Layer 3 Switching
Routers vs Layer 3 Switches
• Both forward on the basis of IP addresses
• But Layer 3 switches are faster and cheaper
• However, Layer 3 switches are more limited
– Usually do not support WAN connections and only
Ethernet for LANs
– Usually support only TCP/IP and sometimes
IPX/SPX, not SNA, AppleTalk, etc.
• There are other differences as well
Routers
• Forward each packet independently
– Ignore previous IP packets to the same destination
– Make a full router forwarding decision (Ch. 3) for each
arriving IP packet
– This takes a great deal of processing time
– This makes routers slow and inexpensive
– However, this allows routers to adjust rapidly to
failures
Layer 3 Switches
• Do NOT forward each packet independently
– For instance decision caching; Remember previous
decisions based on IP address
– No need to make a full router forwarding decision (Ch.
3) for each arriving IP packet
– This saves a great deal of processing time
– This makes Layer 3 switches fast and inexpensive
– However, this may make failure recovery slow
Routers
• IP being extended with MultiProtocol Label
Switching (MPLS)
– A label is added to each packet
– Packets that should follow the same route get the
same label value
– Router learns output port for each label value
– When packet arrives, send it back out the correct
port without a full router forwarding decision
using routing tables
Routers
• Traditionally used a Bus Architecture
– Interfaces are connected to the processor via a
serial bus like the bus in a PC
– The processor and the bus can become
bottlenecks
Processor
Interface
Bus
Interface
Routers
• Traditionally used a Bus Architecture
– Busses only handle one transmission at a time
– Processors only handle one packet at a time
– Problem if there are several interfaces
Processor
Interface
Bus
Interface
Routers
• Traditionally used a Bus Architecture
–
–
–
–
Each packet has to pass over the bus twice
From the incoming interface to the processor
From the processor to the outgoing interface
This ties up the router for two bus cycles
Processor
Interface
Bus
Interface
Layer 3 Switches
• Interfaces are intelligent
– Make forwarding decisions independently
– So no single processor bottleneck
Decision
Interface
IP Packet
Interface
Switching Matrix
Interface
Interface
Layer 3 Switches
• Interfaces are Intelligent
– Can handle forwarding decisions
simultaneously, reducing processing delays
Decision
Interface
IP Packet
Interface
Switching Matrix
Interface
Decision
Interface
Layer 3 Switches
• Interfaces are Connected by a Switching
Matrix
– Forward packet within frame directly to
outgoing interface
– No bus
Interface
IP Packet
Interface
Switching Matrix
Interface
Interface
Layer 3 Switches
• Interfaces are Connected by a Switching
Matrix
– Can handle multiple frame forwardings
simultaneously
Interface
IP Packets
Interface
Switching Matrix
Interface
Interface
Routers
• Traditionally Did all Processing in Software
– Software is slow
• Many commands may be needed to execute router
forwarding decisions
• Must be executed one at a time
• These will take a long time to execute
– In contrast, hardware is fast
• Many processes can be executed in parallel
• Reduces number of cycles
Layer 3 Switches
• In some Layer 3 switches, ASICs handle some
or all processing
– Application-Specific Integrated Circuits
– Designed for particular applications, such as
routing
– Much faster than software
– Can only handle processes of medium complexity
– Layer 3 switching’s reduced protocol support
allows ASICs to be used there
Layer 3 Switches
• ASIC Technology is Receiving Competition
from Network Processors
– ASICS are purely hardware
– Network processors are programmable but have
hardware optimized for network functions
– Network processors are slower than ASICs but
much faster than software processes on general
routers
– Network processors can be programmed for
specific functions less expensively than ASICs can
be built
Routers
• Adopting Many Speed Switch Innovations
– Some use decision caching
– MPLS is already being used somewhat
– Switching matrices
– Uses some ASICs in processing; Will use more as ASIC
technology becomes able to support more processing
– Limited protocol support may become the only
enduring Layer 3 switch distinction