ccna dis2 - chapter 6 – routing_ppt [compatibility mode]

8/3/2019 CCNA Dis2 - Chapter 6 Routing_ppt [Compatibility Mode]

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Chapter 6 Routing


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2Hc vin cng ngh thng tin Bach Khoa - Website: www.bkacad.com

Objectives

Describe the purpose and function of dynamic routing andthe protocols used to implement it

Configure RIPv2 dynamic routing using Cisco IOS Describe the use of exterior routing protocols across the

Internet Enable BGP on a customer site router
http://www.bkacad.com/http://www.bkacad.com/


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Routing

Routing is the method by which network devices direct messages acrossnetworks to arrive at the correct destination.

Router makes routing decisions by looking up information stored in theirrouting tables.

The routing table contain information about the routes that router uses to reachto destination

These routes can be statically assigned to the router by administrator or can bedynamically given to the router via a program called a routing protocol

Hc vin cng ngh thng tin Bach Khoa - Website: www.bkacad.com


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Routing



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Routing



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Every router uses a routing table to make decisions where to sendpackets. A route has four main components:

Destination value Mask Gateway or interface address Route cost or metric

The router looks at the destination IP address in the packet and thenlooks for a matching route in the routing table.

The resulting network address is then compared to the networkaddress of the route in the table. If a match is found, the packet is forwarded out the correct interface When the network address matches more than one route in the routing

table, the router uses the route that has the most specific, or longest,network address match from its routing table. Sometimes there is more than one equal cost route to the same destination

network. There are routing protocol rules that determine which route therouter will use.

If none of the route entries match, the router directs the message to thegateway specified by the default route if it has been configured.

Routing



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Configure static route

1. Connect to the router using a console cable.

2. Open a HyperTerminal window to connect with the first router that you want toconfigure.

3. Enter privileged modeRouter1>enableRouter1#

4. Enter global configuration mode.Router1#config terminal

Router1(config)#

5. Use the ip route IOS command to configure the static route, with the followingformat:ip route [destination_network] [subnet_mask] [gateway_address]

Routing- configure static route



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Routing protocols

Routers use routing protocols to dynamically manage information receivedfrom their own interfaces and from other routers. A dynamic routing protocol learns all available routes, places the best routes

into the routing table, and removes routes when they are no longer valid

The method that a routing protocol uses to determine the best route to adestination network is called a routing algorithm.

In order for two routers to exchange routes, they must be using the samerouting protocol and therefore the same routing algorithm.

There are two main classes of routing algorithms: distance vector and linkstate.



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The distance vector routing algorithm passes periodic copies of a routing tablefrom router to router. Two basic criterias:

Distance - How far away is the network from this router? Vector - In what direction should the packet be sent to reach this network?

The distance component of a route is expressed in terms of a route cost, ormetric, that can represent:

Number of hops Administrative cost Bandwidth Transmission speed Likelihood of delays Reliability

The vector or direction component of a route is the address of the next hopalong the path to the network named in the route

Routing protocols- distance vector



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Neighbor routers share a directly connected network. Each router receives a routing table from its directly connected neighbor

routers.

Each of the network entries in the routing table has an accumulated distancevector to show how far away that network is in a given direction.

As the distance vector discovery process proceeds, routers discover the bestpath to destination networks based on the information they receive from eachneighbor. The best path is the path with the shortest distance or smallestmetric

Routing table updates also occur periodically




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Refer: 6.1.2.3




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Routing protocols- activity



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Common Interior Routing Protocols- RIP

Routing Information Protocol (RIP) is a distance vector routing protocol that isused in thousands of networks throughout the world.

The key characteristics of RIP include the following: Initially specified in RFC 1058

Is a distance vector routing protocol

Uses hop count as the metric for path selection

Defines a hop count greater than 15 as an unreachable route

Sends routing table contents every 30 seconds, by default

Refer 6.1.3.1 When a router receives a routing update that includes a new or changed route,

the router updates its routing table to reflect this new route.

At each router, the hop count value is increased by one. The router uses the local network address of the directly connected router

which sent the update as the next hop address.

Triggered updates: are sent independently when topology changes



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The RIP protocol is simple, easy to implement, and is available free of costwith most routers. These advantages make RIP a widely used and popularrouting protocol.

RIP has several disadvantages: Allows a maximum of 15 hops, so it can only be used for networks that

connect no more than 16 routers in series.

Periodically sends complete copies of the entire routing table to directlyconnected neighbors. In a large network, this can cause a significantamount of network traffic each time there is an update.

Converges slowly on larger networks, when the network changes.

There are currently two versions of RIP available: RIPv1 & RIPv2 The most significant difference between RIP version 1 and 2 is that RIPv2 can

support classless routing, because it includes the subnet mask information in

routing updates. RIPv1 does not send subnet mask information in the updates;therefore it must rely on the classfull default subnet masks.




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Common Interior Routing Protocols-EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietaryenhanced distance vector routing protocol. EIGRP uses a number of metrics, including a configured bandwidth value, and

the delay encountered when a packet travels a particular route.

The key characteristics of EIGRP are: Uses a variety of metrics to calculate the cost of a route

Combines the next hop and metric features of distance vector protocols,with additional database and update features

Has a maximum hop count of 224 hops

EIGRP creates three database tables: the neighbor table, the topology tableand the routing table

The neighbor table stores data about the neighboring routers that are ondirectly connected local networks.

The topology table contains all of the routes advertised by the neighborrouters.

EIGRP depends on a routing algorithm called Diffused Update Algorithm(DUAL) to calculate the shortest path to a destination within a network and toinstall this route into the routing table.



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Common Interior Routing Protocols-EIGRP



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Routing protocols- Link-state

The link-state routing algorithm maintains a full database of distant routers andhow they interconnect. Link-state routing uses the following features:

Routing table - List of the known paths and interfaces Link-state advertisement (LSA) - Small packet of routing information that is

sent between routers. LSAs describe the state of the interfaces (links) of arouter, as well as other information, such as the IP address of each link

Topological database - Collection of information gathered from all the LSAsreceived by the router SPF (Shortest Path First) algorithm - Calculation performed on the

database that results in the SPF tree. The SPF tree is a map of thenetwork as seen from the point of view of the router. The information in thistree is used to build the routing table(Each time a new LSA packet causes a change to the link-state database;SPF recalculates the best paths and updates the routing table.)



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Routing protocols- Link-state



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OSPF

Open Shortest Path First (OSPF) is a non-proprietary link-state routing protocoldescribed in RFC 2328. The key characteristics of OSPF are:

Uses the SPF algorithm to calculate the lowest cost to a destination

Sends routing updates only when the topology changes; does not send periodicupdates of the entire routing table.

Provides fast convergence

Supports VLSM and discontiguous subnets

Provides route authentication



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Routing Within An Organization

Small networks with only one gateway to the Internet can probably use staticroutes. As an organization grows and adds routers to its network topology, RIPv2 can

be used. It is easy to configure and in small networks can work well. When anetwork begins to exceed 15 routers, RIP is no longer a good choice.

For larger networks, EIGRP and OSPF are common. The three main criteria in helping settle on a protocol are:

Ease of management - What information does the protocol keep aboutitself? What show commands are available?

Ease of configuration - How many commands will the averageconfiguration require? Is it possible to configure several routers in yournetwork with the same configuration?

Efficiency - How much bandwidth does the routing protocol take up while in

steady state, and how much could it take up, when converging in responseto a major network event?



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Configure and Verify RIP



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Autonomous Systems

An AS is a set of networks controlled by a single administrative authority usingthe same internal routing policy throughout. Each AS is identified by a unique AS number (ASN). ASNs are controlled and

registered on the Internet.

The most common example of an AS is the ISP



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Autonomous Systems



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AS- Activity



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Routing between Autonomous Systems

Interior Gateway Protocols (IGP) are used to exchange routing informationwithin an autonomous system or individual organization. Examples: RIP,EIGRP and OSPF.

Exterior gateway protocols (EGP) are designed to exchange routinginformation between different Autonomous Systems. Examples: BGPv4

EGP protocols run on the exterior routers, that is, the routers that are located

at the border of an AS. Exterior routers are also called border gateways.



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Routing Across the Internet

Packets are routed across the Internet in several steps:

1. The source host sends a packet destined for a remote host located in another AS.

2. Since the destination IP address of the packet is not a local network, the interiorrouters keep passing the packet along their default routes, until eventually it arrives at anexterior router at the edge of the local AS.

3. The exterior router maintains a database for all the Autonomous Systems with which it

connects. This reachability database tells the router that the path to the destinationnetwork passes through several ASs, and that the next hop on the path is through adirectly-connected exterior router on a neighboring AS.

4. The exterior router directs the packet to its next hop on the path, which is the exteriorrouter at the neighboring AS.

5. The packet arrives at the neighboring AS, where the exterior router checks its ownreachability database and forwards the packet to the next AS on the path.

6. The process is repeated at each AS until the exterior router at the destination ASrecognizes the destination IP address of the packet as an internal network in that AS.

7. The final exterior router then directs the packet to the next hop interior router listed inits routing table. From then on, the packet is treated just like any local packet and isdirected through interior routing protocols through a series of internal next hops until itarrives at the destination host.



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Step 1




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Step 6




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Step 7



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Exterior Routing Protocols and the ISP

EGPs not only allow traffic to be routed across the Internet to remotedestinations, but they also provide the method by which ISPs can set andenforce policies and local preferences so that the traffic flow through the ISP isefficient and that none of the internal routes are overloaded with transit traffic

Business customers insist on reliability for their Internet service, and so ISPsmust make sure that the Internet connection for those customers is alwaysavailable. They do this by providing backup routes and routers in case theregular route fails



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The flow of messages in the Internet is called traffic. Internet traffic can becategorized in one of two ways: Local Traffic - Traffic carried within an AS that either originated in that

same AS, or is intended to be delivered within that AS. This is like localtraffic on a street.

Transit Traffic - Traffic that was generated outside that AS and can travel

through the internal AS network in route to be delivered to destinationsoutside the AS. This is like through traffic on a street.

The flow of traffic between Autonomous Systems is carefully controlled.

Exterior Routing Protocols and the ISP


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Configure and Verify BGP



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Summary

Routing is used to forward messages to the correctdestination Routing can be dynamic or static Distance vector and Link-state dynamic routing protocols

IGP & EGP (BGP)



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ccna dis2 - chapter 6 – routing_ppt [compatibility mode]

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