1 interconnecting lan segments repeaters hubs bridges switches
Post on 21-Dec-2015
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TRANSCRIPT
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Interconnecting LAN segments
• Repeaters• Hubs• Bridges• Switches
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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
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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
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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
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Advantages of hubs• Easy to Understand• Easy to Implement• …so they’re cheap
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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
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Interconnecting with bridges
bridge collision domain
collision domain
= hub
= host
LAN segment 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
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Backbone Bridge
100BaseT
• Recommended configuration• Notice that a bridge can connect a 10BaseT
LAN with a 100BaseT LAN, while a hub can not!
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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
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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
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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
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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
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Bridges: Filtering/ForwardingWhen 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
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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
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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
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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!
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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
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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