 32768

Switch XMAC = 0c0011111111Default priority = 32768

Port 0

Port 1

Port 0

Port 1

10baseT

x

100baseT

Root Bridge

Designated port Root port (F)

Non-designated port (B)Designated port


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Switch YMAC = 0c0022222222Default priority = 32768

Switch XMAC = 0c0011111111Default priority = 32768

Port 0

Port 1

Port 0

Port 1

10baseT

x

100baseT

Root Bridge

Designated port Root port (F)

Non-designated port (B)Designated portBPDU

xMAXAGE

x

Spanning-Tree Recalculation(cont.)


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Key Issue: Time to Convergence

Convergence occurs when all the switchesand bridge ports have transitioned to either

the forwarding or blocking state

When network topology changes, switchesand bridges must re-compute the Spanning-

Tree Protocol, which disrupts user traffic


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One STP Instance Per VLAN!

As if this wasn't complicated enough, there is a separate instance ofSpanning Tree Protocol running for each VLAN. This feature isreferred to as per-VLAN spanning tree (PVST)

So with PVST, each VLAN can have a different Root Bridge and

active topology for the same Layer-2 network


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Primarily software based

One spanning-tree instance per bridge

Usually up to 16 ports per bridge

Bridging

Primarily hardware based (ASIC)

Many spanning-tree instances per switch

More ports on a switch

LAN Switching

Bridging Compared to LAN Switching


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Transmitting Frames Through a Switch

Cut-through

Switch checks destination address andimmediately begins forwarding frame

Frame


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Transmitting Frames through a Switch

Store and forward

Complete frame is received and checkedbefore forwarding

Cut-through


Frame FrameFrame

Frame


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Transmitting Frames through a Switch

Cut-through


Frame

Fragment free(modified cut-through) - Cat1900 Default

Switch checks the first 64 bytes then immediatelybegins forwarding frame

Frame

Store and forward

Complete frame is received and checkedbefore forwarding

Frame

Frame

Frame


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Duplex Overview

Half duplex (CSMA/CD) Unidirectional data flow Higher potential for collison Hubs connectivity

Switch

Hub


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Duplex Overview

Half duplex (CSMA/CD) Unidirectional data flow Higher potential for collison Hubs connectivity

Switch

Hub

Full duplex Point-to-point only Attached to dedicated switched port

Requires full-duplex support on both ends Collision free Collision detect circuit disabled


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