Chapter 3 Part 1 Switching and Bridging Networking CS 3470, Section 1

Why do you need to switch? So far, we’ve seen how to create small networks Switching enables us to build very large networks

What is a switch? A mechanism that allows us to interconnect links to form a large network A multi-input, multi-output device which transfers packets from an input to one or more outputs Adds the star topology to the point-to-point link, bus (Ethernet), and ring (802.5 and FDDI) topologies ** ** Picture copyright Cisco Systems, license GNU 2.5. http://en.wikipedia.org/wiki/File:Linksys48portswitch.jpg

Advantages of Switching We can interconnect switches to connect many hosts to the same network Can use point-to-point links Adding a new host to the network by connecting it to a switch does not necessarily reduce network performance for other attached hosts

The University of Adelaide, School of Computer Science 1 October 2017 Switching How does the switch decide which output port to place each packet on? It looks at the header of the packet for an identifier that it uses to make the decision Two common approaches Datagram or Connectionless approach Virtual circuit or Connection-oriented approach Also called forwarding Chapter 2 — Instructions: Language of the Computer

The University of Adelaide, School of Computer Science 1 October 2017 Forwarding Assumptions Each host has a globally unique address There is some way to identify the input and output ports of each switch We can use numbers We can use names Chapter 2 — Instructions: Language of the Computer

Types of Switching Circuit Switching Packet switching Most commonly used in telephone networks Physical connection is set up from one end to the other Data is transmitted, Connection is torn down Packet switching Data networks, uses datagrams Enables packets to be sent from one host to another without a direct or dedicated physical connection. Asymmetric routing common Asymmetric routing occurs when packets take one route from sender to receiver, but take a different route from receiver to sender.

Circuit Switching Circuit switching dedicates fixed amount of bandwidth from one end to another Example: A DS3 from LA to Cedar Falls consumes a full DS3 from end to end whether it is in use or not

Circuit Switching Virtual Circuit Switching requires set up and tear down of the circuit after each use. No one else can use any of the bandwidth during an active session—even if it is idle! Known as a connection-oriented approach

Packet Switching Differs from circuit switching (obviously) Data link layer — single broadcast and collision domain Data is sent across links when space is available Known as a connectionless (or datagram) approach

Datagram / Connectionless Approach The University of Adelaide, School of Computer Science 1 October 2017 Datagram / Connectionless Approach Key Idea Every packet contains enough information to enable any switch to decide how to get it to destination Every packet contains the complete destination address Chapter 2 — Instructions: Language of the Computer

Packet Switching Bad example: We can have a switch with 100 attached clients at 100Mbps linked through 10Mbps to another switch with 100 clients at 100Mbps Obviously not the best solution because there's contention at the interconnect Bandwidth is not reserved from one end to another.

Interconnecting with Hubs Hub – Repeats incoming signals out of all outgoing ports (like a repeater) Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain

Bridges Link layer device transparent plug-and-play, self-learning stores and forwards Ethernet frames examines frame header and selectively forwards frame based on MAC dest address when frame is to be forwarded on segment, uses CSMA/CD to access segment transparent hosts are unaware of presence of bridges plug-and-play, self-learning bridges do not need to be configured

Bridges: Traffic Isolation Bridge installation breaks LAN into LAN segments Bridges filter packets: Same-LAN-segment frames not usually forwarded onto other LAN segments Segments become separate collision domains bridge collision domain = hub = host LAN segment LAN segment LAN (IP network)

Interconnection without Backbone Not recommended for two reasons: Single point of failure at Computer Science hub All traffic between EE and SE must path over CS segment

Backbone configuration Recommended !

Self Learning A bridge has a bridge table Entry in bridge table: (Node LAN Address, Bridge Interface, Time Stamp) Stale entries in table dropped (TTL can be 60 min) Bridges learn which hosts can be reached through which interfaces When frame received, bridge “learns” location of sender: incoming LAN segment Records sender/location pair in bridge table

Filtering/Forwarding When bridge receives a frame: index bridge table using MAC dest address if entry found for destination { if dest on segment from which frame arrived drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

Bridge Example Suppose C sends frame to D and D replies back with frame to C. Bridge receives frame from C Notes in bridge table that C is on interface 1 Because D is not in table, bridge sends frame into interfaces 2 and 3 Frame received by D

Bridge Learning: Example D generates frame for C, sends Bridge receives frame Notes in bridge table that D is on interface 2 Bridge knows C is on interface 1, so selectively forwards frame to interface 1

Your Turn Assume fresh (empty) address/port table. Through which bridge interfaces are the following messages sent? 1. H sends to F, and F replies back to H 2. Next, A sends to F, and F replies back to A What does the end table look like?

Bridge Learning: Drawback Previous strategy works fine until a LAN has a loop in it Possible bad failure case – frames could loop forever without getting to final destination! How could this happen? In a large network, some administrator could add a bridge that closes a loop without realizing it Could also be built in on purpose to provide redundancy So single link failure does not bring down whole network