1 Computer Networks Internetworking Devices

2 Repeaters Hubs Bridges –Learning algorithms –Problem of closed loops Switches Routers

3 Repeaters Repeaters are purely physical layer devices Single collision domain

4 Functions of a Repeater

5 Shared Hubs Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces Hubs can be arranged in a hierarchy with backbone hub at its top

6 Shared Hubs Limitations Single collision domain results in no increase in maximum throughput –multi-tier throughput same as single segment throughput Cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)

7 Bridges Bridges are MAC/Link layer devices operating on Ethernet frames, examining frame header and selectively forwarding frame based on its destination Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

8 A Bridge in the OSI model

9 LAN Bridges Isolates collision domains resulting in higher total maximum throughput, and does not limit the number of nodes nor geographical coverage Can connect different type Ethernet since it is a store and forward device Transparent: no need for any change to hosts LAN adapters. Hosts do not communicate with bridges

10 Bridged LAN Configuration

11 Bridged LAN with Multiple Segments

12 Bridge Modes of Operation Filtering: Bridges filter frames if source and destination hosts are on the same segment! Other segments will not get such frames Forwarding: Bridges forward frames if source and destination hosts are on different segments and the bridge knows on which segment is the destination host connected to Flooding: Bridges flood frames to all interfaces (except the one it received the frame from) if it doesn’t know where the destination host is

13 Learning Bridges Bridges learn which hosts can be reached through which interfaces by maintain filtering tables –When a frame received, bridge “learns” location of sender: incoming LAN segment –Records sender location in filtering table Filtering table entries –Host MAC address, Bridge interface, Time stamp –Stale entries in filtering table dropped.

14 Example of Learning Bridges Suppose C sends frame to D and D replies back with frame to C C sends frame, bridge has no info about D, so floods to both interfaces 2 & 3 –Bridge learns C is on port 1, add it to its table

15 Example (Continued) D generates reply to C and sends it –Bridge sees frame from D –Bridge learns D is on interface 2, add to table –Bridge knows C on interface 1, so it forwards frame out via interface 1 and filter it from interface 3

16 Closed Loops Host A sends a frame to Host B Bridge 1 receives the frame Not knowing where host B is it forwards it to segment 2 Frame goes to its destination B, but at the same time is picked up by Bridge 2 Bridge 2 – Erroneously sees a frame on Segment 2 from Host A so he updates his tables to include Host A in Segment 2 Because it does not know about Host B it forwards the frame to Segment 1 The frame is then received by Bridge 1 again – and the cycle will repeat itself endlessly Solution: Spanning Tree Algorithm ( insure that there would be one and only one path between any two hosts) Bridge 1Bridge 2 Host A Host B Segment 1 Segment 2

17 Ethernet “Layer 2” Switching layer 2 (frame) forwarding/ filtering/flooding based on MAC addresses Switching: A-to-B and A’- to-B’ simultaneously, no collisions Ethernet but no collisions Store & Forward v.s Cut- through Switching

18 Example of Ethernet Switching

19 Routers Routers are network-layer devices Routers implement routing algorithms and maintain routing tables

20 Example of an Internetwork Routers are used to interconnect arbitrary topologies