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OSI Reference Model

Introduction to TCP/IP

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Understanding Switching

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Understanding Routing

What Is Layer 3 Switching?

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The Internet




OSI Reference Model

Static Versus Dynamic Routes

Routers must be aware of what links, or lines, on the network are up and running, which ones are overloaded, or which ones may even be down and unusable. There are two primary methods routers use to determine the best path to a destination: static and dynamic Static knowledge is administered manually: a network administrator enters it into the router’s configuration. The administrator must manually update this static route entry whenever an internetwork topology change requires an update. Static knowledge is private–it is not conveyed to other routers as part of an update process.

Dynamic knowledge works differently. After the network administrator enters configuration commands to start dynamic routing, route knowledge is updated automatically by a routing process whenever new topology information is received from the internetwork. Changes in dynamic knowledge are exchanged between routers as part of the update process.

Static Route : Uses a protocol route that a network administrator enters into the router

Dynamic Route : Uses a route that a network protocol adjusts automatically for topology or
traffic changes

Dynamic routing tends to reveal everything known about an internetwork. For security reasons, it might be appropriate to conceal parts of an internetwork. Static routing allows an internetwork administrator to specify what is advertised about restricted partitions.
When an internetwork partition is accessible by only one path, a static route to the partition can be sufficient. This type of partition is called a stub network. Configuring static routing to a stub network avoids the overhead of dynamic routing.

Adapting to Topology Change

The internetwork shown in the graphic adapts differently to topology changes depending on whether it uses statically or dynamically configured knowledge.
Static knowledge allows the routers to properly route a packet from network to network. The router refers to its routing table and follows the static knowledge there to relay the packet to Router D. Router D does the same and relays the packet to Router C. Router C delivers the packet to the destination host.

But what happens if the path between Router A and Router D fails? Obviously Router A will not be able to relay the packet to Router D. Until Router A is reconfigured to relay packets by way of Router B, communication with the destination network is impossible.

Dynamic knowledge offers more automatic flexibility. According to the routing table generated by Router A, a packet can reach its destination over the preferred route through Router D. However, a second path to the destination is available by way of Router B. When Router A recognizes the link to Router D is down, it adjusts its routing table, making the path through Router B the preferred path to the destination. The routers continue sending packets over this link.
When the path between Routers A and D is restored to service, Router A can once again change its routing table to indicate a preference for the counter-clockwise path through Routers D and C to the destination network.

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