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Copyright © 1998, Cisco Systems, Inc. All rights reserved. Printed in USA.Presentation_ID.scr 1
13010947_05F9_c2 © 1999, Cisco Systems, Inc.
2© 1999, Cisco Systems, Inc. 3010947_05F9_c2
Introduction to RoutingIntroduction to Routing
Session 301Session 301
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33010947_05F9_c2 © 1999, Cisco Systems, Inc.
AgendaAgenda
•• AddressingAddressing
• Concepts
• Routing Protocols
• Statics and Defaults
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Routing Information Routing Information Protocol (RIP and RIPv2)Protocol (RIP and RIPv2)
Interior Gateway RoutingInterior Gateway RoutingProtocol (IGRP)Protocol (IGRP)
Open Shortest Path First Open Shortest Path First (OSPF) Protocol(OSPF) Protocol
NetWare Link ServicesNetWare Link ServicesProtocol (NLSP)Protocol (NLSP)
Intermediate System to Intermediate System to Intermediate System (IS-IS)Intermediate System (IS-IS)
Enhanced IGRP (EIGRP)Enhanced IGRP (EIGRP)
Border Gateway Protocol (BGP)Border Gateway Protocol (BGP)
Physical LayerPhysical Layer
Network LayerNetwork Layer
TransportTransport
SessionSession
PresentationPresentation
ApplicationApplication
Data Link ControlData Link Control
L1L2L3L4L5L6L7
ISO—OSI Reference ModelISO—OSI Reference Model
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Router FunctionsRouter Functions
• Routing = buildingmaps and givingdirections
• Switching =moving packetsbetween interfaces
• Routers are packetswitches
• Path determinationis overhead
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UNIX HostUNIX HostUNIX HostUNIX Host
Company A Company B
InternetTCP/IP
Introduction to IP AddressesIntroduction to IP Addresses
• Unique addressing allows communicationbetween end stations
• Path choice is based on location
• Location is represented by an address
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32 Bits
NetworkNetwork HostHost
172 . 16 . 122 . 204
IP AddressingIP Addressing
8 Bits 8 Bits 8 Bits 8 Bits
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• Class A:
• Class B:
• Class C:
• Class D: for multicast
N = Network number assigned by NICH = Host number assigned by network administrator
NN HH HH HH
IP Address ClassesIP Address Classes
NN NN HH HH
NN NN NN HH
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172.16.200.11
172.16.3.10
172.16.12.12
IP: 172.16.2.1
10.1.1.1
10.250.8.11
10.180.30.118
IP: 10.6.24.2
E0 E1
172 .16 12 12
Network Host
. . NetworkNetwork InterfaceInterface172.16.0.0172.16.0.0
10.0.0.010.0.0.0
E0E0
E1E1
Routing TableRouting Table
Host AddressesHost Addresses
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172.16.2.11
172.16.2.2
172.16.2.160
IP: 172.16.2.1
172.16.3.5
172.16.3.100
172.16.3.150
IP: 172.16.3.1
NetworkNetwork InterfaceInterface172.16.2.0172.16.2.0
172.16.3.0172.16.3.0
E0E0
E1E1
Routing TableRouting Table
Subnet AddressingSubnet Addressing
Host.160172 .16
Network Subnet.2
E0 E1
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IPAddress
Subnet MaskSubnet Mask
172172 1616 00 00
DefaultSubnet
Mask
Network Host
255255 255255 00 00
Use Host Bits, Starting at the High Order Bit Position
8-bitSubnet
Mask
Network Subnet Host
255255 255255 255255 00
Network Host
123010947_05F9_c2 © 1999, Cisco Systems, Inc.
.6
.13
.9
.10.14
.5
A
C
B172.16172.16.40.1
255.255.255.0
172172.16.50.1255.255.255.0
172.16172.16.60.1255.255.255.0
192.168.1.8255.255.255.252
192.168.1.4255.255.255.252
192.168.1.12255.255.255.252
Where Is 172.16.0.0?
Discontiguous IP SubnetDiscontiguous IP Subnet
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.6
.13
.9
.10.14
.5
A
C
B172.16.40.1
255.255.255.0
172.16.50.1255.255.255.0
172.16.60.1255.255.255.0
172.16.1.8255.255.255.252
172.16.1.4255.255.255.252
172.16.1.12255.255.255.252
Variable Length Subnet MaskVariable Length Subnet Mask
• Conserve IP addresses
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80 Bits
NetworkNetwork NodeNode
32 Bits 48 Bits
000C 15C0 0077.0650.2328
IPX AddressingIPX Addressing
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• Assigns an address and subnet mask
• Starts IP processing on an interface
• Assigns a network number
• Starts IPX processing on an interface
• Must have ipx routing configured
Address ConfigurationAddress Configuration
Router (config-if) #
ip address ip-address subnet-maskip address ip-address subnet-mask
ipx network network ipx network network
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131.108.48.0 /24131.108.48.0 /24
131.108.49.0 /24131.108.49.0 /24
131.108.50.0 /24131.108.50.0 /24
131.108.51.0 /24131.108.51.0 /24
131.108.52.0 /24131.108.52.0 /24
131.108.53.0 /24131.108.53.0 /24
131.108.54.0 /24131.108.54.0 /24
131.108.55.0 /24131.108.55.0 /24
Classless Prefix NotationClassless Prefix Notation
• 131.108.0.0/16 versus 255.255.0.0
• Summarizable blocks of subnets131.108.48.0 /21131.108.48.0 /21
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Router (config) #
IP Address ConfigurationIP Address Configuration
• Sets format of network mask as seen in showshowcommands
• bitcount 172.16.31.6/24
• decimal 172.16.31.6 255.255.255.0
• hexadecimal 172.16.31.6 0xFFFFFF00
ip netmask-format{bitcount | decimal | hexadecimal}
ip netmask-format{bitcount | decimal | hexadecimal}
183010947_05F9_c2 © 1999, Cisco Systems, Inc.
AgendaAgenda
• Addressing
•• ConceptsConcepts
• Routing Protocols
• Statics and Defaults
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x
ConvergenceConvergence
• Time required for router to identify anduse an alternate path
• Dependent on timer values and algorithm
• Difficult to predict precisely
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Load BalancingLoad Balancing
• Equal cost paths
• Rapid failover
N1
R1 R4R3
R2T1
T1T1
T1
N2
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Load BalancingLoad Balancing
• Unequal cost paths
N1
R1 R4R3
R2
T1
512K
N2
768K
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x
HolddownHolddown
• Sets minimum convergence time
• Prevents routing loops
I Will IgnoreI Will IgnoreRoutes to XRoutes to X
While inWhile inHolddownHolddown
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Packets for Network X
Routing Loop:Routing Loop:A Routing DisagreementA Routing Disagreement
• Packets do not get to the destination
• Temporary traffic surge until convergence
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Do not send routing databack in the direction from
which it came
Split HorizonSplit Horizon
“
”
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A
C
BD
PVC
PVC
PVCS0
DD 11
Split HorizonSplit Horizon
Frame Relay Network
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C
BD
PVC
PVC
PVCS0
Turn off Split HorizonTurn off Split Horizon
DD 11
CC 22
BB 22
AA 22DD 22
CC 22
BB 22
AA 1,31,3
A
Frame Relay Network
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Split HorizonSplit Horizon
A
C
BD
PVC
PVC
PVCS0.3
S0.1
Frame Relay Network
DD 11
CC 22
BB 22
DD 11
BB 22
AA 22
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Metrics (Cost)Metrics (Cost)
• Numeric value used to chooseamong paths
• RIP/RIPv2 is hop count and ticks (IPX)
• OSPF/ISIS is interface cost (bandwidth)
• (E)IGRP is compound
• BGP can be complicated
• Path determination depends on metric
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AgendaAgenda
• Addressing
• Concepts
•• Routing ProtocolsRouting Protocols
• Statics and Defaults
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Routing TableRouting Table
198.113.181.0198.113.181.0 [170/304793][170/304793]192.150.42.177192.150.42.177 02:03:5002:03:50 DD
198.113.178.0198.113.178.0
192.168.96.0192.168.96.0
192.168.97.0192.168.97.0
[110/9936][110/9936]192.150.42.177192.150.42.177 02:03:5002:03:50 OO
192.150.42.177192.150.42.177 00:00:2000:00:20 RR
CC
[120/3][120/3]
Ethernet0Ethernet0
Ethernet0Ethernet0
Ethernet0Ethernet0
Ethernet0Ethernet0
AgeAge SourceSourceNetwork #Network # InterfaceInterface Next HopNext Hop MetricMetric
One Forwarding Table Per Protocol (IP, IPX)
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• Hardware state
• DynamicRoutes are learned from a protocol
• StaticRoutes are manually defined
Building the Routing TableBuilding the Routing Table
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I Know About:Network XNetwork YNetwork Z
I Know About:Network ANetwork BNetwork CA
B
C
X
Y
Z
Routing Update
Exchanges Network Knowledge
Routing ProtocolsRouting Protocols
• Routers are packet switches that forward trafficbased on layer 3 logical addresses
• Routing protocol updates are exchanged by routersto learn about paths to other logical networks
• Each routing protocol offers features that can make itdesirable as part of an internetwork design
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Routing Protocol GoalsRouting Protocol Goals
• Optimal path selection
• Loop-free routing
• Fast convergence
• Limited designadministration
• Minimize update traffic
• Handle address limitations
• Support hierarchicaltopology
• Incorporate rapidconvergence
• Easy to configure
• Adapts to changeseasily and quickly
• Does not create a lotof traffic
• Scales to a large size
• Compatible with existinghosts and routers
• Supports variable lengthsubnet masks anddiscontiguous subnets
• Supports policy routing
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IP RIPIP RIP
• RoutingInformationProtocol
• Widely available
• Hop count metric
• Periodic update
• Easy to implement
• Usually free
• RFC 1058
• Simple = limited
• Slow convergence
• No VLSM
• No discontiguoussubnets
• Routing loops
• Count to infinity
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Send Routing Table to Neighbors
Net A
Net B Net C
Net D
E0S0 S1 S0E0
R1 R2 R3
RIP—Distance VectorRIP—Distance Vector
AA E0E0BB S0S0
S0S0DDS0S0CC
NetworkNetwork InterfaceInterfaceBB S0S0CC S1S1
S1S1DDS0S0AA
NetworkNetwork InterfaceInterfaceCC S0S0DD E0E0
S0S0AAS0S0BB
NetworkNetwork InterfaceInterface
S0
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RIP V1
Broadcast Routing UpdatesBroadcast Routing Updates
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R1
R2
R3
T1
56k
T1
0 Hops
1 Hop
Path A
Path B
Hops
RIP MetricRIP Metric
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When to Use RIPWhen to Use RIP
• Implementation in a few hours
• Good for stable links
• Good for small networks
• routed in host environment
• Multivendor environment
• Non-redundant network
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RIP V2RIP V2
• RFC 1723• Cisco IOS® 11.1 support• Advertises masks• Variable length subnet masks• Route summarization• Routing updates use multicast• Authenticated updates using MD5
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Multicast Routing UpdatesMulticast Routing Updates
RIP V2
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When to Use RIPv2When to Use RIPv2
• Same as RIP
• Subnet mask support
• Reduce broadcast load
• Validated updates
• Multivendor environment
• Non-redundant network
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IPX RIPIPX RIP
• Widely available
• Hop count metric
• Ticks (1/18 sec)
• Periodic update
• Easy to implement
• Free on servers
• Tied to SAPprotocol
• Simple = limited
• Slow convergence
• No default route
• Routing loops
• Count to infinity
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IPX RIP—TicksIPX RIP—Ticks
• Ticks are usedto determineserver timeout
• Default for LANinterfaces is 1
• Default for WANinterfaces is 4
• IPXWANcalculates forits interfaces
• can be set viathe ipx delaynumber interfacesub command
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IGRPIGRP
• Interior GatewayRouting Protocol
• Cisco developed
• Distance vector
• Compoundmetric
• Cisco IOS 9.21
• Periodic update
• No VLSM
• Default timersproduce slowconvergence
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• Administrativeweight
• Delay
• Bandwidth
• Reliability
• Load R1
R2
R3
T1
56k
T1
(K2 * BW) (256-load)
K5 (reliability + K4))
= ((K1 * BW + + K3* delay)) *
IGRP Compound MetricIGRP Compound Metric
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Delay Metric-Based on
D1 + D2 + D3
BandwidthMetric-Based
on 64kbps
D1 D2 D3
1.5 Mbps 64 kbps 1.5 Mbps
How the IGRP Metrics WorkHow the IGRP Metrics Work
• Bandwidth dominates short paths
• Delay dominates long paths
• Configure bandwidth on all interfaces
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When to Use IGRPWhen to Use IGRP
• Simplicity of RIP
• Good for small and medium networks
• When metrics are important
• Reduced routing overhead
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Enhanced IGRPEnhanced IGRP
• Extremely fastconvergence
• VLSM support
• Discontiguoussubnets
• Arbitrary routesummarization
• Supports prefix andhost routing
• Best of DV and LS
• Low overhead
• Guaranteedloop-free
• Reliable, incrementalupdate-based
• Multiprotocol:IP, IPX®, AppleTalk
• Easy to configure
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Topology Table
On Startup Routing TablesAre Exchanged; Routing
Table Built Based on BestPaths from Topology Table
X’s Table
Y’s Table
Advanced Distance VectorAdvanced Distance Vector
• Construct neighbor tables• Construct topology tables• Compute routes
AABBCC
1113132020
AABBCC
553333
AABBCC
QQZZXX
2213131313
AA 27271155
ZZQQXX
BB.... 1212.... ZZ....
AABBCC
272712123535ZZ
XX
YYQQ
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EIGRP TablesEIGRP Tables
• Topology table
• Acted upon by DUAL
• All routes advertisedby neighbors
• List of neighbors foreach route
• Routes passiveor active
• Neighbor table
• Keeps adjacentneighbor’s address
• Keeps the hold time
• Information forreliable transport
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Diffusing UpdateDiffusing UpdateAlgorithm (DUAL)Algorithm (DUAL)
• DUAL is a loop-free routing algorithmthat performs a diffused computationof a routing table
Uses a new routing algorithm
Achieves fast convergence
Network changes propagate only to affectednodes (“bounded updates”)
• No need for route holddown
• Researched and developed by SRIInternational
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IPX EIGRPIPX EIGRP
• Automatic redistribution of routesinto RIP/SAP
• Maximum network size is 224 hopsvs 15 for RIP
• Incremental SAPs sent, reducingbandwidth usage
• All other benefits of EIGRP
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When to Use EIGRPWhen to Use EIGRP
• Very large, complex networks
• VLSM
• For fast convergence
• Little network design
• Multiprotocol support
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Topology Information IsKept in a Database Separate
from the Routing Table
X’s Link State
Z’s Link StateQ’s Link State
Link State RoutingLink State Routing
• OSPF• IS-IS• NLSP• DECnet V
ZZ
XX
AABBCC
QQZZXX
2213131313YY
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Link State RoutingLink State Routing
• Neighbor discovery• Constructing an LSA (Link State
Advertisement)• Distribute LSA• Compute routes using SPF
(Shortest Path First)• On network failure
New LSAs flooded
All routers recompute routing tables
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OSPFOSPF
• Open ShortestPath First
• Link state or SPFtechnology
• Developed by OSPFworking group ofIETF (RFC 1253)
• Designed expresslyfor TCP/IP Internetenvironment
• Fast convergence
• Variable-lengthsubnet masks
• Discontiguoussubnets
• No periodic updates
• Route authentication
• Delivered two yearsafter IGRP
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BackboneBackboneRouterRouter
OSPF Areas and RulesOSPF Areas and Rules
Area 1Area 4
Area 0
Area 2 Area 3
InternalInternalRouterRouter
AreaAreaBorderBorderRouterRouter
AutonomousAutonomousSystem (AS)System (AS)
Border RouterBorder Router
Internet
• Backbone area (0)must be present
• All other areasmust haveconnectionto backbone
• Backbone mustbe contiguous
• Do not partitionarea (0)
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When to Use OSPFWhen to Use OSPF
• Large hierarchical networks
• Complex networks, except…Topology restrictive
Additional network design
• VLSM
• Fast convergence
• Multivendor
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IS-ISIS-IS
• IS = IntermediateSystem
• Dual IS-IS
• Integrated IS-IS
• Metric is 6 bitswide (1-63)*
• All interfacesdefault to 10
• ISO 10589
• Two types of areas:Level-1 other areas
Level-2 backbone
• Default foreach level
• Much like OSPF
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NetWare Link Services ProtocolNetWare Link Services Protocol
• Derived from ISIS
• NLSP specs 3 levels of routers
• Only 2 levels are defined
• Spec is Novell NLSP version 1.1http://developer.novell.com
/devres/langrp/specs/nlspspec.exe
http://www.novell.com/documentation
/en/kayak/nw411com/ipxrtenu/docmodul/ch3.html
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BGPBGP
• RFC 1771
• Border GatewayProtocol
• Version 4 is current
• Exterior routingprotocol (vs.interior)
• Uses TCP fortransport
• Many options forpolicy enforcement
• Classless InterDomain Routing(CIDR)
• Widely used forInternet backbone
• Autonomoussystems
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EE
AS 100 AS 101
AS 102
Peering
BGP BasicsBGP Basics
• Runs over TCP
• Path vectorprotocol
• Incremental update
CCAA
BB DD
EE
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• BGP peer within the same AS• Not required to be directly connected• IBGP neighbors should be fully meshed• Few BGP speakers in corporate network
AS 100
Internal BGP (IBGP) PeeringInternal BGP (IBGP) Peering
BB
DD
AA
EE
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AS 100 AS 101
External BGP (EBGP) PeeringExternal BGP (EBGP) Peering
• Between BGP speakers in different AS
• Should be directly connected
• Don’t run an IGP between EBGP peers
AA
BB
CC
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BGP
BGP
BGP
StaticRoute
AS 100
AS 200
AS 400
AS 300
Policy DrivesPolicy DrivesBGP RequirementsBGP Requirements
• Policy for AS 100: Always use AS 300path to reach AS 400
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B
Static
Advertise DefaultNetwork Via IGP Use a Static Route to
Provide Connectivity
NetworkNumber
ISP Runs BGP
When Not to Use BGPWhen Not to Use BGP
• Avoid BGP configuration by usingdefault networks and static routes
Appropriate when the local policy is thesame as the ISP policy
AA BB
CC
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AgendaAgenda
• Addressing
• Concepts
• Routing Protocols
•• StaticsStatics and Defaults and Defaults
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Static RoutesStatic Routes
• Routes configured manually
• Useful when few or just oneroute exist
• Can be administrative burden
• Frequently used for default route
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172.16.3.13
172.16.1.0C15C0
172.16.3.23
T1
ISDN
Floating Static RoutesFloating Static Routes
• A static route with a high distancedistance• Can be overridden by dynamic info
ip route 172.16.1.0 255.255.255.0 172.16.3.1 140140
ipx route C15C0 3.0000.0c15.3628 floating-static
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Default RoutesDefault Routes
• Route used if no match is found inrouting table
• Can be carried by routing protocols• Two models
Special network number:0.0.0.0 (IP)-2 (IPX)
Flagged in routing protocol
• Protocols support multiple models
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Creating a Default RouteCreating a Default Route
• RIP, RIPv2: network 0.0.0.0
• IGRP, EIGRP: ip default-network
• OSPF: default originate
• IPX: ipx route default
• default gateway is for “host mode”
723010947_05F9_c2 © 1999, Cisco Systems, Inc.
172.16.1.0
172.16.0.0
s0 s1
Internet
Default IPDefault IP Subnet Subnet
• Two defaultsFor unknown networks
For unknown subnets
• Controlled by ip classless
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733010947_05F9_c2 © 1999, Cisco Systems, Inc.
Comparison of Routing ProtocolsComparison of Routing Protocols
LinkStateLinkLinkStateState
TraditionalTraditionalDistanceDistanceVectorVector
AdvancedAdvancedDistanceDistanceVectorVector
ScalabilityScalability
BandwidthBandwidth
MemoryMemory
CPUCPU
ConvergenceConvergence
ConfigurationConfiguration
GoodGood
LowLow
LowLow
HighHigh
LowLow
LowLow
HighHigh
HighHigh
ExcellentExcellent
LowLow
ModerateModerate
LowLow
FastFast
EasyEasy
FastFast
ModerateModerate
SlowSlow
EasyEasy
743010947_05F9_c2 © 1999, Cisco Systems, Inc.
NameName TypeType ProprietaryProprietary FunctionFunction UpdatesUpdates MetricMetric VLSMVLSM SummSumm
RIPRIP
RIPv2RIPv2
IGRPIGRP
EIGRPEIGRP
OSPFOSPF
IS-ISIS-IS
BGPBGP
DVDV
DVDV
DVDV
Adv DVAdv DV
LSLS
LSLS
DVDV
NoNo
NoNo
YesYes
YesYes
NoNo
NoNo
NoNo
InteriorInterior
InteriorInterior
InteriorInterior
InteriorInterior
InteriorInterior
Int/ExtInt/Ext
ExteriorExterior
30 Sec30 Sec
30 Sec30 Sec
90 Sec90 Sec
TrigTrig
TrigTrig
TrigTrig
TrigTrig
HopsHops
HopsHops
CompComp
CompComp
CostCost
CostCost
N/AN/A
NoNo
YesYes
NoNo
YesYes
YesYes
YesYes
N/AN/A
AutoAuto
AutoAuto
AutoAuto
BothBoth
ManMan
AutoAuto
ManMan
Internet Routing ProtocolsInternet Routing Protocols
• IP routing protocols arecharacterized as
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Topology/TechnologyTopology/TechnologyConsiderationsConsiderations
• Routing and services overhead is usuallynot a big deal when you have a lot ofbandwidth (i.e. LANs)
• Protect WAN bandwidth using update-basedprotocols—more bandwidth and buffers forapplication traffic
• High densities of sub(interfaces) can cause“hot spots” and router CPU overload
• NBMA (Non-Broadcast Multi-Access)technologies always require gooddesign practices
763010947_05F9_c2 © 1999, Cisco Systems, Inc.
For Further Reference…For Further Reference…
•• Computer Networks, Third EditionComputer Networks, Third Editionby Andrew Tanenbaum (ISBN: 0-13349-945-6)
•• Interconnections : Bridges and RoutersInterconnections : Bridges and Routersby Radia Perlman (ISBN: 0-20156-332-0)
•• Internetworking with TCP / IP, Volume 1:Internetworking with TCP / IP, Volume 1:Principles, Protocols, and ArchitecturePrinciples, Protocols, and Architectureby Douglas Comer (ISBN: 0-13216-987-8)
•• IP Routing FundamentalsIP Routing Fundamentalsby Mark Sportack (ISBN: 1-57870-071-x)
•• IP Routing PrimerIP Routing Primerby Robert Wright (ISBN: 1-57870-108-2)
•• OSPF Network Design SolutionsOSPF Network Design Solutionsby Thomas, Thomas M. (ISBN: 1-57870-046-9)
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773010947_05F9_c2 © 1999, Cisco Systems, Inc.
Thank You!Thank You!
• Please fill out the survey• This was #301 Introduction
to Routing• Related sessions:
304 Intro to IP Switching307 Deploying IGRP/EIGRP308 Deploying OSPF/NLSP/IS-IS
309 Deploying BGP
Introduction to Routing
Session 301
78© 1999, Cisco Systems, Inc. 3010947_05F9_c2