Tài liệu học CCNA kỳ 2 mod6 routing

Module 6 - Routing and Routing ProtocolsCCNA 2 version 3.1 Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com 2 Overview • Explain the significance of static routing • Config

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Module 6 - Routing and Routing Protocols

CCNA 2 version 3.1

Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com 2

Overview

• Explain the significance of static routing

• Configure static and default routes

• Verify and troubleshoot static and default routes

• Identify the classes of routing protocols

• Identify distance vectorrouting protocols

• Identifylink-staterouting protocols

• Describe the basic characteristics of common routing protocols

• Identify interior gatewayprotocols

• Identify exterior gatewayprotocols

• Enable Routing Information Protocol(RIP) on a router

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The Routing Table

directly connected routes.

• This is information which is not in the curriculum, but will

give you a better understanding of what is taking place.

Directly Connected Networks and the IP Routing Table

RTA#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

U - per-user static route, o - ODR

Gateway of last resort is not set

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• Adding an ip address/mask to an interface tells the router that it is a member, “Directly

Connected” to that network – just like when a host computer is configured with an ip

address/mask.

• Notice the route is shown with the subnet mask and the “exit-interface.”

• Don’t forget the “no shutdown”

• Don’t forget the interface must be in “up” and “up”

RTA(config)#inter e 0

RTA(config-if)#ip add 192.168.2.1 255.255.255.0

RTA(config-if)#no shutdown

Codes: C - connected, <Other codes and gateway information omitted>

C 192.168.2.0/24 is directly connected, Ethernet0

C 172.16.0.0/16 is directly connected, Serial0

RTB#show ip route

RTC#show ip route

The Routing Tables

• Notice that the routers only know about their own directly connected networks.

• They are not sharing routing information because we have not configured any static routes or dynamic

routing protocols.

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Configuring an interface which has a subnet mask greater than the classful mask

• We will discuss this in much more detail later using the presentation – “The Routing Table.”

• For now, notice that when the subnet mask is not a classful mask, but a subnetted /16 mask

• The routing table information shows the route to the subnetted network

• The mask is shown in the above, “parent” classful network.

C 10.1.0.0 is directly connected, Ethernet0

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Routing– Routing tables must have the necessary network routes

• Question: If RTA can ping RTB’s 172.16.0.2 interface why can’t it ping RTB’s 192.168.1.1

interface? - RTA does not have a route to it in its routing table.

• Question: Would an extended ping from RTA, using the source IP address of 192.168.2.1 be able

to ping 172.16.0.2 on RTB? Why or why not? Where does the echo request or echo reply fail?

Routing– Routing tables must have the necessary network routes

• Question: Would an extended ping from RTA, using the source IP address of 192.168.2.1 be able

to ping 172.16.0.2 on RTB? Why or why not?

• The echo request from RTA reaches RTB because RTA has a route to 172.16.0.0/16 in its routing

table.

• However, the echo reply from RTB back to RTA fails, because RTB does not have a route for

192.168.2.0/24 in its routing table.

RTA#ping

Protocol [ip]:

Target IP address: 172.16.0.2

Extended commands [n]: y

Source address or interface: 192.168.2.1

Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:

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Routing Table Principles Revisited (Zinin, Cisco IP Routing)

• Every router makes its decision alone , based on the information it has in its own routing table

• The fact that one router has certain information in its routing table does not mean that other routers

have the same information

• Routing information about a path from one network to another does not provide routing information

about the reverse, or return path.

RTC#show ip route

10.0.0.0/16 is subnetted, 1 subnets

C 10.1.0.0 is directly connected, Ethernet0

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Routing Types

Static Route Operation

Hoboken#show ip route

Codes: C - connected, S - static,

S 172.16.1.0/24 [1/0] is directly connected, Serial0

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RTR(config)# ip route prefix mask {address |

interface} [distance] [tag tag] [permanent]

• prefix IP route prefix for the destination.

• mask Prefix mask for the destination.

• address IP address of the “next hop” that can be used to reach

that network

• interface Network interface to use (exit-interface)

• distance (Optional) An administrative distance.

• tag tag (Optional) Tag value that can be used as a "match" value

for controlling redistribution via route maps (CCNP Advanced Routing)

• Permanent (Optional) Specifies that the route will not be removed,

even if the interface shuts down (CCNP Advanced Routing)

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• If the router cannot reach the outgoing interface that is being used in the route,

the route will not be installed in the routing table

• This means if that interface is down, the route will not be placed in the routing

table

Administrative Distance and Metric

• [ administrative distance / routing metric (or cost) ]

• The cost for all static routes is “0”

• The default administrative distance for static routes is “1”

Hoboken#show ip route

S 172.16.1.0/24 [1/0] is directly connected, Serial0

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Administrative Distance

• Administrative Distance is the “trustworthiness” of the routing information

• Lower the administrativedistance the more trustworthy the information

• If the router hears about a route to the same network from more than one

source it will use the administrative distance to decide which route to put in the

routing table

Examples from the curriculum

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“default route”

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Static Routing

• Some extra information on static routing that is not in the

curriculum

Static Routing

Router(config)#ip route prefix

destination-prefix-mask {address | interface} [distance] [tag tag]

[permanent]

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Static Routing

Configuring static routes

• Routers do not need to configure static routes for their own directly connected

networks

• We need to configure static routes for networks this router needs to reach

• We will need to configure static routes for the other routers as well, as “routing

information about a path from one network to another does not provide routing

information about the reverse, or return path.”

• Convergence–When all the routers in the network (AS) have accurate and

consistent information, so that proper routing and packet forwarding can take

place

• Convergence will not happen until all the routers have complete and accurate

routing information, meaning we must configure static routes on all the routers

before packets will be correctly delivered

Basic static route example

• Be sure to use the proper subnet mask!

RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2

S 192.168.1.0/24 [1/0] via 172.16.0.2

Network/subnet route

Intermediate-Address (usually “next-hop”)

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Basic static route example (continued)

• [1/0] – [ Administrative Distance / Metric ]

• Administrative Distance– This is the “trustworthiness” of the routing information The

default administrative distance of static routes is 1.

• The Administrative Distance of a directly connected route is 0.

• Lower the AD the more trustworthy.

• If the router learns about a route to a network from more than one source, it will install the

route with the lower administrative distance in the routing table.

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Recursive Lookup

• The router knows it can get to 192.168.1.0/24 network by forwarding the packets to the

router at the ip address of 172.16.0.2

• How does the router know how to get to the ip address 172.16.0.2?

• It does a recursive lookup – first (1) by looking up the 192.168.1.0/24 network and

finding it needs to forward the packet to 172.16.0.2 – the router then (2) looks up the

172.16.0.0 network and sees it can forward it out the interface Serial 0.

Configuring all of the static routes

• Notice that the intermediate-address is always the next-hop ip

address.

• Good idea to do a “copy running-config startup-config” if everything is

working right.

• To verify the routes are in there, you can do a:

Router# show running-config

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Recursive Lookup (continued)

• We can take this even further.

• One route can be used to resolve the route of another.

• It doesn’t matter how the routes are resolved, whether they are directly connected,

static or dynamic.

• Note: If an intermediate address cannot be resolved, that route and any routes it

affects are not installed in the routing table.

Static Routing – Recursive Lookups

Note regarding recursive route lookups

• Every route that does not reference an exit-interface must finally be resolved

via a route with an interface descriptor reference in the corresponding path

descriptor – a route with an exit-interface

• Static routes cannot be recursively resolved and will not be in the routing table

• Consider these three static routes:

Route1: ip route 10.1.0.0 255.255.0.0 20.1.1.1

Route2: ip route 20.1.0.0 255.255.0.0 30.1.1.1

Route3: ip route 30.1.0.0 255.255.0.0 10.1.1.1

• Route1 is resolved by Route2 which is resolved by Route3

• None of these routes are finally resolved via a route with an exit-interface

• This leads to endless recursion

• The routing table process will not permit these static routes to be entered in the

routing table

Static Routing – Recursive Lookups

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• Need only to use only an exit interface

• For point-to-point serial interfaces, the next-hop address in the routing table

is never used by the packet-delivery procedure, so it is not needed (It could

even reference a bogus IP address.)

• Notice that the static route appears in the routing table as directly connected,

however it is still a static route with an administrative distance of 1

S 192.168.1.0/24 is directly connected, Serial0

Static Routing – Point-to-Point Links

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Using both an intermediate address instead and an exit-interface:

• Notice we changed 172.16.0.0 to an Ethernet link

• Static routes via broadcast links, it is best to use both an exit interface and

intermediate address

• This saves the router from having to do a recursive route lookup for the

intermediate address of 172.16.0.2, knowing the exit interface is Ethernet 1

S 192.168.1.0/24 [1/0] via 172.16.0.2 Ethernet1

Static Routing – Ethernet interfaces

Static Route Rule of Thumb

Static routes via point-to-point links

• It is best to configure static routes with only the exit interface

• For point-to-point serial interfaces, the next-hop address in the routing table is

never used by the packet-delivery procedure, so it is not needed (It could

even reference a bogus IP address.)

Static routes via broadcast networks such as Ethernet

• It is best to configure static routes with both the next-hop addressand the

exit-interface

Using only an intermediate address

• “What about static routes referencing only intermediate network address? In

short, try to avoid using them The reason is that these static routes are not

bound to any interface, rely on intermediate address resolvability, and thus

converge more slowly They can also create unexpected routing loops.” Alex

Zinin, Cisco IP Routing

NOTE: Most of our examples in this course do not follow either of these

rules-of-thumb – but you may want to use it on your network

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