Basic Routing Principles V1.2

Objectives Understand the function of router Know the basic conception in routing Know the working principle of router Understand IP routing process

Course Outline Router Definition and Function Basic Conception Working Principle of Router IP Routing Process

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-finding

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.

Course Outline Router Definition and Function Basic Conception Working Principle of Router IP Routing Process

Basic concepts Routed protocol Routing protocol Connected route Static route Dynamic route Default route

Network Protocol Destination Network Connected RIP OSPF Exit Interface fei_0/1 e1_1 e1_2 Routed protocol: IP Routing protocol: RIP, OSPF E0 S0 Routing protocol and routed protocol

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

Route table composition Route table composition: Destination address Mask Next hop address (Gateway) Forwarding physical interface Source of the information(Owner) Route priority metric

Route table composition Example Destination network address Destination network mask Next hop address fei_0/ Route learning interface and forwarding interface static -- how does router learns this entry 1-- route priority 0-- Metric Dest Mask Gw Interface Owner pri metric fei_0/1.1 static 1 0

Route table establishing and refreshing How is route established at the beginning? How to maintain the route table after it is established?

Route Types Default route Static routeDynamic route Directly connected route

/ / / /24 IPv4 Routing Table: Dest Mask Gw Interface Owner pri metric fei_0/1 direct fei_0/1 address fei_1/1 direct fei_1/1 address 0 0 ZXR10# Directly connected route When network address is configured and the status of the interface 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. B A

Static route configuration This is a unidirectional route, there should be an opposite route configured on the counterpart router. ip route Stub Network SO B Network B A

Example Default route configuration Default route is configured on the egress router in a stub network. Stub Network ip route SO Network A B

RIPOSPFRIPOSPF / / /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.

RIPOSPF Route table /16 fei_1/1 Notice ： the comparison of route priority must be between identical routes /16 fei_1/3 Route priority Select the route that has the highest priority to the route table.

Connected interface 0 Static route 1 External BGP20 OSPF110 IS-IS 115 RIP v1, v2120 Internal BGP200 Special (For inner process)255 Connected interface 0 Static route 1 External BGP20 OSPF110 IS-IS 115 RIP v1, v2120 Internal BGP200 Special (For inner process)255 Route Source Default priority Default route priority

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?

ZXR10(config)#ip route tag150 ZXR10(config)#ip route tag155 Network DDN PSTN fei_1/1 e1_2/1.1 serial_3/1 Float static route

Longest match principle  ? ZXR10#show ip route IPv4 Routing Table: Dest Mask Gw Interface Owner pri metric fei_1/1.1 direct fei_1/1.1 address fei_1/1.2 direct fei_1/1.2 address fei_1/1.3 direct fei_1/1.3 address fei_1/1.1 ospf fei_1/1.2 static fei_1/1.3 rip fei_1/1.1 static 0 0

Course Outline Router Definition and Function Basic Conception Working Principle of Router IP Routing Process

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)

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 ？

＝ 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

Routing process Routing process Routing process Routing process Route table Forwarding process Search entry in route table Based on destination address Longest match principles Add route based on route priority Routing protocol study possible route,select the best route based on metric Route working process summary

Course Outline Router Definition and Function Basic Conception Working Principle of Router IP Routing Process

Communication in the same network Network A ┉┉┉ To router1 Communication requirement Hub/switch

Communication process IP address ： MAC address ： 00:20:AF:00:00:02 IP address ： MAC 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 Ethernet IP layer Encapsulate MAC address in frame and send to MAC address 00:20:AF:00:00:02 Decapsulate MAC frame Receiving MAC frame ARP decapsulateIP packet Receiving IP packet A B Network Protocol layer

Communication between different network Network B Network A Router Communication requirements Ethernet Router Ethernet DDN PPP

Communication between different network Ethernet interface Ethernet IP layer MAC frame IP layer Serial interface Ethernet interface IP layer Network Protocol PPP packet IP packet Send host A Receiving host B router Ethernet PPP HOST A HOST B fei-1/1e1_2/1.1 Ethernet fei-1/1e1_2/1.1 IP layer router PPP frame IP packet Serial interface MAC frame IP packet No need to consider higher layer protocols

Whether in the same network with peer Is the peer MAC address in ARP table? Data link layer encapsulation Send data at physical layer Get peer MAC Address by ARP YES NO YES NO Is gateway configured ？ YES Is the gateway MAC address in ARP? Data link layer encapsulation Get gateway MAC address by ARP NO Send error message YES NO End Get peer IP address Communication process

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 will be a new encapsulation of the packet The return path is not related to the forwarding path

Network protocol layer process X Y Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical A B C X Y Network Data Link Physical Network Data Link Physical Network Data Link Physical A B C

Routing process example / /24 e1_1/1.1 fei_1/ / / /24 R1 R2 R3 R1 route table e1_1/1.1 direct fei_1/1 direct e1_1/1.1 rip e1_1/1.1 rip e1_1/1.1 rip e1_2/1.1 e1_1/1.1 fei_1/1 Destination address: A B

1.Destination address: / / / / /24 R1 R2 R3 R2 route table e1_1/1.1 direct e1_2/1.1 direct fei_1/1 direct e1_1/1.1 rip e1_1/1.1 rip e1_1/1.1 fei_1/1 e1_2/1.1 e1_1/1.1 fei_1/1 Destination Address: Routing process example

/ / / / /24 R1 R2 R3 R3 route table fei_1/1 direct e1_1/1.1 direct e1_1/1.1 rip e1_1/1.1 rip e1_1/1.1 rip Return path ？ e1_1/1.1 fei_1/1 e1_2/1.1 e1_1/1.1 fei_1/1 Destination address:

Review Definition of router Router functions Basic conceptions in routing IP routing process within the same subnet IP routing process between different subnet

Questions What’s the two main functions of router? How is the route table established? What is the standard when a router is selecting the route to route table? When forwarding IP packet, If there are many matches, what will router do? Describe the encapsulation and decapsulation during the process of IP routing.