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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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8-3CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
Terminology
?
?
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8-4CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
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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What Is Layer 2 Switching?
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.
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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
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8/168-8CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
What Is Layer 3 Switching?
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
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9/168-9CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
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
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10/168-10CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
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
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11/168-11CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
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
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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
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13/168-13CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
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
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14/168-14CSE: Networking FundamentalsLayer 3 Switching 1999, Cisco Systems, Inc.www.cisco.com
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
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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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