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28 Οκτ 2013 (πριν από 4 χρόνια και 5 μήνες)

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Routing Theory


process that a router uses to forward packets toward
the destination network

based upon the destination IP address of a packet

routers must learn the direction to remote networks

dynamic routing

static routing


Static Route Operation

Network administrator configures the route using a

local interface


hop address

as their gateway

Router installs the route in the routing table

Administrative distance defaults to 0 when using local
interface as gateway

Administrative distance defaults to 1 when using next
hop as gateway

Packets are routed using the static route

Local interface as gatewy

Next Hop address as gateway

Administrative Distance

Optional parameter that gives a measure of
the reliability of the route

Lower value

more reliable route

A value between 0 and 255 is entered after
next hop | outgoing interface if value other
than default is desired

Static routes

Requires network administrator to add and
delete static routes to account for topology

In large network this requires tremendous
amount of administrative time

Often configured in conjunction with
dynamic routing protocol

Default Route

a special static route

used to route packets with destinations that do not
match any of the other routes in the routing table

default route for Internet
bound traffic

ip route [next
address |
outgoing interface]

Default Route

Verifying Static Route

show running
config is used to view the active
configuration in RAM

show ip route command is used to make sure that
the static route is present in the routing table

Verifying Static Route with show

Verifying Static Route with show
ip route

Routing Protocols

A routing protocol is the communication used
between routers

Routing Information Protocol (RIP)

Interior Gateway Routing Protocol (IGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Open Shortest Path First (OSPF)

A routed protocol is used to direct user traffic

Internet Protocol (IP)

Internetwork Packet Exchange (IPX)

Autonomous systems

collection of networks under a common
administration sharing a common routing strategy

Routing protocols, such as Cisco’s IGRP, require
assignment of a unique, autonomous system

Purpose of a routing protocol and
autonomous systems

goal of a routing protocol is to build and maintain
the routing table

it learns all available routes, places the best routes
into the routing table, and removes routes when
they are no longer valid

Whenever the topology of a network changes
because of growth, reconfiguration, or failure, the
network knowledgebase must also change

Autonomous systems (AS) provide the division of
the global internetwork into smaller and more
manageable networks

classes of routing protocols

routing algorithms can be classified into one of two

distance vector

determines the direction (vector) and distance to any link in the


recreates the exact topology of the entire internetwork