Interconnecting LAN segments

1. Interconnecting LAN segments • Repeaters • Hubs • Bridges • Switches

2. Repeater LAN segment 2 LAN segment 1 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

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

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. collision domain collision domain bridge = hub = host LAN segment LAN segment Interconnecting with bridges • 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 separatecollision domains • Transparent: hosts are unaware of presence of bridges • Plug-and-play, self-learning: bridges do not need to be configured

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

9. 100BaseT Bridges: Forwarding • 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. 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. 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. 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. Bridges: Filtering/Forwarding When bridge receives a frame: index bridge table using destinationMAC address • if entry found for destinationthen { if dest on segment from which frame arrivedthen drop the frame else forward the frame on interface indicated } else flood:forward on all but the interface on which the frame arrived • Ifdestination 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. Disabled 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

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. 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. 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. 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 Summary