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Basic Routing Principles
V1.2
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Objectives
Understand the function of router Know the basic conception in routing Know the working principle of router Understand IP routing process
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Course Outline
Router Definition and Function Basic Conception Working Principle of Router IP Routing Process
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Router definition(1)
Router——A kind of computer device used to interconnect network
Router must have the following features: Multiple three layer interfaces to interconnect different
networks Implement the protocol up to the network layer Have the function of storing、 forwarding、 path-findi
ng
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Router functions(2)
The core function of router is interconnecting networks and data forwarding. routing: building route table and refreshing switching: forwarding packet between networks. Insulate broadcast packets, define access rule. Connecting different kinds of networks. Rate adapting between networks.
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Course Outline
Router Definition and Function Basic Conception Working Principle of Router IP Routing Process
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Basic concepts
Routed protocol Routing protocol
Connected route Static route Dynamic route Default route
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NetworkProtocol
DestinationNetwork
ConnectedRIP
OSPF
10.120.2.0172.16.2.0172.17.3.0
Exit Interface
fei_0/1e1_1e1_2
Routed protocol: IPRouting protocol: RIP, OSPF
172.17.3.0
172.16.2.010.120.2.0
E0S0
Routing protocol and routed protocol
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Route table
The information that router need to forward data is stored in a table, called ’route table’
Router check the destination address of the packet,and choose the next hop based on the information in route table.
Route table is stored in RAM
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Route table composition
Route table composition: Destination address Mask Next hop address (Gateway) Forwarding physical interface Source of the information(Owner) Route priority metric
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Route table composition
Example
172.16.8.0 -- Destination network address255.255.255.0 -- Destination network mask1.1.1.1 -- Next hop address fei_0/1.1 -- Route learning interface and forwarding interfacestatic -- how does router learns this entry1 -- route priority0 -- Metric
Dest Mask Gw Interface Owner pri metric
172.16.8.0 255.255.255.0 1.1.1.1 fei_0/1.1 static 1 0
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Route table establishing and refreshing
How is route established at the beginning? How to maintain the route table after it is
established?
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Route Types
Default route
Static route Dynamic route
Directly connected route
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192.168.0.1/30
192.168.0.2/30
10.0.0.1/24
172.16.0.1/24
IPv4 Routing Table: Dest Mask Gw Interface Owner pri metric
10.0.0.0 255.255.255.0 10.0.0.1 fei_0/1 direct 0 010.0.0.1 255.255.255.255 10.0.0.1 fei_0/1 address 0 0192.168.0.0 255.255.255.252 192.168.0.1 fei_1/1 direct 0 0192.168.0.1 255.255.255.255 192.168.0.1 fei_1/1 address 0 0ZXR10#
Directly connected route
Directly connected route When network address is configured and the status of the interfa
ce is valid, the network configured on this network will appear in the route table and associated with this interface, the route will change with the status of the interface.
BA
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Static route configuration
This is a unidirectional route, there should be an opposite route configured on the counterpart router.
ip route 10.0.0.0 255.0.0.0 172.16.2.2
Stub Network
172.16.2.1
SO
172.16.1.0
B172.16.2.2Network
10.0.0.0B A
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Example
Default route configuration
Default route is configured on the egress router in a stub network.
Stub Network
ip route 0.0.0.0 0.0.0.0 172.16.2.2
172.16.2.1
SO
172.16.1.0
172.16.2.2Network A B
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RIPOSPF RIPOSPF
1.0.0.0/8 2.0.0.0/8 3.0.0.0/8
Dynamic route
Routing protocol is a software process running on the router. It will exchange routing information with other routers, studying route information of network not directly connected and adjusting the route information when topology changes.
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RIP OSPF
Route table
10.0.0.0/16 fei_1/1
Notice : the comparison of route priority must be between identical routes.
10.0.0.0/16 fei_1/3
10.0.0.0/16 fei_1/3
Route priority
Select the route that has the highest priority to the route table.
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Connected interface 0Static route 1External BGP 20OSPF 110IS-IS 115RIP v1, v2 120Internal BGP 200Special (For inner process) 255
Connected interface 0Static route 1External BGP 20OSPF 110IS-IS 115RIP v1, v2 120Internal BGP 200Special (For inner process) 255
Route SourceRoute SourceRoute SourceRoute Source Default priorityDefault priorityDefault priorityDefault priority
Default route priority
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Network
DDN
PSTN
fei_1/1 e1_2/1.1
serial_3/1
Float static route
How to configure static route when there are redundant links?
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ZXR10(config)#ip route 10.0.0.0 255.0.0.0 20.0.0.1 tag150ZXR10(config)#ip route 10.0.0.0 255.0.0.0 30.0.0.1 tag155
Network
DDN
PSTN
fei_1/1 e1_2/1.1
serial_3/1
Float static route
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Longest match principle
10.1.1.1?
ZXR10#show ip routeIPv4 Routing Table: Dest Mask Gw Interface Owner pri metric
1.0.0.0 255.0.0.0 1.1.1.1 fei_1/1.1 direct 0 01.1.1.1 255.255.255.255 1.1.1.1 fei_1/1.1 address 0 02.0.0.0 255.0.0.0 2.1.1.1 fei_1/1.2 direct 0 02.1.1.1 255.255.255.255 2.1.1.1 fei_1/1.2 address 0 03.0.0.0 255.0.0.0 3.1.1.1 fei_1/1.3 direct 0 03.1.1.1 255.255.255.255 3.1.1.1 fei_1/1.3 address 0 010.0.0.0 255.0.0.0 1.1.1.1 fei_1/1.1 ospf 110 1010.1.0.0 255.255.0.0 2.1.1.1 fei_1/1.2 static 1 010.1.1.0 255.255.255.0 3.1.1.1 fei_1/1.3 rip 120 50.0.0.0 0.0.0.0 1.1.1.1 fei_1/1.1 static 0 0
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Course Outline
Router Definition and Function Basic Conception Working Principle of Router IP Routing Process
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Router working principle
Routing function: Study and maintain the network topology,produce and
maintain route table
Forwarding function: Data transferring and processing procedure(Receiving
data on one interface,then choose an appropriate interface to send it out, including the work of frame encapsulation and decapsulation)
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Route function
Study and maintain the network topology mechanism is regarded as the function of routing.It requires several basic information: What kind of protocols to route? Is the destination network address in the route table? Which interface to send out the packet? What is the next hop address?
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=Frame checking and cache the packet
Coming interface
Associate destination address with the next hop interface
Route table
Create the frame head with the physical address
ARP cache( LAN)
Packet encapsulation and forwarding
Out interface
=
=
=
Maintained by routing protocol
Maintained by ARP or RARP process
Mapping list(WAN)
Forwarding Function
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Routing processRouting processRouting processRouting
processRoute table
Forwardingprocess
Search entry in route tableBased on destination address
Longest matchprinciples
Add route based on route priority
Routing protocol study possible route,select the best
route based on metric
Route working process summary
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Course Outline
Router Definition and Function Basic Conception Working Principle of Router IP Routing Process
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Communication in the same network
Network A
┉┉┉
To router1
Communication requirement
Hub/switch
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Communication process
HubIP address: 192.168.1.2MAC address: 00:20:AF:00:00:02
IP address: 192.168.1.1MAC address: 00:20:AF:00:00:01
Ethernet
IP layer
No need to consider higher layer protocols
Encapsulate IP address in packet
and send to 192.168.1.2
Ethernet
IP layer
Encapsulate MAC address in frame
and send to MAC address00:20:AF:00:00:02
Decapsulate MAC frame
Receiving MAC frame
ARP
decapsulateIP packet
Receiving IP packet
A B
Network
Protocol layer
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Communication between different network
Network BNetwork A
Router
Communication requirements
Ethernet
Router
Ethernet
DDN
PPP
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Communication between different network
EthernetEthernetinterface
Ethernet
IP layer
MAC frame
IP layer
Serial interface
Ethernetinterface
IP layer
Network
Protocol
PPP packet
IP packetIP packet
Send host A
Receiving host B
router
Ethernet PPP
HOST A HOST B
fei-1/1 e1_2/1.1
Ethernet
fei-1/1e1_2/1.1
IP layer
router
PPP frame
IP packet
Serial interface
MAC frame
IP packet IP packet IP packet
No need to consider higher layer protocols
No need to consider higher layer protocols
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Whether in the same networkwith peer
Is the peer MACaddress in ARP table?
Data link layerencapsulation
Send data at physical layer
Get peer MAC Address by ARP
YES
NO
YES
NO
Is gatewayconfigured?
YES Is the gateway MACaddress in ARP?
Data link layerencapsulation
Get gateway MACaddress by ARP
NO
Send error message
YES
NO
End
Get peer IP address
Communication process
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IP communication process
IP communication process basic conception: IP communication is hop by hop communication Source IP and destination IP address is unchanged Every time the packet passed a data link layer,there wi
ll be a new encapsulation of the packet The return path is not related to the forwarding path
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Network protocol layer process
XY
ApplicationPresentationSessionTransportNetworkData LinkPhysical
ApplicationPresentationSessionTransportNetworkData LinkPhysical
A B C
X Y
NetworkData LinkPhysical
NetworkData LinkPhysical
NetworkData LinkPhysical
AA
BB
CC
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Routing process example
192.168.6.0 /24
192.168.2.0 /24
e1_1/1.1
e1_1/1.1
fei_1/1
192.168.2.3
192.168.1.0 /24
192.168.3.0 /24
192.168.3.3
192.168.4.0 /24
R1
R2
R3
R1 route table192.168.4.0 255.255.255.0 192.168.4.1 e1_1/1.1 direct 0 0 192.168.2.0 255.255.255.0 192.168.2.1 fei_1/1 direct 0 0192.168.1.0 255.255.255.0 192.168.4.2 e1_1/1.1 rip 120 1192.168.6.0 255.255.255.0 192.168.4.2 e1_1/1.1 rip 120 1 192.168.3.0 255.255.255.0 192.168.4.2 e1_1/1.1 rip 120 2
e1_2/1.1
e1_1/1.1
fei_1/1
fei_1/1
Destination address:192.168.3.3
A B
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1.Destination address:192.168.3.3
192.168.6.0 /24
192.168.2.0 /24
192.168.2.3
192.168.1.0 /24
192.168.3.0 /24
192.168.3.3
192.168.4.0 /24
R1
R2
R3
R2 route table192.168.4.0 255.255.255.0 192.168.4.2 e1_1/1.1 direct 0 0 192.168.6.0 255.255.255.0 192.168.6.1 e1_2/1.1 direct 0 0192.168.1.0 255.255.255.0 192.168.1.1 fei_1/1 direct 0 0192.168.2.0 255.255.255.0 192.168.4.1 e1_1/1.1 rip 120 1192.168.3.0 255.255.255.0 192.168.6.2 e1_1/1.1 rip 120 1
e1_1/1.1
e1_1/1.1
fei_1/1
e1_2/1.1
e1_1/1.1
fei_1/1
fei_1/1
Destination Address:192.168.3.3
Routing process example
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Routing process example
192.168.6.0 /24
192.168.2.0 /24
192.168.2.3
192.168.1.0 /24
192.168.3.0 /24
192.168.3.3
192.168.4.0 /24
R1
R2
R3
R3 route table192.168.3.0 255.255.255.0 192.168.3.1 fei_1/1 direct 0 0 192.168.6.0 255.255.255.0 192.168.6.2 e1_1/1.1 direct 0 0192.168.1.0 255.255.255.0 192.168.6.1 e1_1/1.1 rip 120 1192.168.4.0 255.255.255.0 192.168.6.1 e1_1/1.1 rip 120 1192.168.2.0 255.255.255.0 192.168.6.1 e1_1/1.1 rip 120 2
Return path ?
e1_1/1.1
e1_1/1.1
fei_1/1
e1_2/1.1
e1_1/1.1
fei_1/1
fei_1/1
Destination address:192.168.3.3
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Review
Definition of router Router functions Basic conceptions in routing IP routing process within the same subnet IP routing process between different subnet
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Questions
What’s the two main functions of router? How is the route table established? What is the standard when a router is selecting t
he route to route table? When forwarding IP packet, If there are many ma
tches, what will router do? Describe the encapsulation and decapsulation d
uring the process of IP routing .
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