Download Routers

Routers
• Operate in a Mesh
– Many possible alternative routes between two
stations
Packet
Only One of Many
Possible Alternative Routes
Routers
• Benefits of Alternative Routes
– Can route around a router or trunk line failure
– Can route around congestion
– Can select an optimal route based on cost, latency, security,
or other goal
• In Contrast
– Ethernet deals poorly with failures
– ATM can change virtual circuits in case of failure, but this
takes time
Routing is Complex
• For each packet, router must consider all possible
alternative routes
• As noted earlier, it must do complex calculations
on each router forwarding table row
• This complexity requires a complex and expensive
router for relatively low packet forwarding rates
?
Switches Versus Routers
•
•
•
•
Switches
Fast
Inexpensive
No benefits of
alternative routing
•
•
•
•
Routers
Slow
Expensive
benefits of alternative
routing
“Switch where you can; route where you must”
Early Site Networks
• Organization
– LANs (subnets) based on hubs
– Routers link hubs
– Hierarchy of Routers
Router
Hub
The Switching Revolution
• Switches Push Routers to the Edge
– Switches replace most routers in site networks
– Because switches are cheaper than routers
External
Switch
The Switching Revolution
• Switches Push Routers to the Edge
– Router still needed at the edge of the site network to
communicate with outside world because routers
handle expensive long-distance links very well
External
Switch
The Switching Revolution
• Layer 3 Switches
– Traditional switches operate at Layer 2; Switch based
on MAC addresses
– Layer 3 switches switch based on internet layer IP
addresses
External
Layer 3
Switch
The Switching Revolution
• Layer 3 Switches
– Layer 3 switches are replacing many Layer 2 switches
in site networks because of their ability to switch based
on IP addresses
External
Layer 3
Switch
The Switching Revolution
• Layer 3 Switches versus Routers
– Layer 3 switches are much faster than routers
– Layer 3 switches cost less than routers
External
Layer 3
Switch
The Switching Revolution
• Layer 3 Switches versus Routers
– At the internetworking layer, Layer 3 switches normally
only support IP and sometimes IPX; Routers route
many more internet layer protocols, including those of
AppleTalk, SNA, and others
– At the data link layer, Layer 3 switches normally
support only Ethernet; Rarely ATM
The Switching Revolution
• Layer 3 Switches versus Routers
– Layer 3 switches rarely support Layer 2 WAN protocols
– Routers usually are still needed at the edge of the site
network, to communicate with external links
External
Layer 3
Switch
The Switching Revolution
• Routers
– Forward based on IP
addresses and other
internet layer addresses
– Expensive and slow
– Handle multiple internet
layer protocols
– Handle multiple LAN
and WAN subnet
protocols
• Layer 3 Switches
– Forward based on IP
addresses, sometimes
IPX addresses
– Inexpensive and Fast
– Do not handle multiple
internet layer protocols
– Do not handle multiple
LAN and WAN subnet
protocols
The Switching Revolution
• Layer 4 Switches
– Examine port fields in TCP and UDP
– These fields describe the application
– Therefore, can switch based on application (to give
priority by application, etc.)
Layer 4
Switch