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
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
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.