08 Layer 3 Switching

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1999, Cisco Systems, Inc.www.cisco.com

Module 8: Layer 3Switching


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8-2CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com

Agenda

What Is Layer 3 Switching?

What is the DifferenceBetween Layer 2Switching, Layer 3

Switching, and Routing?


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Terminology

?

?


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Why Do We Need Layer 3Switching?

Inversion of 80/20 rule Intranets have changed traffic patterns so that 80%

of traffic now crosses network backbones

Trend to centralize servers exacerbates problem Web has turned PCs into both subscribers and

publishers

Advanced network services such as QoS, IP

multicasting, intranets, extranets, and VPNsare better handled at Layer 3

Layer 3 performance must scale toaccommodate new, increased traffic patterns


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Data link layer (Layer 2) forwarding Forwards based on MAC layer address

Wire-speed multiport bridge

Transparent to upper layers

LAN

SwitchingIP Address: 1.1.1.1

IP Address: 1.1.1.2

MAC Address:0002.ABCD.EF12

MAC Address:0001.2345.6789

E0 E6

Switching TableMACAddress

OutputInterface

AAAA.1111.BBBB Ethernet 10

3215.2511.AFFC Ethernet 5

0001.2345.6789 Ethernet 0

3005.6798.AA05 Ethernet 0

0002.ABCD.EF12 Ethernet 6

0005.3ADB.1112 Ethernet 6

Frame Packet

Destination0002.ABCD.EF12


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What Is Routing?

Network layer (Layer 3) forwarding

Forwards packets using CPU processing

based on network layer address Fundamental building block of

Layer 3 switching

Network reachability learned byrouting protocols

EIGRP, OSPF, etc.


7/168-7CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com

DST

SRC

FCS

DST

SRC

TTL

Layer 3 switching/routing performs packetrewrite

Modify MAC addresses

Decrement TTL to prevent packet looping Recompute FCS of modified frame

Layer 3 switches and routers perform the samefunctions to a packet being switched

EthernetFrame

IPPacket Data

X Y

MAC Addresses

A

EthernetFrame

IPPacket Data

DST

SRC

FCS

DST

SRC

TTL

BX A B Y9 8

Packet Manipulation at Layer 3




Network layer (Layer 3) forwarding

Forwards packets using ASICs basedon network layer address

Wire-speed routing for the LAN

Network reachability learned by routing protocols

Router

IP Address: 1.1.1.1

IP Address: 2.1.1.1

E0

Routing TableDestination

Subnet

E2

SwitchInterface

FrameHeader

Packet Header

2.1.1.1

DataDST

SR

CLayer 3 Switch

5.X Fast Ether 1

6.x Gigabit Ether 3

2.X Ethernet 2

1.X Ethernet 0

1.5.X Ethernet 6



ASICs: High-performance, hardware-based

Layer 3 switching and services withconsistent low latency

Routing software: Routing protocols to provide

scalability:

Backbone redundancy

Dynamic load balancing and fast

convergence in the backbone Reachability information

Multiprotocol support for the campus

A Layer 3 Switch HasTwo Distinct Components

Route

Processing

PacketSwitching

IntelligentNetworkServices



What Is the Difference BetweenLayer 3 Switching and Routing?

Layer 3 switches forward packets usingASICs

Routers forward packets using CPUprocessing power

A Layer 3 switch is the sameas a campus router



A Layer 3 or a Layer 2 Switch?Scalability Advantages

Spanning tree limitations

Broadcast storms

Slow convergence

Layer 3 at Layer 2 speeds

Routing components

provides redundancy Advanced services

Layer 2 Flat Network

SiSi SiSi

SiSi SiSi

SiSi

Layer 3 Switching/Routing



A Layer 3 or a Layer 2 Switch?Other Advantages

Enhances security

Improves manageability

Allows VLAN implementation withoutan external router

Offers increased redundancy/resiliency

Other Considerations Cost Complexity



Guidelines

Use a traditional router to:

Provide better WAN aggregation

Use a Layer 3 switch to:

Scale Layer 3 performance to handle

changing traffic patterns



Route

Processing(OSPF, EIGRP,

BGP4, PIM, RIP,etc.)

PacketSwitching

(NetFlow, ExpressForwarding)

Intelligent

NetworkServices

(HSRP, AccessLists, Mobility,

TACACS, Debug,

QoS, NTP, etc.)

Packet-by-packet switching inhardware, Layer 2 = Layer 3 = Layer 4performance

Policy enablers, access lists, proxy

services, QoS features, debugging

Keys to manageability, troubleshooting,and applications availability

Path determination, load balancing,

and summarization, multiprotocolroutingKeys to scalability andstability

Not All Layer 3 SwitchesAre Created Equal



Summary

Layer 3 switching is ASIC-based routing

Traditional routers are better for WANaggregation

Layer 3 switches are more appropriate forscaling Layer 3 performance

Layer 2 switches are more appropriatewhen the additional cost and complexityare not warranted


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08 Layer 3 Switching

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