Understanding Routing. Agenda What Is Routing? Network Addressing Routing Protocols.

Understanding Routing

Agenda

What Is Routing?

Network Addressing

Routing Protocols

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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 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?

Multiprotocol Routing

IP 15.17.132.6

IP 15.16.42.8

IP 15.16.50.3

Routing Tables

IP

TokenRing

TokenRing

AppleTalk 200.167

AppleTalk 100.110

Apple

IPX 4b.0800.0121.ab13

IPX 3a.0800.5678.12ab

Novell

DECnet 5.8

DECnet 10.1

DEC

VAX

VAX

Routing Tables

Routing algorithms Initialize and maintain

routing tables to help with path determination

Route information types Destination/next-hop associations Path desirability Vary depending on routing algorithm

Message = Routing table maintenance communications Routing update messages Link-state advertisement

To Reach SendNetwork: To:

27 Node A

57 Node B

17 Node C

24 Node B

11 Node B

72 Node A

To Reach SendNetwork: To:

27 Node A

57 Node B

17 Node C

24 Node B

11 Node B

72 Node A

Routing 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 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

Network Addressing

Network and Node Addresses

Network Node

1

2

123

1

3 1

1.1

2.1

3.1

1.2

1.3

• Network address—Path part used by the router

• Node address—Specific port or device on the network

1

2

3

Addressing Examples

Network Node/HostProtocol Address Address

General 1. 4

TCP/IP 10. 8.2.48

Novell IPX 1aceb0b 0000.0c00.6e25

AppleTalk 10. 1.

X.25 DNIC NTN

NTN: National Terminal Number

131.108.0.0INTERNET

131.108.1.0

131.108.2.0131.108.3.0

131.108.5.0

131.108.4.0

131.108.8.0131.108.6.0

131.108.7.0

131.108.10.0

131.108.9.0

Subnetwork Addressing

Manufacturing

R&D

Subdividing address space into smaller blocks

Helps organize network Security (keeps HR separately

addressable) Scalability—Keeps traffic to

appropriate segments Allows single, summarized

routing entry (131.108.0.0) to be advertised to external networks

Specific route entries (131.108.8.0) required only for routers in the subnetted block

HR

Routing Algorithm Types

Single-path versus multi-path

Flat versus hierarchical

Host-intelligent versus router-intelligent

Intradomain versus interdomain

Static versus dynamic routing

Link state versus distance vector

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

Distance Vector versus Link State

Distance vector Sends routing table info only to neighbors, so change

communication may need one min/router Also called “routing by rumor” Easy to configure, but slow

Link state Floods routing information about itself to all nodes, so changes

are known immediately Efficient, but complex to configure

Cisco’s EIGRP hybrid Efficient and easy to configure

Routing 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 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

RIP and IGRP

RIP

Industry standard that selects the path with the

fewest hops

19.2 k

64k64k 64k

IGRP

Cisco protocol that selects the fastest path (using

load, distance, etc.)

19.2 k

64k64k 64k

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