A wont ping B before B pings A first!!

Hewlett-packard Proliant dl380 g3 2u rac...
September 10, 2009 at 02:56:39
Specs: Windows XP to Linux CentOS
This is just crazy! How is it possible that a machine A (windows xp) wont be able to join machine B (dns server, linux centos) unless machine B makes the first contact? there is no firewall between them, just 3 routers. Where could the problem be?

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September 10, 2009 at 08:02:10
You need to provide some more details as to exactly what's going on.

Contact how? Are you trying to ping and if so, are you trying to ping by IP or hostname?

Are you trying to connect to a share?

there is no firewall between them, just 3 routers.

Why are there 3 routers in between? You might want to also detail your network so I can try to understand how it's setup.

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September 10, 2009 at 08:33:49
nic set to power off? Your b ping very well may be waking the interface up.

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September 11, 2009 at 01:35:28
Thanks for all the replies! I will try to see if the nic set to
power off it's the issue

Here's the deal

Contact how? Are you trying to ping and if so, are you trying
to ping by IP or hostname?

I ping the IP's, no hostname. It's a simple ping between IP

I am not trying to connect to a share, it's a client pc windows
XP and a DNS Server, Linux CentOS. The windows PC tries
to ping the DNS server but it doesnt work. Once the DNS
server pings the windows PC, the windows PC can ping back.

The PC is connected to a cisco router 7750 through a radio
interface. Then there's a link to an Alcatel router, which is the
core router of the network, and then there is the server room
router, to which the DNS is connected. This is an Alcatel
router too.

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September 11, 2009 at 07:52:16
Does the "core router" have ACLs or a Firewall. If so does it use Stateful Packet Inspection. It sound like you have to establish a connection to allow communications through a Firewall but this is just a Guess.

Another thing it could be is separate subnets. Can you provide the first two octets of the address you are pinging and the address you are pinging from?

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September 11, 2009 at 07:57:02
OP says there are three routers between so they have to be in different subnets. Does not appear acls or any routing has anything to do with the issue.

It is a matter of one pc not being able to ping until it is pinged.

Once done ping works fine. To me this points to station specific not network specific issue.

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September 11, 2009 at 14:05:55
That doesn't make any sense. A Ping is a TWO way communication. It shouldn't matter who pings who first.

Is this a netbios or ip address ping? Try tracert or pingpath also.

Playing to the angels
Les Paul (1915-2009)

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September 11, 2009 at 14:16:16
You don't ping netbios names. You ping host names. They are not the same. OP says ping ip address which takes out name resolution from the issue.

I have seen this before personally, where ping doesn't work until you ping from the workstation being pinged then it works.

Again this has nothing to do with what is in between. If it did ping would be consistant.

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September 11, 2009 at 14:18:58
As for ACLs not affecting pings this is dead wrong. If an ACL is not setup to allow ICMP on both the out bound and Inbound ports on your router then you will only be able to ping one way. Talk to you OP again an ask him if ICMP is allowed both way on the routers or hook a second computer (laptop is easiest) on the same ethernet cord and try the same test.

If it happens with a different computer then your problem is in the configuration of those routers.
If it does not then there is a local configuration problem.

Also, it sounds like a Routing Problem. Routing tables constantly change with each connection made. Try doing a TraceRT for both when it does ping and when it does not and see if they match. If they do not then it is a routing problem. At the very least you can see if the ICMP packet is getting dropped somewhere and know where it is dropped.

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September 14, 2009 at 00:52:39
Well, the problem went away, and I did nothing about it. There
are no firewalls or ACLs between the endpoints of
communication, it's an Internal network. What i did do,
BEFORE the problem showed up, was that I "converted" one
of the routers to a switch, c'est-à-dire, I moved all layer 3
config to the next router in line (vlan interfaces, static routes)
and I qtagged everything. So I had 3 routers, then I had 2
routers and a switch, then the problem showed up, then I had
3 routers again, and 3 days later the problem was solved by
itself. Does the chnage in architecture may have had some
impact on this issue? and if yes, how come the problem
persisted for 3 days after the rollback was performed?

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September 14, 2009 at 08:56:52
The question both ace and myself have is, was this effect only on one machine or all machines on that side of the router.

Your original post had only machine a and machine b and never have you mentioned multiple machines ajacent to machine b.

If only one machine then the effect is only on that machine. If it was all machines where b was then it would be router/network config.

Many or one? Or is there only one machine b at the end of those three routers?

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September 15, 2009 at 01:42:01
There are several machines on both sides of the network.
Machine A is in a /24 network, machine B is in a /26 network.
There are at least 10 machines on A side and 10 on B side.
However, since the problem dissapeared when I pinged from B to
A, I didn't check for connectivity with the other machines.

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September 15, 2009 at 07:59:48
How you have it subnetted should not really make a difference as long as you have the masks setup right in your router configurations. I am guessing you do or nothing would work.

So you configured one of your routers to be a switch and that is when the problem began. Is there any way you can post the configuration? Just redact the last two octets of the IP addresses and if you have a config password, that too.

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