lec04b - Hubs physical-layer ( dumb ) repeaters: ! bits...

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Data Link Layer 5-1 Hubs … physical-layer ( l dumb z ) repeaters: ! bits coming in one link go out all other links at same rate ! all nodes connected to hub can collide with one another ! no frame buffering ! no CSMA/CD at hub: host NICs detect collisions twisted pair hub
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Data Link Layer 5-2 Switch ! link-layer device: smarter than hubs, take active role " store, forward Ethernet frames " examine incoming frame ` s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment ! transparent " hosts are unaware of presence of switches ! plug-and-play, self-learning " switches do not need to be configured
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Data Link Layer 5-3 Switch: allows multiple simultaneous transmissions ! hosts have dedicated, direct connection to switch ! switches buffer packets ! Ethernet protocol used on each incoming link, but no collisions; full duplex " each link is its own collision domain ! switching: A-to-A ` and B- to-B ` simultaneously, without collisions " not possible with dumb hub A A ` B B ` C C ` switch with six interfaces ( 1,2,3,4,5,6 ) 1 2 3 4 5 6
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Data Link Layer 5-4 Switch Table ! Q: how does switch know that A ` reachable via interface 4, B ` reachable via interface 5? ! A: each switch has a switch table, each entry: " (MAC address of host, interface to reach host, time stamp) ! looks like a routing table! ! how are entries created, maintained in switch table? " something like a routing protocol? A A ` B B ` C C ` switch with six interfaces ( 1,2,3,4,5,6 ) 1 2 3 4 5 6
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Data Link Layer 5-5 Switch: self-learning ! switch learns which hosts can be reached through which interfaces " when frame received, switch l learns z location of sender: incoming LAN segment " records sender/location pair in switch table A A ` B B ` C C ` 1 2 3 4 5 6 A A ` Source: A Dest: A ` MAC addr interface TTL Switch table (initially empty) A 1 60
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Data Link Layer 5-6 Switch: frame filtering/forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. 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
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Data Link Layer 5-7 Self-learning, forwarding: example A A ` B B ` C C ` 1 2 4 5 A A ` Source: A Dest: A ` MAC addr interface TTL Switch table (initially empty) A 1 60 A A ` ! frame destination unknown: flood A ` A ! destination A location known: A ` 4 selective send
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Data Link Layer 5-8 Interconnecting switches ! switches can be connected together A B ! Q: sending from A to G - how does S 1 know to forward frame destined to F via S 4 and S 3 ? ! A: self learning! (works exactly the same as in single-switch case!) S 1 C D E F S 2 S 4 S 3 H I G
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Data Link Layer 5-9 Self-learning multi-switch example Suppose C sends frame to I, I responds to C !
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lec04b - Hubs physical-layer ( dumb ) repeaters: ! bits...

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