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1 Interconnecting LAN segments Repeaters Hubs Bridges Switches.

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1 1 Interconnecting LAN segments Repeaters Hubs Bridges Switches

2 2 Interconnecting with repeaters Repeaters used to connect multiple LAN segments A repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! Ethernet: Max 4 repeaters per LAN Total 5 LAN segments  5*30 = 150 nodes max. Repeaters have become a legacy technology Repeater LAN segment 1 LAN segment 2

3 3 Interconnecting with hubs Effectively a physical layer device –Multi-port repeater –Operates at bit level –Repeat received bits on one interface to all other interfaces

4 4 Interconnecting with hubs Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top Better than repeaters –Hubs can detect malfunctioning node adapters and disconnect them from the network thereby increasing reliability –Can collect statistics such as collision rate, network usage, average frame size Provide network management functionality

5 5 Advantages of hubs Easy to Understand Easy to Implement …so they’re cheap

6 6 Limitation of hubs Can’t interconnect 10BaseT & 100BaseT Individual segment collision domains become one large collision domain –if a node in CS and a node EE transmit at same time: collision Poor security –Why should host B get to share its link with a conversation between A and D? –“Packet sniffer” on one port can monitor the traffic of all of the ports Can we do better? –Use bridges

7 7 Interconnecting with bridges bridge collision domain collision domain = hub = host LAN segment Link layer device –stores and forwards LL, e.g., Ethernet, frames –examines frame header and selectively forwards frame based on MAC destination address –when frame is to be forwarded on segment, uses CSMA/CD to access segment –segments become separate collision domains Transparent: hosts are unaware of presence of bridges Plug-and-play, self-learning: bridges do not need to be configured

8 8 Backbone Bridge 100BaseT Recommended configuration Notice that a bridge can connect a 10BaseT LAN with a 100BaseT LAN, while a hub can not!

9 9 Bridges: Forwarding 100BaseT How does the bridge determine to which LAN segment to forward a frame to? Notice that this has to be done transparent to the hosts. That is, hosts should not be aware that there is a bridge connecting several LANs together

10 10 Bridges: Self Learning Basic idea: Build cache (called the bridge table) of which nodes are downstream of which ports –entry in bridge table: (Node MAC Address, Bridge Interface, Time Stamp) stale entries in table dropped (TTL can be 60 min) How? Bridge monitors source MAC address on all packets that it forwards –when frame received, bridge “learns” location of sender: incoming LAN segment –records sender/location pair in bridge table What to do with unknown sources? –Flood network, i.e., forward the frame on all interfaces except over the one from which the frame was received

11 11 Bridge Learning: Example 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 LANs –bridge notes that C is on port 1 –frame ignored on upper LAN –frame received by D

12 12 Bridge Learning: Example D generates reply to C, sends –bridge sees frame from D –bridge notes that D is on interface 2 –bridge knows C on interface 1, so selectively forwards frame out via interface 1

13 13 Bridges: Filtering/Forwarding When bridge receives a frame: index bridge table using destination MAC address if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood:forward on all but the interface on which the frame arrived If destination MAC is FF-FF-FF-FF-FF-FF, that is, the packet is being broadcast to all hosts, then –forward the frame on all but the interface on which the frame arrived

14 14 Eliminating Loops in Bridged Networks: Spanning Tree Desirable to have redundant, alternate paths from source to destination for increased reliability, availability with multiple simultaneous paths, cycles result - bridges may multiply and forward frame forever solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

15 15 Interconnecting with Switches Switches –“multi-port bridge” –Each port acts as a bridge –Each port determines MAC addresses connected to itself –Master list within switch determines forwarding behavior

16 16 Switches (more) A-to-B and A’-to-B’ communication simultaneously: no collisions large number of interfaces versus bridges (which typically have only two) Typically star-shaped topology Cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame –slight reduction in latency Combinations of shared/dedicated, 10/100/1000 Mbps interfaces LAN, e.g., Ethernet, but no collisions!

17 17 Switched Network Advantages Higher link bandwidth –Point to point electrically simpler than bus Much greater aggregate bandwidth –Separate segments can send simultaneously –Data backplane of switches typically large to support simultaneous transfers amongst ports Challenge –Learning which packets to copy across links Forwarding table based on destination MAC address –Avoiding forwarding loops Perlman’s Spanning Tree Algorithm

18 18 Summary Covered how to extend LAN segments Repeaters –Physical Layer Devices Hubs –Multi-port repeaters Bridges –Link Layer Devices: Store & forward frames based on the destination MAC address of the frame –Build packet forwarding table on the fly by observing passing packets –Spanning Tree to eliminate loops Switches –Multi-port bridges

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