Router Architecture

High level view of generic router architecture:

• Set of input/output ports for a router
  • Physically the same, but distinct functions
• High-speed switching fabric between ports forwards from input to output ports
• Routing processor takes care of maintaining/creating forwarding rules
  • Runs in control plane (implemented in software)
• Forwarding data plane operates in hardware

Input Port Functionality

• Physical Layer Functionality
  • terminates incoming physical link at router
  • Receives bits and passes them to link layer protocol
    • Rebuild frame, etc.
• Decentralized Switching
  • With headers, lookup output port using forwarding table in input port memory (i.e. “match plus action”)
    • Should process as fast as “line speed”
  • Input port queueing occurs if datagrams arrive faster than forwarding rate into switch fabric
  • Destination-based forwarding
    • Forward only based on destination IP address
    • Traditional approach
  • Generalized forwarding
    • Forward based on any set of header field values

Longest Prefix Matching

When looking for the forwarding table entry for a given destination address, use the longest address prefix that matches the destination address.

• e.g. Given the following table:

  • IP addresses are 4 bytes long

    

  • The first address will be forwarded to link interface 0

    

  • Second address will be forwarded to link interface 1, as it matches a longer prefix than 2

    

Longest prefix matching is often performed using ternary content addressable memories (TCAMs).

• Content addressable ⇒ Can retrieve address in 1 clock cycle, regardless of table size
  • Cisco Catalyst has around 1 million entries in TCAM