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Community Member

Ping problems

To simplify: I can ping from network host on network A: E0 on RouterA, S0 on RouterA, S0 on RouterB, E0 on RouterB, but no hosts on network B. This is all while ping is executed from command prompt in DOS. But, I CAN ping a host on network B from the Cisco IOS on RouterA. What is happening?

Thanks.

5 REPLIES
Community Member

Re: Ping problems

You may well have an access list on Router A.

Any traffic originating from Router A would normally be exempt from an access list applied on that router.

So, if you had an access list stating that all ping traffic going out of the serial interface will be blocked, the ping from the router would be granted.

That is one thing worth looking at.

Regards

Barry Hart

Community Member

Re: Ping problems

Thank you both for your comments. There is no restriction on ping on either router and no access-list on either router that would cause this. Is there someway to ping a host on network b from router a but not from a host on network a? Are there routes that need to be added or something else I have not yet considered? Again, thanks.

Community Member

Re: Ping problems

I also think it does matter with access-list of router or firewall.

Community Member

Re: Ping problems

Dennis,

Here is one difference to keep in mind as you troubleshoot:

When you are pinging from router a, the source address of that ping is the interface on which the ping leaves the router. So when you ping from hosts on network a, you are pinging from a different network altogether. I was tempted to say that router b probably just doesn't have a good route back to network a. However, you said that you can ping from hosts on network a to interfaces of router b. So router b is routing ICMP replies back towards network a.

I would first of all try an extended ping from router a and specify a source address from network a. Where you go from there depends on whether the results are the same or different. There are some pretty good debug commands for ICMP that might help to determine where and why what is being dropped.

It is possible that your hosts on network b have a route to the network that exists between routers a and b (and hence the good ping) but maybe they have no default route and no specific route to network a (and hence the bad ping). Something strange along those lines?

Regards,

Scott

Community Member

Re: Ping problems

Do a traceroute or tracert from both sides of the network (from workstations) and see where things 'stop' working. That will give you a clue where the problem is. Most probably router b is not advertising the IP subnet (that you are trying to ping) that exists behind router b E0, to router A. The previous posts had some good ideas about default routes too. Do a line by line match between both router configuration files. They should 'mirror' eachother. Please check your workstations and make sure that they are getting the appropriate setup information that was setup by hand the received from the DHCP server. Their default route must be the respective E0 of the attached router. It would have been nice if you had the results of the ping coming from network B towards A.

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