routing behaviour question (ip classful)

[Deleted]

so I get this question:

packets destined for servers on the 172.16.254.0 network are being dropped. What command should the administrator issue to ensure that these packets are sent out the gateway of last resort, Serial 0/0/1?

with this routing table(only wrote down the needed info):

172.16.0.0/24 is subnetted, 3 subnets
R 172.16.1.0
C 172.16.2.0
C 172.16.3.0
C 192.168.1.0/24
s* 0.0.0.0/0

the answer should be that the administrator has to use ip classless. But I dont understand why.
172.16.254.0 doesn't fit in the parent route since its a /24, so why doesn't it automaticly go to the default route?

[Entizer]

so I get this question:

packets destined for servers on the 172.16.254.0 network are being dropped. What command should the administrator issue to ensure that these packets are sent out the gateway of last resort, Serial 0/0/1?

with this routing table(only wrote down the needed info):

172.16.0.0/24 is subnetted, 3 subnets
R 172.16.1.0
C 172.16.2.0
C 172.16.3.0
C 192.168.1.0/24
s* 0.0.0.0/0

the answer should be that the administrator has to use ip classless. But I dont understand why.
172.16.254.0 doesn't fit in the parent route since its a /24, so why doesn't it automaticly go to the default route?

It has been a while since I have done this stuff, so I might be a little slow on it.

This is how I would understand it:

In your routing table, there is no 172.16.254.0 route. If there is no route to that address it will take your default route (I assume that is your default route at the end of the table you listed.)

However, if it is dropping packets (and the default route is for sure up and running properly) then that means it is going some other direction. The only way for it to want to take a different direction is if it thinks that a direction may contain that subnet, so you infer that classless is not on, otherwise it should not confuse the routes.

Not sure if it is worded different on the question you answered or what but from the info here and the expected answer, that is what I would assume.

EDIT: Check this link out, I think ti explains it well http://www.ccna4.com/2010/06/differe...ssful-and.html

When it is classful, the subnet information is not put into the routing table. Thus, it probably cannot distinguish between different networks that have the same 172.16.0.0 /16 subnet. With the classless, it can tell the difference between the different networks because the proper subnet information is propagated with the networks. I know I had this problem with RIP before when RIPv1 does not support classless and RIPv2 does.

[Deleted]

dropping means it doesn't send it, but descards it, so it wont send it in another direction, the general rule for ip classful is that it will assume the routing table is complete. So if the route would fit a parent route, but it doesn't fit a child route, it would be dropped, it would not look any further.
I think thats whats happening, but I dont understand why it would be said to fit a parent route... or I might be looking at it wrong.

I dont think the explenation you gave is correct, or I dont understand what you mean, thanks for trying though

EDIT: Check this link out, I think ti explains it well http://www.ccna4.com/2010/06/differe...ssful-and.html

When it is classful, the subnet information is not put into the routing table. Thus, it probably cannot distinguish between different networks that have the same 172.16.0.0 /16 subnet. With the classless, it can tell the difference between the different networks because the proper subnet information is propagated with the networks. I know I had this problem with RIP before when RIPv1 does not support classless and RIPv2 does.

well the subnet mask is not send, but there will still be a subnet mask in the routing table. hmm


edit 2: You're thinking about routing protocols, but here for my question we need to look at routing behaviour.

[Yohassakura]

I believe it's because, as you're on your last subnet (254) it will only do the ip addresses after there - 172.16.254.###

If you sent it to 172.16.1 instead, it should try to route through 172.16.1.### , 172.16.2.### and 172.16.3.###

[Entizer]

Upon more investigation, it appears you are correct. From what I am reading, if you do not have the ip classless command (meaning you are using classful routing) then default routes will not work in this scenario. So when it looks at the routing table, it does not have a suitable direction to go, so it discards the packets.

If packets were going to the 192.168.1.0 network, it would send it via it's route. If packets were to go to 172.16.1.0, it would send them via it's route. If something were sent for anything but those (10.0.1.0 for example) it would send it to the default route because there is no parent network listed (10.0.0.0). However, when the 172.16.254.0 has packets headed to it, it sees the parent network (172.16.0.0) and assumes that it can be no other way than one of those directions so it does not take the default route. It seems this is just the nature of Cisco routers and how they operate.

Using these sites as a reference:

http://www.cisco.com/en/US/tech/tk36...80094823.shtml (go to the classless section)
https://learningnetwork.cisco.com/thread/8085
http://visualland.net/view.php?cid=1...te-%20classful

[Deleted]

I looked at the cisco site, and there its the same example, only there it makes sence because its a /16, so it will only check the first bits to see if it fits.
But here, with a /24 it should check the first 24 bits, and then it shouldn't fit. I'll check the other sites to see if they have the right example explained.

EDIT: Ok I understand now, the default route is NEVER used in classful routing behaviour. (seems like you said that, but I read over it )

Thanks for the help (examn 2nd part of my CCNA course tomorrow )

[Entizer]

I'm trying to think how to put this into words and I'm sure this will come out terrible. With classfull, it only thinks in /8 /16 /24 unless it is specifically in the routing table. So when it sees 172.16 it thinks class B subnet and if there is no 172.16.254.0 listed under that 172.16 parent network, it does what I liked above. With class less it would do almost the same except it would send it to the default route.

When I send an IP address a ping, I don't tell it what subnet to go to. If a classful router receives it, it has to assume that it should go to 172.16.0.0 /16 because that is all it knows to do. Once classless is turned on, it realizes that there might just be other networks than just that 172.16.0.0 /16 so it sends it to the default route.

That is the best explanation I can do, sorry if it does not get my point across well.

EDIT: Cool, glad to see it answered it. It is good for me to know because I have never ran into that. Although, I've never not used classless cause classful is the way of the old

[Deleted]

what you say isn't true though, the class A, B, C has nothing to do with routing behaviour, only with classful routing protocols.
Routing behaviour starts when the routing table is already complete, and so the masks are already given. It will not look at classes, but at the mask given.
If you would use any classless routing protocol but you use classfull routing behaviour you will have a 10.0.0.0/18 for example.. it will compare the recieved IP with the 10.0.0.0 looking at /18, not /8 .
But as you said, no one uses classful, but we still have to learn it ofcourse ^^.