Router

Question 1

Router Architecture

Answer 1

• Data Plane– Handles the actual movement of packets from an
  input interface to an output interface.
  – Deals with how packets get forwarded
• Control Plane– How routing protocols establish routes etc.

Question 2

Router Components

Answer 2

• Hardware components of a router:
  – Network interfaces
  – Interconnection network
  – Processor with a memory and CPU
• PC router:
  – interconnection network is the (PCI)
  bus and interface cards are NICs
  – All forwarding and routing is done on
  central processor
• Commercial routers:
  – Interconnection network and
  interface cards are sophisticated
  – Processor is only responsible for
  control functions (route processor)
  – Almost all forwarding is done on
  interface cards

Question 3

Routing functions include:

Answer 3

– route calculation
– maintenance of the routing table
– execution of routing protocols
* Commercial routers handled by a single general purpose
processor, called route processor

Question 4

IP forwarding is per-packet processing

Answer 4

• On high-end commercial routers, IP forwarding is distributed
• Most work is done on the interface cards– Each interface card has its own lookup tables.
  – When a packet enters an interface card, it is processed locally and
  forwarded out the appropriate interface, without needing to ask the
  central CPU.

Question 5

Slotted Chassis

Answer 5

• Large routers are built as a slotted chassis
  – Interface cards are inserted in the slots
  – Route processor is also inserted in a slot
  7
• This simplifies repairs and upgrades of components

Question 6

Evolution of Router Architectures

Answer 6

• Early routers were essentially general purpose computers
• Today, high-performance routers resemble supercomputers
• Exploit parallelism
• Special hardware components
• Until 1980s (1st generation): standard computer
• Early 1990s (2nd generation): delegate to interfaces
• Late 1990s (3rd generation): Distributed architecture
• Today: Distributed over multiple racks

Question 7

1st Generation Routers

Answer 7

• This architecture is still used in low
  end routers
• Arriving packets are copied to main
  memory via direct memory access
  (DMA)
• Interconnection network is a backplane
  (shared bus)
• All IP forwarding functions are
  performed in the central processor.
• Routing cache at processor can
  accelerate the routing table lookup.
• Drawbacks:
  – Forwarding Performance is
  limited by CPU
  – Capacity of shared bus limits
  the number of interface cards
  that can be connected

Question 8

2nd Generation Routers

Answer 8

• Keeps shared bus architecture,
  but offloads most IP forwarding to interface cards
• Interface cards have local route cache and processing elements
• Fast path: If routing entry is found in local cache, forward packet directly to outgoing interface
• Slow path: If routing table entry is not in cache, packet must be handled by central CPU

Question 9

3rd Generation Architecture

Answer 9

• Interconnection network is a switch fabric (e.g., a crossbar switch)
• Distributed architecture:
  
  • Interface cards operate independent of each other
  • No centralized processing for IP forwarding
• These routers can be scaled to many hundred interface cards and to aggregate capacity of > 1
  Terabit per second