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Bridges, Routers and Switches Part One

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  1. Bridges, Routers and SwitchesPart One Three things are certain: Death, taxes, and lost data. Guess which has occurred. Haiku error message

  2. Bridging • Bridges connect two independent LANs to form internetworks

  3. Types of Bridges • Transparent • Source Routing • Transnational

  4. Bridges • Each LAN is referred to as a Network Segment

  5. Bridge Ports • Bridge Ports connect the Bridge to the Network • Consist of a Network Interface (Ethernet, Token Ring, FDDI)

  6. Bridges Form One Logical Network

  7. Bridging and ISO Model • Bridges function at at the Media Access (MAC) layer

  8. Bridging and ISO Model • Bridges “see” the network at the MAC layer • Bridges make forwarding decisions based on MAC (Physical) addresses

  9. Bridging and ISO Model • Bridges have no knowledge of paths between address • Bridges are transparent to higher level protocol • Bridges are protocol independent, the same bridge can pass IP, IPX, DecNet traffic

  10. Transparent Bridges • Usually used on Ethernet networks • Called Transparent because nodes are not aware of its presence • Defined by IEEE 802.1 standard • Forwards frames between networks • Self learning

  11. Transparent Bridges • Maintains a data base of all known node address on each port

  12. Transparent Bridges • Transparent Bridges are self learning • Examine every frame which passes a port • Determines the source address • Compares each address to its data base • If address is not in data base then the bridge adds the address

  13. Transparent Bridges

  14. Transparent Bridge Frame Forwarding • Examines Destination address • If address is on the same LAN as the incoming port, ignores the frame • If Not • Looks up destination address in its Data Base • If Address is found • Forward out the appropriate Port • If Address is not found • Forwards out all Ports

  15. Spanning Tree Algorithm

  16. Spanning Tree Algorithm

  17. Spanning Tree Algorithm

  18. Spanning Tree Algorithm

  19. Spanning Tree Algorithm • Eliminates all active loops in the network by creating a set of paths with only one path between each node • Bridges negotiate which paths are to remain open and which paths will be blocked • If a path becomes inaccessible, the bridges will reconfigure the network, creating a new spanning tree and new set of paths

  20. Source Route Bridging • Only used in Token Ring environments

  21. Source Route Bridging • NOTE: Despite its name Source Routing is Bridging and takes place at the MAC layer

  22. Source Route Bridging • Node maintain their own routing table • When a frame is transmitted to a node on another ring, the packet must contain the route or path • Node “discovers” the route by sending out an Explorer Frame with the address of the destination Node • Depending on the protocols used the node may send an All Route Exploder Frame or a Singe Route Explorer Frame

  23. Source Route Bridging • The route explorer frame records the Bridge Number and Ring Number of all bridges and networks crossed • When it reaches the destination node, the destination node sets a flag and transmits the frame back to the sending node • The Route Explorer Frame now has the route to the destination station • If two Routes return to the sending node, it usually uses the first route received

  24. Source Route Bridging

  25. Source Route Bridging • Source Route Bridges can connect rings running at different speeds

  26. What’s Wrong with Source Routing? • Takes processing resources of end nodes • Each Ring and Bridge must be assigned a Ring Number and Bridge Number - Any error in assigning these numbers causes fatal errors • Fatal errors happen often

  27. Source Route Bridging

  28. Source Route Bridging • Allows loops • How many paths though this network?

  29. Source Route Bridging • What else is wrong with Source Routing? • What is good about Source Routing?

  30. Source Route and Transparent Bridges in Same Network • Usually not very reliable - may work in some simple networks • Source Routed traffic can only cross Source Route Bridges

  31. Transnational Bridges • Connect two dissimilar topologies

  32. Advantages of Bridging • Easy to install and configure • Inexpensive • Transparent to Network • Automatically adapt to Network Changes • Bridge unroutable protocols

  33. Disadvantages of Bridges • Can not take advantage of redundant paths in the network • Can not prevent Broadcast Storms • Because bridges form a single logical network the network becomes large and therefore difficult to manage

  34. Routers What makes a Protocol Routable • To be routable a protocol must assign addresses so that the address contain Network and Node address • Routable Protocols include • IP • IPX • DecNet • Vines • Non-Routable Protocols • NetBios

  35. Routers • Function of a Router is to efficiently forward packets between networks • Very often more than one path exists, a router must chose the “best” path • Routers function at the Network layer of the OSI Model, routing decisions are made using the address assigned by a Network layer protocol such as IP

  36. Routers • Since routers function at the Network layer routers are protocol dependent devices • Must support IP, IPX, AppleTalk, DecNet, etc.

  37. Routers • Multiple Protocols • Most routers can route a verity of protocols • The ability to route multiple protocols is implemented in software

  38. Routers

  39. Routers • Routers can connect dissimilar LAN topologies • Routers can connect LANs to the WAN

  40. Routers - Why use Routers • Segment Networks • Congestion Control • Source Quench technique - Router can request a source to stop transmitting until congestion is relieved

  41. Routers • Bridges examine each frame on the network segment • Routers only receive packets addressed to them • Packets are addressed to routers from workstations or other routers

  42. Routers Node Configuration

  43. Routers Routing Tables • Routers maintain a Routing Table • The Routing Table instructs the router where (out which interface) to send the packet • Packets are routed one hop at a time

  44. Routers Routing Table

  45. Routers - Multiple Routes

  46. Routers Multiple Routes Routing Table Router 1 Target Net Next Hop Router Metric Net4 Attached 0 Net 1 Router 2 1 Net 1 Router 3 2 Net 3 Router 3 1 Net 3 Router 2 2 Net 2 Router 3 1 Net2 Router 2 2

  47. Routers Metric Values • Chose route with lowest Metric value • Metric value can represent: • Hops • Transmission Delay • Administrative Considerations (Cost) • Line Capacity

  48. Routers - Backup Links

  49. Routers - Routing Protocol • Routing Tables are dynamically created by the router • Router communicate with other routers to learn all paths through the network • This communication consists of all or portions of each routers routing table

  50. Routers - Routing Protocol • Routers use these Routing Tables to build a map of the network topology • The time it takes for all routers in a network to exchange routing tables is called Convergence • Router to Router communication is implemented according to Routing Protocols