Day 3.1 basic routing

1-1CSE-Network Management—Introduction © 1999, Cisco Systems, Inc. www.cisco.com

Understanding Routing

7-2CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com

Agenda

• What Is Routing?

• Routing Protocols

1-3CSE-Network Management—Introduction © 1999, Cisco Systems, Inc. www.cisco.comwww.cisco.com© 1999, Cisco Systems, Inc.

What is Routing?What is Routing?


What Is Routing?What Is Routing?

• Routing is: – Finding a path between a source and

destination (path determination)

– Moving information across an internetwork from a source to a destination (switching*)

– Very complex in large networks because of the many potential intermediate nodes

• A router is: – A network layer device that forwards packets

from one network to another and determines the optimal path for forwarding network traffic

* The term “switching”, when used to describe a router’s function, is different from a switch (the network device).


Routing TableRouting Table192.168.3.0192.168.3.0 Frame RelayFrame Relay192.168.1.0192.168.1.0 EthernetEthernet192.168.2.0192.168.2.0 FDDIFDDI

Network 192.168.2.0FDDI

Network 192.168.3.0Frame Relay

Remote Location

Network 192.168.1.0Ethernet

Main Site

Routers—Layer 3Routers—Layer 3


• LAN-to-LAN connectivity

• LAN-to-WAN connectivity

• Remote access

Where are Routers Used?


LAN-to-LAN Connectivity

• Routers encapsulate and de-encapsulate data packets as they are transferred from system X to system Y

X Y

AA

BB

CC

Presentation

Data LinkPhysical

Data LinkPhysical

A B C

Data LinkPhysical

Data LinkNetworkTransportSessionPresentationApplication

PhysicalData LinkNetworkTransportSession

Application

Physical

Network Network Network


Path Determination

1

2

3

4

5

6

7

8 9

10 11

• Routers find the best path through the network

– Routing tables contain route information

– Network addresses represent the path of media connections to a destination

Which Path?Which Path?


Routing Algorithm GoalsRouting Algorithm Goals

• Optimality– Selecting the best route based on metrics and

metric weightings used in the calculation

• Simplicity and low overhead– Efficient routing algorithm functionality with a

minimum of software and utilization overhead

• Robustness and stability– Correct performance in the face of unusual

or unforeseen circumstances (e.g., high load)

• Rapid convergence– Fast agreement, by all routers, on optimal routes

• Flexibility– Quick and accurate adaptation to changes in

router availability, bandwidth, queue size, etc.


Routing MetricsRouting Metrics

• Path length – Total hop count or sum of cost per network link

• Reliability – Dependability (bit error rate) of each network link

• Delay – Useful because it depends on bandwidth, queues, network congestion, and physical

distance

• Communication cost

– Operating expenses of links (private versus public)

• Bandwidth and load


Types of Routing.Types of Routing.

Static routing

Default routing

Dynamic routing.


Static Routing

“Stub” Network

A

B

• Manual table updates by a network administrator

• Benefits– Reflects administrator’s special

topology knowledge

– Private—Not conveyed to other routers in updates

– Avoids the overhead of dynamic routing

• Stub network– When a node is accessible by

only one path, a static route is sufficient

– Point-to-point or circuit-switched connection


Dynamic Routing

A network change blocks the established path...

A B

CD

XA B

CD

X

…and an alternate route is found dynamically.

• Most internetworks use dynamic routing

1-14CSE-Network Management—Introduction © 1999, Cisco Systems, Inc. www.cisco.comwww.cisco.com© 1999, Cisco Systems, Inc.

Routing ProtocolsRouting Protocols


Routed versus Routing Protocols

• Routed protocols used between routers to direct user traffic; also called network protocols– Examples: IP, IPX,

DECnet, AppleTalk, NetWare, OSI, VINES

1.02.03.0

1.12.13.1

DestinationNetwork

NetworkProtocol

Protocol name

Exit Port to Use

• Routing protocols used between routers to maintain routing tables– Examples: RIP, IGRP,

OSPF, BGP, EIGRP


Routing Protocol EvolutionsRouting Protocol Evolutions

Distance Vector

Link State

RIP• Distance vector• Most common IGP• Uses hop count

RIP• Distance vector• Most common IGP• Uses hop count

IGRP• Distance vector• Developed by Cisco • Addresses problems in large, heterogeneous networks

IGRP• Distance vector• Developed by Cisco • Addresses problems in large, heterogeneous networks

OSPF• Link state, hierarchical • Successor to RIP • Uses least-cost routing,

multipath routing, and load balancing

• Derived from IS-IS

OSPF• Link state, hierarchical • Successor to RIP • Uses least-cost routing,

multipath routing, and load balancing

• Derived from IS-IS

EIGRP• Hybrid protocol• Developed by Cisco• Superior convergence

and operating efficiency• Merges benefits of link

state & distance vector

EIGRP• Hybrid protocol• Developed by Cisco• Superior convergence

and operating efficiency• Merges benefits of link

state & distance vector

Hybrid


OSPF and EIGRP

Aspect OSPF EIGRPTopology Hierarchical Not restricted

Memory & CPUrequirements High Moderate

Routing table size Large Moderate

Controlling body Industry standard Cisco proprietary

Convergence Fast Fast

Supported protocols IP

IPIPX

AppleTalk

Configuration Difficult Easy


SummarySummary

• Routers move data across networks from a source to a destination

• Routers determine the optimal path for forwarding network traffic

• Routing protocols communicate reachability information between routers

Day 3.1 basic routing

Education

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