Packet Tracer issues.

I was at my cousin's and he was showing me some little problems that he thought I must be proficient at solving at this point of my training towards the CCNA. I met his expectations but then i started messing up with packet tracer and now im confused.

The thing is I was setting up a little internetwork that I thought was not going to work because the use of VLSM using RIP v1 as routing protocol. To my surprise, the internetwork could effectively communicate with all hosts from all routers. I dont get it. I have three ROUTERS connected with serial links and i used /30 CIDR AT all of them. I have two host connected to one of them and one at each of the other two, with no subnets on the networks where the hosts are...

Isnt this supposed to not work? It is working and i just dont get it...

I was considering installing packet tracer because dynamips makes it hard to simulate hosts and also it is too heavy when the topology has more than a 3-4 routers.

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Comments

EdTheLad

AlexMR wrote:

I have three serial links and i used /30 CIDR AT all of them. The I have two host connected to one of them and one to other two, with no subnets on the networks where the hosts are...

tech-airman

AlexMR wrote:

I was at my cousin's and he was showing me some little problems that he thought I must be proficient at solving at this point of my training towards the CCNA. I met his expectations but then i started messing up with packet tracer and now im confused.

The thing is I was setting up a little internetwork that I thought was not going to work because the use of VLSM using RIP v1 as routing protocol. To my surprise, the internetwork could effectively communicate with all hosts from all routers. I dont get it. I have three ROUTERS connected with serial links and i used /30 CIDR AT all of them. I have two host connected to one of them and one at each of the other two, with no subnets on the networks where the hosts are...

Isnt this supposed to not work? It is working and i just dont get it...

I was considering installing packet tracer because dynamips makes it hard to simulate hosts and also it is too heavy when the topology has more than a 3-4 routers.

AlexMR,

You mentioned that physically, three routers are connected to each other through serial links. So I'm visualizing the three routers are the corners of a triangle. So I was wondering if you can tell me what three subnetwork addresses with the subnet mask of /30 that you used on each of the three serial links?

c0d3_w0lf

Depending on your IP addressing scheme, it could still work.

It's not so much the fact that you're using /30s that would mess it up, it's WHERE those /30s are coming from. For instance, if all of the /30s were coming from the same classful network, like 192.168.1.0, then I would question what's going on. But if you have three routers, let's call them A, B, and C, and they're set up to where:

Link AB is using a /30 on 192.168.1.0
Link BC is using a /30 on 192.168.2.0
Link CA is using a /30 on 192.168.3.0

Then it should run just fine. Hopefully that explanation made some sort of sense. It's very late and I am braintarded.

EDIT: I guess my main point is that the reason that RIP v1 doesn't support classless routing and VLSM is because it doesn't advertise the subnet mask in its updates. You can still configure all of your interfaces to use /30 subnet masks and if your IP scheme is set up right, it won't interfere with RIP v1 routing.

AlexMR

c0d3_w0lf wrote:

Depending on your IP addressing scheme, it could still work.

It's not so much the fact that you're using /30s that would mess it up, it's WHERE those /30s are coming from. For instance, if all of the /30s were coming from the same classful network, like 192.168.1.0, then I would question what's going on. But if you have three routers, let's call them A, B, and C, and they're set up to where:

Link AB is using a /30 on 192.168.1.0
Link BC is using a /30 on 192.168.2.0
Link CA is using a /30 on 192.168.3.0

Then it should run just fine. Hopefully that explanation made some sort of sense. It's very late and I am braintarded.

EDIT: I guess my main point is that the reason that RIP v1 doesn't support classless routing and VLSM is because it doesn't advertise the subnet mask in its updates. You can still configure all of your interfaces to use /30 subnet masks and if your IP scheme is set up right, it won't interfere with RIP v1 routing.

Well that makes sense. Thx for your contribution. The thing is that I have ANOTHER lan connected to all routers and those are not subnetted. Maybe RIPv1 works in this scenario? I am going to be patient and check it with dynamips just to confirm. I am also going to subnet the lan with the hosts at all routers just to check if RIP 1 works if the new IP scheme...im just going through your post again and I think there is something worng there. I mean, if you subnet all lans with the same subnet you wont have a problem. The thing here is that I have /30 on all serial links but I also have three /24 LANs connected to each router.

Anybody else?

tech-airman

AlexMR wrote:

c0d3_w0lf wrote:

Depending on your IP addressing scheme, it could still work.

It's not so much the fact that you're using /30s that would mess it up, it's WHERE those /30s are coming from. For instance, if all of the /30s were coming from the same classful network, like 192.168.1.0, then I would question what's going on. But if you have three routers, let's call them A, B, and C, and they're set up to where:

Link AB is using a /30 on 192.168.1.0
Link BC is using a /30 on 192.168.2.0
Link CA is using a /30 on 192.168.3.0

Then it should run just fine. Hopefully that explanation made some sort of sense. It's very late and I am braintarded.

EDIT: I guess my main point is that the reason that RIP v1 doesn't support classless routing and VLSM is because it doesn't advertise the subnet mask in its updates. You can still configure all of your interfaces to use /30 subnet masks and if your IP scheme is set up right, it won't interfere with RIP v1 routing.

Well that makes sense. Thx for your contribution. The thing is that I have ANOTHER lan connected to all routers and those are not subnetted. Maybe RIPv1 works in this scenario? I am going to be patient and check it with dynamips just to confirm. I am also going to subnet the lan with the hosts at all routers just to check if RIP 1 works if the new IP scheme...im just going through your post again and I think there is something worng there. I mean, if you subnet all lans with the same subnet you wont have a problem. The thing here is that I have /30 on all serial links but I also have three /24 LANs connected to each router.

Anybody else?

AlexMR,

Your answer to my question is what I'm waiting for.

c0d3_w0lf

AlexMR wrote:

c0d3_w0lf wrote:

Depending on your IP addressing scheme, it could still work.

It's not so much the fact that you're using /30s that would mess it up, it's WHERE those /30s are coming from. For instance, if all of the /30s were coming from the same classful network, like 192.168.1.0, then I would question what's going on. But if you have three routers, let's call them A, B, and C, and they're set up to where:

Link AB is using a /30 on 192.168.1.0
Link BC is using a /30 on 192.168.2.0
Link CA is using a /30 on 192.168.3.0

Then it should run just fine. Hopefully that explanation made some sort of sense. It's very late and I am braintarded.

EDIT: I guess my main point is that the reason that RIP v1 doesn't support classless routing and VLSM is because it doesn't advertise the subnet mask in its updates. You can still configure all of your interfaces to use /30 subnet masks and if your IP scheme is set up right, it won't interfere with RIP v1 routing.

Well that makes sense. Thx for your contribution. The thing is that I have ANOTHER lan connected to all routers and those are not subnetted. Maybe RIPv1 works in this scenario? I am going to be patient and check it with dynamips just to confirm. I am also going to subnet the lan with the hosts at all routers just to check if RIP 1 works if the new IP scheme...im just going through your post again and I think there is something worng there. I mean, if you subnet all lans with the same subnet you wont have a problem. The thing here is that I have /30 on all serial links but I also have three /24 LANs connected to each router.

Anybody else?

Well, in all honesty, without seeing the actual IP addressing scheme it's hard to say for sure. But, I think my original point still stands. For instance, lets say you have the scheme I described above for your serial links. Now let's add three lans into the equation, lans A, B, and C (named obviously for the routers they attach to). If your addressing scheme as a whole looks like:

Link AB is using a /30 on 192.168.1.0
Link BC is using a /30 on 192.168.2.0
Link CA is using a /30 on 192.168.3.0
Lan A is using 192.168.4.0/24
Lan B is using 192.168.5.0/24
Lan C is using 192.168.6.0/24

RIP v1 will still work properly. The whole idea behind RIPv1 getting confused is discontiguous subnets. If you're using the above addressing scheme, there are going to be 6 different routing table entries, all for /24 blocks since it's classful routing. For instance, the advertisement for the AB serial link would be 192.168.1.0/24. You may have configured the interfaces with a /30 mask, but RIPv1 doesn't pay attention to that.

Now, if you had it set up to where you were using just a singe /24 to feed all of your serial links, say like

Link AB using 192.168.1.0/30
Link BC using 192.168.1.4/30
Link CA using 192.168.1.8/30

This would NOT work in RIPv1. RIP would be seeing a bunch of 192.168.1.0/24 routes and would go crazy. Hopefully this explanation was a little bit clearer...I'm not used to trying to explain this stuff, but I want to become a more active participant in these forums so I'm giving it a shot. Some of the more experienced folk at the forums may have a better way of putting it. :P

And, of course it could be that all that is moot and it's just packet tracer being weird :P. But I'm pretty sure my theory is sound on this one.

AlexMR

tech-airman wrote:

AlexMR wrote:

I was at my cousin's and he was showing me some little problems that he thought I must be proficient at solving at this point of my training towards the CCNA. I met his expectations but then i started messing up with packet tracer and now im confused.

The thing is I was setting up a little internetwork that I thought was not going to work because the use of VLSM using RIP v1 as routing protocol. To my surprise, the internetwork could effectively communicate with all hosts from all routers. I dont get it. I have three ROUTERS connected with serial links and i used /30 CIDR AT all of them. I have two host connected to one of them and one at each of the other two, with no subnets on the networks where the hosts are...

Isnt this supposed to not work? It is working and i just dont get it...

I was considering installing packet tracer because dynamips makes it hard to simulate hosts and also it is too heavy when the topology has more than a 3-4 routers.

AlexMR,

You mentioned that physically, three routers are connected to each other through serial links. So I'm visualizing the three routers are the corners of a triangle. So I was wondering if you can tell me what three subnetwork addresses with the subnet mask of /30 that you used on each of the three serial links?

Sorry man. I didnt see your post. Apparently we posted about at the same time...

OK. You're right about the triangle. The subnetworks are 192.168.2.0/30, 92.168.3.0/30 and 192.168.4.0/30. The LANs connected to FE ports of the routers are 192.168.1.0/24(a switch and two hosts), 192.168.5.0/24 (one host) and 192.168.6.0/24 (one host).

c0d3_w0lf wrote:

AlexMR wrote:

c0d3_w0lf wrote:

Depending on your IP addressing scheme, it could still work.

It's not so much the fact that you're using /30s that would mess it up, it's WHERE those /30s are coming from. For instance, if all of the /30s were coming from the same classful network, like 192.168.1.0, then I would question what's going on. But if you have three routers, let's call them A, B, and C, and they're set up to where:

Link AB is using a /30 on 192.168.1.0
Link BC is using a /30 on 192.168.2.0
Link CA is using a /30 on 192.168.3.0

Then it should run just fine. Hopefully that explanation made some sort of sense. It's very late and I am braintarded.

EDIT: I guess my main point is that the reason that RIP v1 doesn't support classless routing and VLSM is because it doesn't advertise the subnet mask in its updates. You can still configure all of your interfaces to use /30 subnet masks and if your IP scheme is set up right, it won't interfere with RIP v1 routing.

Well that makes sense. Thx for your contribution. The thing is that I have ANOTHER lan connected to all routers and those are not subnetted. Maybe RIPv1 works in this scenario? I am going to be patient and check it with dynamips just to confirm. I am also going to subnet the lan with the hosts at all routers just to check if RIP 1 works if the new IP scheme...im just going through your post again and I think there is something worng there. I mean, if you subnet all lans with the same subnet you wont have a problem. The thing here is that I have /30 on all serial links but I also have three /24 LANs connected to each router.

Anybody else?

Well, in all honesty, without seeing the actual IP addressing scheme it's hard to say for sure. But, I think my original point still stands. For instance, lets say you have the scheme I described above for your serial links. Now let's add three lans into the equation, lans A, B, and C (named obviously for the routers they attach to). If your addressing scheme as a whole looks like:

Link AB is using a /30 on 192.168.1.0
Link BC is using a /30 on 192.168.2.0
Link CA is using a /30 on 192.168.3.0
Lan A is using 192.168.4.0/24
Lan B is using 192.168.5.0/24
Lan C is using 192.168.6.0/24

RIP v1 will still work properly. The whole idea behind RIPv1 getting confused is discontiguous subnets. If you're using the above addressing scheme, there are going to be 6 different routing table entries, all for /24 blocks since it's classful routing. For instance, the advertisement for the AB serial link would be 192.168.1.0/24. You may have configured the interfaces with a /30 mask, but RIPv1 doesn't pay attention to that.

Now, if you had it set up to where you were using just a singe /24 to feed all of your serial links, say like

Link AB using 192.168.1.0/30
Link BC using 192.168.1.4/30
Link CA using 192.168.1.8/30

This would NOT work in RIPv1. RIP would be seeing a bunch of 192.168.1.0/24 routes and would go crazy. Hopefully this explanation was a little bit clearer...I'm not used to trying to explain this stuff, but I want to become a more active participant in these forums so I'm giving it a shot. Some of the more experienced folk at the forums may have a better way of putting it. :P

And, of course it could be that all that is moot and it's just packet tracer being weird :P. But I'm pretty sure my theory is sound on this one.

OK. I get it now. I mean, what i get is how it works and Im going to use the addressing scheme of your example just to see.

MY point is that is kinda confusing that my scheme is working IMO. Why? Because if it works as you are saying, why does it automatically assume you assign default masks if that is not what you are typing?

Im currently restudying OSPF but i will input your addressing scheme just to see how it works. Im going to use dynamips with mine too.

Thank you!