3_BridgesLANs%26IOS_spring10

3_BridgesLANs%26IOS_spring10 - Chapter 3 Bridges, LANs, and...

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TCP/IP Essentials A Lab-Based Approach Chapter 3 Chapter 3 Bridges, LANs, and Bridges, LANs, and the Cisco IOS the Cisco IOS Spring 2009
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Polytechnic Institute of NYU Slide 2 zjzhao Bridges •Interconnect multiple LANs, possibly of different types •Bridges operate at the Data Link Layer (Layer 2) •Pass frames to a different LAN if the destination is not on the local LAN. LAN 1 Bridge LAN 2 Bridge Token Ring Ethernet Bus
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Polytechnic Institute of NYU Slide 3 zjzhao Why Bridges Bridges allow to build a local area network with multiple small LANs instead of one single LAN, which increases reliability throughput security geography Transparent bridges are not seen by hosts. A frame is simply copied to the destination network Å not flooding to all networks/segments No change in the header and data section. Source Route bridges , another form of bridging, use a field in frame header, ex. the Routing Information Field (RIF) in token ring header, to indicate the series of bridges along the routing path
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Polytechnic Institute of NYU Slide 4 zjzhao Bridge Function – an Example For frames from Host 1 • Those to Host 2 … Host 10 are forwarded on LAN A • Those to Host 11 … Host 20 are forwarded on LAN A, accepted and repeated further on LAN B ... ... LAN A LAN B Host1 Host2 Host10 Host11 Host12 Host20 Bridge
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Polytechnic Institute of NYU Slide 5 zjzhao Filtering Database of a Bridge •MAC addresses of the hosts are stored in a Filtering Database in the bridge. •Elements of each entry of the filtering database – The destination MAC address – The bridge port where frames for this destination MAC address should be forwarded to – The age of this entry •The filtering database could be set statically. •In an IEEE 802.1d bridge, the filtering database is maintained automatically by an address learning process.
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Polytechnic Institute of NYU Slide 6 zjzhao Address Learning Process • When a frame is received, its source MAC address and the incoming port are updated in the bridge’s filtering database . • The default age of a new entry is 300 sec.
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Polytechnic Institute of NYU Slide 7 zjzhao Address Learning – an Example •Consider the following three packets: <Src=A, Dest=F>, <Src=C, Dest=A>, <Src=E, Dest=C> •What have the bridges learned? Bridge 1 Port1 LAN 1 A LAN 2 C B D LAN 3 E F Port2 Bridge 2 Port1 Port2 2 E 2 C 1 A Age Port Dest. 2 E 1 C 1 A Age Port Dest.
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Slide 8 zjzhao Bridge Operations • A bridge makes forwarding decisions by filtering database lookups. –If an entry is found, the bridge forwards the frame to the network segment indicated by the entry. –Otherwise,
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3_BridgesLANs%26IOS_spring10 - Chapter 3 Bridges, LANs, and...

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