Exp 2 Ch 8 Child Parent Route TonyChen

8/3/2019 Exp 2 Ch 8 Child Parent Route TonyChen

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2007 Cisco Systems, Inc. All rights reserved. Cisco PublicITE PC v4.0Chapter 1 1

The Routing Table: A

Closer Look

Routing Protocols and Concepts Chapter 8Modified by Tony Chen

04/01/2008


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ITE PC v4.0Chapter 1 2 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Notes:

If you see any mistake on my PowerPoint slides or ifyou have any questions about the materials, pleasefeel free to email me at [email protected].

Thanks!

Tony Chen

College of DuPage

Cisco Networking Academy


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Objectives

Describe the various route types found in the routingtable structure

Describe the routing table lookup process.

Describe routing behavior in routed networks.


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Introduction

Chapter Focus

Structure of the routing table

Will examine the format of the routing table and learn about

level 1 and level 2 routes.

Lookup process of the routing table

Classless and classful routing behaviors

Cisco IP Routing, by Alex Zinin (ISBN 0-201-60473-6).


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Routing Table Structure Lab Topology

3 router setup

-R1 and R2 share a common 172.16.0.0/16 network with172.16.2.0/24 subnets.

-R2 and R3 are connected by the 192.168.1.0/24 network.

-R3 also has a 172.16.4.0/24 subnet, which is disconnected, ordiscontiguous, from the 172.16.0.0 network that R1 and R2share.

In a later section, we will configurethe interfaces for R2


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Routing Table Structure

The figure shows routing table entries come from thefollowing sources

-Directly connected networks

-Static routes

-Dynamic routing protocols


7/46ITE PC v4.0Chapter 1 7 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Routing Table Structure The figure shows what happens as the Serial 0/0/1 interface for R2 is

configured with the 192.168.1.1/24 address. R1 and R3 already have their interfaces configured with the appropriateIP addresses and subnet masks.

We will now configure the interfaces for R2 and use debug ip routing toview the routing table process that is used to add these entries.

As soon as the no shutdown command is issued the route is addedto routing table

debug ip routing




Cisco IP routing tableis a hierarchicalstructure

The reason for this isto speed up lookupprocess

The hierarchy

includes several levels.level 1

level 2



Routing Table Structure Level 1 Routes

Have a subnet mask equal to or less than the classfulmask of the network address.

192.168.1.0/24 is a level 1 network route, because thesubnet mask is equal to the network's classful mask. /24for class C networks, such as the 192.168.1.0 network.

Level 1 route can function as

Default route

A default route is a static route with the address0.0.0.0/0.

Supernet route

A supernet route is a network address with a mask

less than the classful mask.Network route

A network route is a route that has a subnet maskequal to that of the classful mask.

The source of the level 1 route can be a directlyconnected network, static route, or a dynamic routingprotocol.



Routing Table Structure The level 1 route 192.168.1.0/24 can be further defined as an

ultimate route.ultimate route includes either:

-A next-hop ip address (another path)

OR

-An exit interface

The directly connected network 192.168.1.0/24It is a level 1 network route because it has a subnet mask that is the same asits classful mask.

This same route is also an ultimate route because it contains the exit interfaceSerial 0/0/1.



A parent route is a level 1 route

A parent routedoes not containany next-hop IP address orexitinterface information

When the 172.16.3.0 subnet wasadded to the routing table, anotherroute, 172.16.0.0, was also added.

The first entry, 172.16.0.0/24, doesnot contain any next-hop IP addressor exit interface information.

This route is known as a level 1

parent route.

A parent route is actually a headingthat indicates the presence of level 2routes, also known as child routes.

Parent and Child Routes



Routing Table Structure A level 1 parent route is automatically

created any time a subnet is added tothe routing table.

In other words, a parent route iscreated whenever a route with a maskgreater than the classful mask is enteredinto the routing table.

172.16.0.0/24 is subnetted, 1 subnets

A level 2 route is a route that is asubnet of a classful network address.

Child routes are level 2 routes

Child routes are a subnet of aclassful network address

C 172.16.3.0 is directly connected,FastEthernet0/0


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Routing Table Structure The parent route contains the 172.16.0.0 - The classful network

address for our subnet. Level 2 child routes contain 172.16.3.0, route source & the

network address of the route

Notice that the subnet mask is not included with the subnet, the level2 child route. The subnet mask for this child route (subnet) is the /24mask included in its parent route, 172.16.0.0

Level 2 child routesare also considered ultimate routes

Reason: they contain the next hop address &/or exit interface


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Routing Table Structure The figure shows the

configuration of the Serial0/0/0 interface on R2.

The routing table showstwo child routes for the

same 172.16.0.0/24 parentroute.

Both 172.16.2.0 and172.16.3.0 are membersof the same parent route,

because they are bothmembers of the172.16.0.0/16 classfulnetwork


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Both child routes have the same subnet mask

-This means the parent route maintains the /24 mask

Note: If there is only asingle level 2 child routeand that route isremoved, the level 1

parent route will beautomatically deleted. Alevel 1 parent routeexists only when thereis at least one level 2child route.

The role of the parent route will be examined when we discuss the route lookup process.


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Inclassless networks, child routes do not have to sharethe same subnet mask

Whenever there are two or more child routes with differentsubnet masks belonging to the same classful network, therouting table presents a slightly different view, which states that

this parent network is variably subnetted.


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Parent & Child Routes: Classless Networks


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Parent & Child Routes: classful and classless Networks

NetworkType

Parent routesClassfulmask is

Displayed

Termvariably

subnettedis seen in

parent routein routing

table

Includes the #of differentmasks of

child routes

Subnet maskincluded with

each childroute entry

Class-ful

No No No No

Class-less Yes Yes Yes Yes

classful

classless


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Routing Table Lookup Process The Route Lookup Process

1. Examine level 1 routes If best match a level 1 ultimate route

and is not a parent route this route isused to forward packet

If the best match is a level 1 parentroute, proceed to Step 2

2. Router examines level 2 (child) routes

If there is a match with level 2 childroute then that subnet is used toforward packet

If no match then proceed to Step 3

3. Router determines classful orclassless routing behavior

If classful then packet is dropped If classless then router searches level

one supernet and default routes

4. If there exists a level 1 supernet ordefault route match then Packet isforwarded

5. If not packet is dropped


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Routing Table Lookup Process Longest Match: Level 1 Network Routes

Best match is also known as the longest match

The best match is the one that has the most number of left mostbits matching between the destination IP address and the route inthe routing table.

For example, in the figure we have a packet destined for 172.16.0.10.Many possible routes could match this packet. Three possible routesare shown that do match this packet: 172.16.0.0/12, 172.16.0.0/18,and 172.16.0.0/26. Of the three routes, 172.16.0.0/26 has the longestmatch.


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Routing Table Lookup Process Finding the subnet mask

used to determine thelongest match

Scenario:

PC1 pings 192.168.1.2

Router examines level 1route for best match

There exist a matchbetween192.168.1.2 &192.168.1.0 / 24

Router forwards packets outs0/0/0


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Routing Table Lookup Process The process of matching

1st there must be a match made between the parent route &destination IP

If a match is made then an attempt at finding a match

between the destination IP and the child route is made.

Do at least 16 of the left-most bits of the parent route match thefirst 16 bits of the packet's destination IP address of 192.168.1.2?

The answer, no,


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Routing Table Lookup Process

Finding a match between packets destination IP address andthe next route in the routing table

The figure shows a match between the destination IP of 192.168.1.0and the level one IP of 192.168.1.0 / 24 then packet forwarded out

s0/0/0Not only does the minimum of 24 bits match, but a total of 30 bitsmatch, as shown in the figure.


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In the example in the figure, PC1 sends a ping to PC2 at172.16.3.10. What happens when there is a match with alevel 1 parent route?

Before level 2 child routes are examined

-There must be a match between classful level oneparent route and destination IP address.


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Routing Table Lookup Process After the match with parent route has been made Level 2 child

routes will be examined for a match

-Route lookup process searches for childroutes with a match with destination IP


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Routing Table Lookup Process How a router finds a match with one of the level 2

child routesFirst router examines parent routes for a match

If a match exists then:

Child routes are examined

Child route chosen is the one with the

longest match First, the router examines the parent route for a

match.

The router checks the last child route for172.16.3.0/24 and finds a match. The first 24 bits

do match. The routing table process will use thisroute, 172.16.3.0/24, to forward the packet withthe destination IP address of 172.16.3.10 out theexit interface of Serial 0/0/0.

R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25,Serial0/0/0


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Example: Route LookupProcess with VLSM

-The use of VLSM does notchange the lookup process

-If there is a match betweendestination IP address and thelevel 1 parent route then

-Level 2 child routes will be

searched


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Routing Behavior Classful & classless routing protocols

Influence how routing table is populated

Classful & classless routing behaviors

Determines how routing table is searched after it isfilled


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Routing Behavior Classful Routing

Behavior: no ip classless

What happens if there isnot a match with anylevel 2 child routes of the

parent?-Router must determine ifthe routing behavior isclassless or classful

-If router is utilizing

classful routing behaviorthen

-Lookup process isterminated andpacket is dropped

ip classless and no ip classless


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Using the ip classless command (cont.)

What is IP Classless?

The "ip classless" command prevents the existence of a single "subnet" route from blocking access via thedefault route to other subnets of the same old-style network. Default only works with single-homed ISPs.

RFC 1879

IP classless command is not easy to understand, we know that. But I bet, after you read the following lines, you willunderstand what it is all about.

First, you must understand a very simple logic. Here is the logic: Me and you are on a journey. If you break my leg, thenyou must carry me all the way! If you understand this logic, you will understand "IP classless".

RIP is telling you: I am classful, if you break my class, then you have to show me every route there is, or I will drop your

packet. I will drop it even though there is a default route (0.0.0.0). What is classful? Classful means that a class A subnet should be shown as x.0.0.0 such as 10.0.0.0 255.0.0.0

If you show it as 10.44.0.0 255.255.0.0, you are breaking its class.

Or, a class B subnet should be shown as x.x.0.0 255.255.0.0 such as 172.29.0.0 255.255.0.0

If you show it as 172.29.26.0 255.255.255.0, you are breaking its class.

Lets assume RIP knows about 10.0.0.0

If you break 10.0.0.0 into three, for example to 10.1.0.0 and 10.2.0.0 and 10.3.0.0, and then give RIP a packet with adestination of10.4.0.1, RIP will drop it. Why? Why doesnt RIP send the packet to the default route?

Because RIP told you, if you break my class, then you have to show me every damn route, otherwise I will drop it. Here,you broke RIP's class so you must show him the way to 10.4.0.1 and every other 10.x.x.x route in the universe.Otherwise RIP will drop the packet, even if there is a default route. RIP will not care about your default route or lastresort gateway; it will drop your packet.

How do you ask RIP not to drop your packet and send the unknown destinations to the default route, although you havebeen so mean to him and have broken its class? You tell him: please, please, ip classless!

If no ip classless, drop the packet

If ip classless, send the packet to the default.

http://www.networkking.net/out/IPClassless.htm


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Routing Behavior

ip Classless Beginning with IOS 11.3, ip classless was

configured by default

The command no ip classless means that theroute lookup process uses classful routing tablelookups by default.

Classless routing behaviorworks for

-Discontiguous networks

And-CIDR supernets


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Routing Behavior Classful Routing Behavior Search Process

when classful routing behavior is in effect (no ipclassless) the process will not continue searchinglevel 1 routes in the routing table. If a packet doesn'tmatch a child route for the parent network route, thenthe router drops the packet.

R2 receives a packet destined for PC3 at

172.16.4.10.Even with the default route configured.

The destinations subnet mask is a /24 and none ofthe child routes left most bits match the first 24 bits.This means packet is dropped

no ip classless


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Routing Behavior

Classful Routing Behavior SearchProcess

The reason why the router will not searchbeyond the child routes

At the beginning of the Internet'sgrowth, networks were all classful

This meant an organization couldsubnet a major network address andenlighten all the organizationsrouters about the subnetting

Therefore, if the subnet was not in the

routing table, the subnet did not existand packet was dropped

The routing table process will not use thedefault route, 0.0.0.0/0, or any otherroute.

no ip classless


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Routing Behavior

The routing table process will notuse the default route, 0.0.0.0/0, orany other route.

A common error is to assume that adefault route will always be used if the

router does not have a better route.In our example, R2's default route isnot examined nor used, although it is amatch.

This is often a very surprising result

when a network administrator does notunderstand the difference betweenclassful and classless routingbehavior.

no ip classless


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Routing Behavior

Classless Routing Behavior-

ip lassless

Step 3: Ifclassless routing behaviorineffect then, continue searching level 1supernet routes in the routing table for amatch, including the default route, if there isone.

Step 4: Match with supernet or default

Supernet routes Checked first

If a match exists then forward packet

Default routes Checked second

Step 5: If there is no match or no default

route then the Packet is dropped

ip classless


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Routing Behavior

Classless Routing Behavior Search Process

Router begins search process by finding a match betweendestination IP and parent route

After finding the above mentioned match, then there is asearch of the child route

There is no match with the level 2 child routes.

ip classless


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Routing Behavior Ifno match is found in child routes of

previous slide thenRouter continues to search therouting table for a match that mayhave fewer than 16 bits in the match

The 192.168.1.0/24 route does nothave 24 left-most bits that match thedestination IP address.

C 192.168.1.0/24 is directly connected, Serial0/0/1

ip classless


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Routing Behavior

S* 0.0.0.0/0 is directly connected, Serial0/0/1

The mask is /0, which means thatzero or no bits need to match.

A default route will be the lowest-

bit match. In classless routingbehavior, if no other routematches, the default route willmatch.

In this case the router will use thedefault route, because it is the bestmatch. The packet will be forwardedout the Serial 0/0/1 interface.

ip classless


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Routing Behavior

What does R3 do with returntraffic back to PC2 at172.16.2.10?

In this case, R3 uses the172.16.0.0/16 child route andforwards the traffic out Serial 0/0/1back to R2.


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Routing Behavior Classfulvs. ClasslessRouting Behavior

-It is recommended to use classless routingbehavior

Reason: so supernet and default routes can

be used whenever needed


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Longest Match

http://www.cisco.com/warp/public/105/21.html Let's look at the three routes we just installed in the routing table, and see

how they look on the router.

router# show ip route

....

D 192.168.32.0/26 [90/25789217] via 10.1.1.1 ---- (192.168.32.0 to 192.168.32.63)

R 192.168.32.0/24 [120/4] via 10.1.1.2 ---- (192.168.32.0 to 192.168.32.255)

O 192.168.32.0/19 [110/229840] via 10.1.1.3 ---- (192.168.32.0 to 192.168.63.255)

....

If a packet arrives on a router interface destined for192.168.32.1, whichroute would the router choose?

If a packet arrives on a router interface destined for192.168.32.100,which route would the router choose?

Answers are on the next page

POP QUIZ


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Longest Matchhttp://www.cisco.com/warp/public/105/21.html

Let's look at the three routes we just installed in the routing table, and see howthey look on the router.

router# show ip route

....

D 192.168.32.0/26 [90/25789217] via 10.1.1.1 ---- (192.168.32.0 to 192.168.32.63)

R 192.168.32.0/24 [120/4] via 10.1.1.2 ---- (192.168.32.0 to 192.168.32.255)

O 192.168.32.0/19 [110/229840] via 10.1.1.3 ---- (192.168.32.0 to 192.168.63.255)....

If a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because192.168.32.1 falls within the 192.168.32.0/26 network (192.168.32.0 to192.168.32.63). It also falls within the other two routes available, but the192.168.32.0/26 has the longest prefix within the routing table (26 bits verses 24or 19 bits).

if a packet destined for192.168.32.100 arrives on one of the router's interfaces,it's forwarded to 10.1.1.2, because 192.168.32.100 doesn't fall within192.168.32.0/26 (192.168.32.0 through 192.168.32.63), but it does fall within the192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it alsofalls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longerprefix length


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Summary

Content/structure of a routing table Routing table entries

-Directly connected networks

-Static route

-Dynamic routing protocols

Routing tables are hierarchical

-Level 1 route

Have a subnet mask that is less than or equal to

classful subnet mask for the network address-Level 2 route

These are subnets of a network address


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SummaryRouting table lookup process

Begins with examining level 1 routes for best match with packets destination IP If the best match = an ultimate route then

-Packet is forwarded -Else-

-Parent route is examined

If parent route & destination IP match then Level 2 (child)routes are examined

Level 2 route examination

If a match between destination IP and child route found thenPacket forwarded -Else

If Router is using classful routing behavior thenPacket is dropped -Else

If router is using classless routing behavior then

Router searches Level 1 supernet & default routes for amatch

If a match is found then Packet if forwarded -Else Packet is dropped


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Summary

Routing behaviors

-This refers to how a routing table is searched

Classful routing behavior

-Indicated by the use of the no ip classless command

-Routerwill not look beyond child routes for a lessermatch

Classless routing behavior

-Indicated by the use of the ip classless command

-Router will look beyond child routes for a lesser match


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Exp 2 Ch 8 Child Parent Route TonyChen

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