Routers

1
Routers

• Operate in a Mesh
• Many possible alternative routes between two
  stations

Packet
Only One of Many Possible Alternative Routes
2
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

3
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

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4
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
5
Early Site Networks

• Organization
• LANs (subnets) based on hubs
• Routers link hubs
• Hierarchy of Routers

Router
Hub
6
The Switching Revolution

• Switches Push Routers to the Edge
• Switches replace most routers in site networks
• Because switches are cheaper than routers

External
Switch
7
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
8
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
9
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
10
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
11
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

12
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
13
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

14
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