Transcript Semester 1 Chapter 11 - Institute of Technology Sligo

Institute of Technology Sligo - Dept of Computing
Chapter 11
Layer 3 Protocols
Paul Flynn
Institute of Technology Sligo - Dept of Computing
Layer 2 Devices –
Bridges and Switches
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uses physical, or MAC addresses
(assigned by NIC card manufacturer), to
make data forwarding decisions
primarily used to connect segments of a
network.
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Layer 3 Devices - Routers
• passes data packets between networks, based on Layer 3
addresses (IP, protocol addresses, logical addresses or
network addresses).
• has the ability to make intelligent decisions regarding the
best path for delivery of data on the network
• Assigned by the Network Administrator
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Routers and Data Relaying
Interface
•Routers connect two or more networks, each of which must
have a unique network number in order for routing to be
successful.
•Each interface must have a separate, unique network (or
subnetwork) address that is part of the network attached to
that interface.
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Addressing Schemes
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Static
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Network administrator must go to each individual device and
configure it with an IP address. This method requires you to keep
very meticulous records, because problems can occur on the
network if you use duplicate IP addresses
Dynamic
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Reverse Address Resolution Protocol (RARP)
binds MAC addresses to IP addresses (Diskless workstations)
BOOTstrap Protocol (BOOTP)
used by a device when it starts up, to obtain an IP address.
Dynamic Host Configuration Protocol (DHCP)
has been proposed as a successor to BOOTP. Unlike BOOTP,
DHCP allows a host to obtain an IP address quickly and
dynamically.
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Address Resolution Protocol (ARP)
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In order for devices to communicate, the sending devices need
both the IP addresses and the MAC addresses of the
destination devices.
When they try to communicate with devices whose IP addresses
they know, they must determine the MAC addresses. ARP
enables a computer to find the MAC address of the computer
that is associated with an IP address.
Some keep tables that contain all the MAC addresses and IP
addresses of other devices that are connected to the same LAN.
They are called Address Resolution Protocol (ARP) tables, and
they map IP addresses to the corresponding MAC addresses
If it lacks one or the other, the data will not pass from Layer 3 to
the upper layers. In this way, MAC addresses and IP addresses
act as checks and balances for each other.
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Default Gateway
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IP address of the interface on the router that
connects to the network segment on which
the source host is located.
The default gateway’s IP address must be in
the same network segment as the source
host.
Without a default gateway, the source
computer has no way to look up destination
MAC addresses on other segments of the
network, and the message is undeliverable;
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Proxy ARP
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variation of the ARP protocol
Basically, a router acts as the default
gateway, allowing a device on one
subnet to obtain the MAC address of a
host on another subnet.
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Routed vs. Routing Protocols
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Routed Protocols
Characteristics
provide support for the network layer
Have the ability to assign a network
number as well as the host number
IP, IPX/SPX, AppleTalk
IP relies on the Transport layer
protocol to determine whether packets
have been lost, and to request
retransmission
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Routing Protocols
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Determines the paths that routed protocols (IP, etc)
follow to their destinations.
Used to exchange routing tables and share routing
information by enabling routers that are connected
to create a map, internally, of other routers in the
network or on the Internet
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Routing Information Protocol (RIP)
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Interior Gateway Routing Protocol (IGRP)
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Developed by CISCO
Enhanced Interior Gateway Routing Protocol (EIGRP)
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(Most Common used between routers on the same network)
Developed by CISCO
Open Shortest Path First (OSPF).
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Routing cont’d
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In addition to IP addresses and MAC
addresses of devices located on networks to
which it connects, a router also possesses IP
addresses and MAC addresses of other
routers.
It uses these addresses to direct data toward
its final destination.
If a router receives a packet whose
destination address is not in its routing table,
it forwards it to the address of another router
that most likely does contain information
about the destination host in its routing table.
This DOES NOT change the hosts
destination IP address in the packet.
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Routing Metrics
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Bandwidth (Data capacity of a link)
Delay (length of time required to move a packet from source to
destination)
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Load (Amount of activity on a network resource)
Reliability (refers to error rate of each network link)
Hop Count (No. of passages of a packet)
Ticks (delay on a data link – approx. 35 milliseconds)
Cost (or path cost - arbitrary value assigned by an NA)
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RIP
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Distance vector routing protocol
Only metric is number of hops
Max. hops is 15
Updates every 30 seconds
Doesn’t always select fastest path
Generates lots of network traffic with constant
updates
Most commonly used routing protocol for
routers on same network
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Interior Gateway Protocols (IGP)
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Route data in an autonomous system.
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RIP (See previous slide)
IGRP
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EIGRP
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a distance-vector protocol; however, when determining the best
path, it also takes into consideration such things as bandwidth,
load, delay, and reliability. Network administrators can determine
the importance given to any one of these metrics, or, allow IGRP to
automatically calculate the optimal path.
provides superior operating efficiency and combines the
advantages of link-state protocols with those of distance-vector
protocols
OSPF
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means "open shortest path first". A better description, however,
might be "determination of optimum path", because this Interior
Gateway Protocol actually uses several criteria to determine the
best route to a destination. These criteria include cost metrics,
which factor in such things as route speed, traffic, reliability, and
security
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Exterior Gateway Protocols
(EGP)
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Route data between autonomous systems.
An example of an EGP is BGP (Border
Gateway Protocol), the primary exterior
routing protocol of the Internet.
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Static Routing
•Manual entries in the routing table
•Requires less overhead than dynamic routing
Uses:
•For hiding parts of an internetwork
•To test a particular link in a network
•It is the preferred method for maintaining routing tables whenever there is
only one path to a destination network (stub network)
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Dynamic Routing
•routers send periodic routing update messages to each other
•Relies on routing protocol to share knowledge
•Allows updates to occur for changing network conditions
•Routing protocol defines the set of rules used by router when it
communicates with neighboring routers.
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Other Network Services
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Connectionless Network Services (Packet switched)
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In a connectionless system the destination is not contacted before a packet
is sent (Postal Service analogy)
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Connection-Oriented
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(Circuit switched)
a connection is established between the sender and the recipient before any data
is transferred (Phone system analogy)