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• Each computer and router interface maintains
an ARP table for Layer 2 communication
• The ARP table is only effective for the
broadcast domain (or LAN) that it is
connected to
• The router also maintains a routing table that
allows it to route data outside of the
broadcast domain.
• Each ARP table contains an IP-MAC address
pair
• The routing tables knows the network IP
address for reachable networks, the hop
count or distance to those networks, and the
interface the data must be sent out to get to
the destination network.
• The routing tables also track how the route
was learned (in this case either directly
connected [C] or learned by RIP [R])
• The Layer 2 switch can only recognize its
own local MAC addresses and cannot handle
Layer 3 IP addresses
• When a host has data for a non-local IP
address, it sends the frame to the closest
router, also known as its default gateway
• The host uses the MAC address of the router
as the destination MAC address
• The router examines the Layer 3
destination address of the packet to
make the forwarding decision
• Just as a Layer 2 switch keeps a table
of known MAC addresses, the router
keeps a table of IP addresses known as
a routing table
• A Layer 2 device can handle a
reasonable number of unorganized
MAC addresses, because it will only
have to search its table for those
addresses within its segment.
• Routers need to handle a greater
volume of addresses.
• Protocols used at the network layer that
transfer data from one host to another
across a router are called routed or
routable protocols
• Routed protocols transport data across
a network. Routing protocols allow
routers to choose the best path for data
from source to destination
• Routers use routing protocols to
exchange routing tables and share
routing information.
• Path determination occurs at the network
layer
• Path determination enables a router to
compare the destination address to the
available routes in its routing table, and to
select the best path
• The routers learn of these available routes
through static routing or dynamic routing
• Routers can make decisions based on the
load, bandwidth, delay, cost, and reliability
of a network link.
• Routers use routing protocols to build and
maintain routing tables that contain route
information
• Routing protocols fill routing tables with a
variety of route information
• Routing tables contain the information
necessary to forward data packets across
connected networks
• Routers communicate with one another to
maintain their routing tables through the
transmission of routing update messages
• Routers keep track of important information
in their routing tables, including the
following:
• Protocol type
• Destination/next-hop associations
• Routing metric
• Outbound interfaces
• The following are the metrics that are most
commonly used by routing protocols:
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Bandwidth
Delay
Load
Reliability
Hop count
Ticks
Cost
• An autonomous system is a network or set
of networks under common administrative
control
• The distance-vector routing approach
determines the distance and direction,
vector, to any link in the internetwork
• Routing Information Protocol (RIP)
• Interior Gateway Routing Protocol (IGRP)
• Enhanced IGRP (EIGRP)
• Link-state routing protocols send periodic updates,
known as link-state refreshes
• Link-state routing protocols respond quickly to
network changes sending trigger updates only
when a network change has occurred
• When a route or link changes, the device that
detected the change creates a link-state
advertisement (LSA) concerning that link
• Open Shortest Path First (OSPF)
• Intermediate System-to-Intermediate System (IS-IS)
• RIP is a distance vector routing protocol
that uses hop count as its metric to
determine the direction and distance to any
link in the internetwork
• RIP selects the path with the least number
of hops
• RIP cannot route a packet beyond 15 hops
• IGRP is a distance-vector routing protocol
developed by Cisco
• IGRP was developed specifically to address
problems associated with routing in large
networks that were beyond the range of protocols
such as RIP
• IGRP can select the fastest available path based on
delay, bandwidth, load, and reliability
• IGRP also has a much higher maximum hop count
limit than RIP