cannot ping router

[nightfall_gr]

I have 2 ethernet subnets(A and B) connected and I have connected a new router's interface to subnet B. Although the hosts from subnet B be can ping the new router(and vice versa) the hosts from subnet A cannot(yet they ping the hosts of B). Why is this happening? Any ideas?

[Richard Burts]

I think that the new router does not have a route to subnet A. If you are on the new router and do a show ip route I think you will see subnet B as a connected network and will not see subnet A.

To explain what is happening: devices on subnet B can get to the new router because they are in a common subnet. And the new router can get to the devices on subnet B because they are in a common subnet.

Devices in subnet A can send data (including ping) to the new router because they know how to get to the router that is connecting A and B. However the new router can not respond to the ping from subnet A because it does not know how to get to subnet A.

You could fix this problem by running a dynamic routing protocol between the routers or you can fix the problem by defining a static route on the new router for subnet A.

HTH

Rick

[nightfall_gr]

Actually I didn't make something enough clear. Neither the router can see subnet A nor subnet A the router! Subnet A can ping only to the computers of subnet B and not to the router.

[Kevin Dorrell]

I am confused. Do I understand this right? You have two subnets, A and B, that are physically connected together? That is, they are the same LAN, but different IP subnets. You have a router connected to this network, that you have given an address in subnet B, along with its correct mask. Is this the topology?

Now, you describe A and B as being different subnets. Hosts in subnet A can ping hosts in subnet B. Is the mask on the hosts correct? Please could you tell us the subnet addresses and masks you are using on the router and on the PCs.

The PCs in subnet B can ping the router because the router has an address in subnet B. If you want PCs in subnet A to be able to get to the router, then you must give the router an address in subnet A and tell the PCs about it by setting it in the default gateway.

Please could you clarify the topology. Thanks.

Kevin Dorrell

Luxembourg.

[michael.armstrong]

How are subnets A and B connected?

[nightfall_gr]

Nothing special, they are both connected to a router in the lab and they are both ethernet

[mgeorccna]

So, you have 1 router with 2 ehternet interfaces, Right?

Each interface connects to a different subnet. Correct?

Can you sent me the configuration of the router?

Thanx

Marinos GR

[nightfall_gr]

Exactly! It's so simple, still it doesn't work!

Unfortunately I don't have access to the router which connects the two subnets but it is definately configured correct.

Could it be some misconfiguration of the ARP protocol in the router? Because although there is access to the subnet B yet the router doesn't answer to the packets with its address. Maybe that's why the router also fails to ping since it never answers to the ping replies. Is there any way to check it?

Note: If I asign the one router interface to subnet A and the other to subnet B it works fine but this is pointless since the subnets are already connected!

[mgeorccna]

I don't think ARP has anything to do with this. Maybe we are missing something here.

Can you try from a PC at subnet A to tracert to subnet B and vice versa.

Please sent me the output of these 2 tracert commands.

[mgeorccna]

Are there 2 routers on your network?

One router connecting subnet A snd subnet B, and another router connecting subnet B to somewhere else?

If that's your topology, then your problem is that hosts from subnet A connot ping router 2. Right?

If that is true then you need to add a route at router 2 for subnet A because subnet A is not connected (or konwn) at router 2. Router 2 should know where to find subnet A.

Marinos

[nightfall_gr]

But for the time being the second router is nothing more than a host, just like the other computers of the subnet. It's like telling me that each computer in subnet B should have a route for subnet A!

The tracert command from a PC of subnet B to A shows:

1 <10 ms <10 ms 10 ms

router.ccslab.aueb.gr [195.251.252.126]

2 20 ms 10 ms 10 ms c90.ccslab.aueb.gr [195.251.252.90]

Similarly from subnet A to B. However when I give the rooter's address everything fails.

[a.awan]

You were provided with the correct answer in the very first response by Rick and the last post repeated the same suggestion however for some reason you are not trying to implement that. Your arguement is that the router is just a host on the subnet. Well even hosts have routing tables. Even though you do not explicitly configure them with specific routes you still need to configure them with a default gateway so that they know how to reach non-connected or unknown subnets. For the second router that is acting as a 'host' the default gateway requirement still exists. There are two ways you can achieve that. One is to turn off routing on the router and use the ip default-gateway command and the other is to keep routing enabled and use the ip route 0.0.0.0 0.0.0.0 . Another option is to configure the router with a specific route rather than the default route. I am sure either of these recommendations will work out for you.

[Richard Burts]

If I understand your topology correctly the new router is connected to subnet B but not connected to subnet A. As I said early in this thread, the issue seems to be that the new router does not have a route to subnet A because it is not connected to it. The easy way to check this is to post the results of show ip route on the new router. I think you will see that it has an entry for subnet B but no entry for subnet A.

HTH

Rick

[mgeorccna]

Each computer in subnet B have a gateway configured. This gateway should be the router. The router(s) then handles the further routing.

Yes, router 2 when first installed, could be acting like a host, but with what gateway configured? Do you have access to router 2? Can you do a show ip route? Does router 2 know where to find subnet A?

Does a traceroute from router 2 go through 195.251.252.126?

Can you send me a drawing of your network in mgeor@freemail.gr ?