Explain the routing principles and
its algorithms.
The main functions of the network layer is routing packets from
the source machine to the destination machine. The routing algorithm is that
part of the network layer software responsible for deciding which output line
an incoming packet should be transmitted … on.
Routing
principle : making the decision which routes to use. Also it is responsible for
filling in and updating the routing tables.
Properties
of routing algorithm:
§ correctness
§ simplicity
§ robustness
§ stability
§ fairness
§ optimality
Routing
algorithms can be grouped into two major classes: Non-adaptive (static) and
adaptive(dynamic)
Non-adaptive:
do not base their routing decisions on measurements or estimates of the current
traffic and topology. Instead, the choice of the route is computed in
advance,offline
§ a network administrator
configures information about remote networks manually (add/delete static
routes)
§ manual maintenance of
routing table require lot of administrative time
§ not scalable
§ default administrative
distance=1
§ sometimes used for backup
purpose
Adaptive
algorithm:
§ uses a route that a
routing protocol adjusts automatically for topology or traffic changes
§ avoids the time-consuming
and exacting process of configuring static routes.
§ two types: Distance
vector and link state
Distance
Vector:
§ passes periodic copies of
a routing table from router to router. Each router receives a routing table
from its directly connected neighbor routers.
§ also called Bellman-ford
algorithm
§ Eg: Routing Information
protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced IGRP
(EIGRP), Border Gateway Protocol (BGP)
Link
state routing:
§ also called Dijkastra’s
algorithm or the shortest path first (SPF) algorithm
§ maintains a complex
database of topology information
§ maintains full
knowledge of distant routers and how they interconnect
§ eg: open shortest path
first (OSPF)
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