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Unformatted text preview: t LAN segment LAN segment LAN (IP network) Forwarding How do determine to which LAN segment to forward frame? • Looks like a routing problem... Self learning A bridge has a bridge table bridge entry in bridge table: entry (Node LAN Address, Bridge Interface, Time Stamp) (Node stale entries in table dropped (TTL can be 60 min) stale bridges learn which hosts can be reached through which interfaces bridges learn which when frame received, bridge “learns” location of sender: incoming LAN when segment records sender/location pair in bridge table records Filtering/Forwarding When bridge receives a frame: index bridge table using MAC dest address if entry found for destination entry then{ if dest on segment from which frame arrived dest then drop the frame then drop else forward the frame on interface indicated else forward } else flood else forward on all but the interface on which the frame arrived Bridge example Suppose C sends frame to D and D replies back with frame to C. Bridge receives frame from from C Bridge notes in bridge table that C is on interface 1 notes because D is not in table, bridge sends frame into interfaces 2 and 3 because frame received by D frame Bridge Learning: example D generates frame for C, sends generates bridge receives frame bridge notes in bridge table that D is on interface 2 notes bridge knows C is on interface 1, so selectively forwards frame to interface bridge selectively forwards 1 Bridges Spanning Tree For increased reliability, it is desirable to have redundant, For alternate paths from a source to a destination With multiple simultaneous paths however, cycles result on With which bridges may multiply and forward a frame forever Solution is organizing the set of bridges in a spanning tree by Solution disabling a subset of the interfaces in the bridges: Disabled Bridges Vs. Routers Both are store-and-forward devices, but Routers are Network Both Layer devices (examine network layer headers) and Bridges are Link Layer devices Routers maintain routing tables and implement routing Routers algorithms, bridges maintain filtering tables and implement filtering, learning and spanning tree algorithms Routers Vs. Bridges (Cont d) Bridges + and Bridges + Bridge operation is simpler, requiring less processing bandwidth - Topologies are restr...
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This note was uploaded on 12/08/2010 for the course IEG IEG3310 taught by Professor Wingc.lau during the Spring '10 term at CUHK.

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