Is this question wrong?

borumas

Hey guys, studying for the ICND exam and in some of my test prep questions I have this one:
Which of the following is affected by whether a router is performing classful or classless routing?
a) When to use mask in routing updates
b) When to use a default route
c) When to perform queuing based on the classification of a packet into a particular queu
d) When to convert a packet's destination address into a network number

I believe it is a), because in classless routing a mask is included in routing updates and is not in classful updates but their answer is b). I have updated the test to hopefully fix errors but don't know if this is right or wrong, thanks in advance for any input.

markzab

I agree with your assesment as well. The MOST logical answer would be (A) to me.

markzab

Although...

"The way routers treat summary and default routes, as well as the algorithm of router operations, depends on the kind of environment—classful or classless—the routers work in."

http://www.awprofessional.com/articles/article.asp?p=24922&seqNum=7&rl=1

I would still then the MOST logical answer would be A.

Paul Boz

The best answer is definitely A.

"When to use default routes" is a poor answer because you can use default routes for static routes and with routing protocols.

Cucumber

The answer is B, a router operating in classful routing mode will not choose the defaul route correctly.

The term classful or classles routing is not the same as classful/classless routing protocols

A router may operate in classful or classless mode, regardles of the routing protocol being run.

When routing a packet the router will look at the destination address of the packet and will try to match that destination address with an entry in the routing table. The way the Router finds a match in the routing table depends on wether it is operating on Classful or Classles mode.

Classful Routing Mode

The router will compare classful networks. That means the router will figure out to which clasful network the packet pertains and will try to find a match using clasful networks from the routing table. This means the router will not pay attention to netmasks in the routing table.

Classless Routing Mode

The router will try to find the most exact match for the IP address paying attention to the netmasks included in each entry in the the routing table.

Example: Suppose a router has the following routing table:

Gateway of last resort is 172.17.1.2
10.1.0.0/16 is directly connected Ethernet 0
172.16.1.0/24, is available through serial 0
172.17.1.0/24, is available through serial 1


Lets assume an IP datagram arrives, having destination address 172.16.3.x.

Using classful routing, the packet would be sent through serial 0.
Because the router will try to match the incoming packet classful network: 172.16.0.0
To the network table: 10.0.0.0 ethernet 0
172.16.0.0 serial 0
172.17.0.0 serial 1
Using classless routing, the packet would be sent through the default gateway,
because the destination address 172.16.3.x is not in any subnet
in the routing table.
That would be the correct behaviour.

In order to make the router operate in classles mode you issue:

(config)#ip classless

Which is the default in later IOSes

update: corrected some typos sorry about being so verbose

EdTheLad

Yep B!