CCNA Exp2 - Chapter08 - The Routing Table_A Closer Look

8/3/2019 CCNA Exp2 - Chapter08 - The Routing Table_A Closer Look

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Chapter 8 -The Routing Table: ACloser Look

CCNA Exploration 4.0



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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.




Introduction

• Chapter Focus- Structure of the routing table.- Lookup process of the routing table.- Classless and classful routing behaviors.




Routing Table Structure




Lab Topologies





• Routing table entries come from the following sources – Directly connected networks – Static routes

– Dynamic routing protocols


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Level 1 Routes• As soon as the no shutdown command is issued the route is

added to routing table




• Cisco IP routing tableis a hierarchicalstructure.The reasonfor this is to speed uplookup process



• A level 1 route is a route with a subnet mask equal to or lessthan the classful mask of the network address. 192.168.1.0/24is a level 1 network route, because the subnet mask is equal tothe network's classful mask. /24 is the classful mask for class Cnetworks, such as the 192.168.1.0 network.

• A level 1 route can function as a: – Default route - A default route is a static route with the

address 0.0.0.0/0. – 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

mask equal to that of the classful mask. A network routecan also be a parent route . Parent routes will be discussedin the next section.




Ultimate Route• The level 1 route 192.168.1.0/24 can be further defined as an

ultimate route. An ultimate route is a route that includes: – Either a next-hop IP address (another path) – And/or an exit interface




• Parent and ChildRoutes – A parent route is

a level 1 route

– A parent routedoes not containany next-hop IPaddress or exitinterface

information• The subnet is the

level 2 child route ofthe parent route.




Classful Networks

• Automatic creation ofparent routes – Occurs any time a

subnet is added to therouting table

• Child routes – Child routes are level2 routes.

– Child routes are asubnet of a classful

network address.






• Both child routes have the same subnet mask – This means the parent route maintains the /24 mask




• In classless networks, child routes do not have to share the same

subnet mask




NetworkType

Parent route’sClassful mask is

Displayed

Termvariably subnettedis seen in parentroute in routing

table

Includes the #of different

masks of childroutes

Subnet maskincluded with

each childroute entry

Class-ful

No No No No

Class-less Yes Yes Yes Yes

Parent & Child Routes in classless & classful network



• Parent & Child Routes: Classless Networks

Parent & Child Routes in classless & classful network



Routing Table Lookup Process



Discontiguous

• Classfulrouting withdiscontiguous



Result routing





Examine level 1 routes

– If best match a level 1 ultimate route and is not a parent routethis route is used to forward packet – If the best match is a level 1 parent route examines level 2

routesRouter examines level 2 (child) routes – If there is a match with level 2 child route then that subnet is

used to forward packet – If no match then – Determine routing behavior typeRouter determines classful or classless routing behavior – If classful then packet is dropped – If classless then router searches level one supernet and default

routes – If there exists a level 1 supernet or default route match then

packet is forwarded. If not packet is dropped




• Longest Match: Level 1 Network Routes

– Best match is a lso known a s the longest match. – The best match is the one that has the most number of left

most bits matching between the destination IP address andthe route in the routing table.




• Finding the subnet maskused 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 packetsout s0/0/0





• 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.






• Finding a match between packet’s destination IP address and the nextroute in the routing tableThe figure shows a match between the destination IP of 192.168.1.0 andthe level one IP of 192.168.1.0 / 24 then packet forwarded out s0/0/0




Level 1 Parent & Level 2 Child Routes• Before level 2 child routes are examined, there must be a match

between classful level one parent route and destination IPaddress.





• After the match with parent route has been made Level 2 child

routes will be examined for a match.• Route lookup process searches for child routes with a match

with destination IP.









Matches Parent Route23 rd bit does not match. First 24 bitmuch match next route entry

24 rd bit does not match. First 24 bit

much match next route entryFirst 24 bits match


• How a router finds a match with one of the level 2 child routes

– First 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

Destination ofIP Packet

172.16.3.10 10101100 00010000 00000011 00001010

Level 1 ParentRoute 172.16.0.0/16 10101100 00010000 00000011 00001010

Level 2 ChildRoute 172.16.1.0/24 10101100 00010000 00000001 00001010






• Example: Route LookupProcess with VLSM

– The use of VLSM does notchange the lookupprocess.

– If there is a matchbetween destination IPaddress and the level 1parent route then Level 2

child routes will besearched.





Routing Behavior




Routing Behavior

• Classful & classless routingprotocols

– Influence how routing table ispopulated

• Classful & classless routingbehaviors – Determines how routing table is

searched after it is filled




Routing Behavior




• Classful RoutingBehavior: no ipclassless

• What happens if thereis not a match with anylevel 2 child routes ofthe parent? – Router must determine

if the routing behavioris classless or classful.

– If router is utilizingclassful routingbehavior then lookupprocess is terminatedand packet is dropped.

• The commands no ip classless and ip

classless are global configurationcommands and can be viewed bytyping show running-config.

• In IOS versions 11.3 and later, thecommand ip classless is the default,implementing a classless route lookupprocess.

Routing Behavior




• Classful Routing Behavior – Search Process

• An example of when classful routing behavior is in effect and whythe router drops the Packet – The destination’s subnet mask is a /24 and none of the child

routes left most bits match the first 24 bits. This meanspacket is dropped

Routing Behavior




First 24 bits must match

Routing Behavior




Classful Routing Behavior – Search Process:

• The routing table process will not use the default route, 0.0.0.0/0,or any other route.

• A common error is to assume that a default route will always beused if the router does not have a better route. In our example,R2's default route is not examined nor used, although it is a

match. This is often a very surprising result when a networkadministrator does not understand the difference between classfuland classless routing behavior.

• The reason why the router will not search beyond the child routesOriginally networks were all classful.

This meant an organization could subnet a major networkaddress and “enlighten” all the organization’s routers aboutthe subnetting.Therefore, if the subnet was not in the routing table, thesubnet did not exist and packet was dropped.

Routing Behavior






• Route lookup process when ip classless is in use – If classless routing behavior in effect then

Search level 1 routesSupernet routes Checked first

– If a match exists then forward packet

– Default routes Checked second – If there is no match or no default route then the packet is dropped.

Routing Behavior: ip classless




Routing Behavior

h




Routing Behavior

h




Routing Behavior



R i B h i




• Classful vs . Classless Routing Behavior – It is recommended to use classless routing behavior so supernet

and default routes can be used whenever needed – Remember that classful and classless routing behaviors are

independent from classful and classless routing protocols . A routercould be configured with classful routing behavior ( no ip classless )and a classless routing protocol, such as RIPv2. A router could alsobe configured with classless routing behavior ( ip classless ) and aclassful routing protocol, such as RIPv1.

Routing Behavior

S




Summary






Documents

CCNA Exp2 - Chapter08 - The Routing Table_A Closer Look