Interconnecting LAN segments Repeaters Hubs Bridges Switches 1
Interconnecting LAN segments • • Repeaters Hubs Bridges Switches 1
Interconnecting with repeaters Repeater LAN segment 1 LAN segment 2 • 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 2
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 3
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 4
Advantages of hubs • Easy to Understand • Easy to Implement • …so they’re cheap 5
Limitation of hubs • Can’t interconnect 10 Base. T & 100 Base. T • 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 6
Interconnecting with bridges collision domain LAN segment bridge 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 7
Backbone Bridge 100 Base. T • Recommended configuration • Notice that a bridge can connect a 10 Base. T LAN with a 100 Base. T LAN, while a hub can not! 8
Bridges: Forwarding 100 Base. T • 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 9
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 10
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 11
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 12
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, that is, the packet is being broadcast to all hosts, then – forward the frame on all but the interface on which the frame arrived 13
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 14
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 15
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, 10/1000 Mbps interfaces • LAN, e. g. , Ethernet, but no collisions! 16
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 17
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 18
- Slides: 18