Lectures 17&18 Fast packet switching Eytan Modiano Massachusetts Institute of Technolog
Lectures 17 & 18 Fast packet switching Eytan Modiano Massachusetts Institute of Technology Eytan Modiano Slide 1
Packet switches engine Switch Scheduler Packet DataReader→[ Packet Tag DestinationAddress Output port number or vc number a packet switch consists of a routing engine(table look-up),a switch scheduler and a switch fabric The routing engine looks-up the packet address in a routing table and determines which output port to send the packet Packet is tagged with port number The switch uses the tag to send the packet to the proper output port
Packet switches Packet Routing engine Switch Scheduler Packet Data Header Packet Tag DestinationAddress Output port number or VC number • A packet switch consists of a routing engine (table look-up), a switch scheduler, and a switch fabric. • The routing engine looks-up the packet address in a routing table and determines which output port to send the packet. – Packet is tagged with port number – The switch uses the tag to send the packet to the proper output port Eytan Modiano Slide 2
First Generation Switches CPU output buffer LC-1 LC-2 LC-3 Input buffer Computer with multiple line cards CPU polls the line cards CPU processes the packets Simple, but performance is limited by processor speeds and bus speeds Examples: Ethernet bridges and low end routers
First Generation Switches CPU LC-1 LC-2 LC-3 output buffer Input buffer • Computer with multiple line cards – CPU polls the line cards – CPU processes the packets • Simple, but performance is limited by processor speeds and bus speeds • Examples: Ethernet bridges and low end routers Eytan Modiano Slide 3
Second Generation switches Bus LC LC LC Most of the processing is now done in the line cards Route table look-up etc Line cards buffer the packets Line card send packets to proper output port Advantages: CPU and main Memory are no longer the bottleneck Disadvantage: Performance limited by bus speeds Bus Bw must be n times LC speed(N ports Example: cisco 7500 series router
Second Generation switches Computer LC LC LC LC Bus • Most of the processing is now done in the line cards – Route table look-up, etc. – Line cards buff er the packets – Line card send packets to proper output port • Advantages: CPU and main Memory are no longer the bottleneck • Disadvantage: Performance limited by bus speeds – Bus BW must be N times LC speed (N ports) • Example: CISCO 7500 series router Eytan Modiano Slide 4
Third generation switches Input LC Output LC Input LC SWITCH Output LC FABRIC Input LC Output LC Controll Replace shared bus with a switch fabric Performance depends on the switch fabric, but potentially can alleviate the bus bottleneck
Third generation switches N by N SWITCH FABRIC Input LC Input LC Input LC Output LC Output LC Output LC Controller • Replace shared bus with a switch fabric • Performance depends on the switch fabric, but potentially can alleviate the bus bottleneck Eytan Modiano Slide 5