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cannot ping back

Posted on 2014-03-28
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Last Modified: 2014-04-08
Can the experts help me in understanding this? I have 2 sites (A & B)  connecting through a MPLS network. A can ping B. But B cannot ping A. Why? If A can ping B then B should have a route back to A. Correct?

I checked the BGP route on my two routers and they have a route to each other. When I do the traceroute to A from B router, I see that it stops at the 3rd hop, I believe that is the PE MPLS router.
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Question by:leblanc
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Expert Comment

by:Soulja
ID: 39962155
Looks like your ISP is blocking ICMP in one of the directions or is disabling TTL propagation.
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Expert Comment

by:BigPapaGotti
ID: 39962159
If B is able to Reply to A's Ping request what this tells me is that it does have a route back to reach A. But since this is not working when originated from B this tells me that something on A's side is not allowing the ping to come through. Check your ACL's and Firewalls to see if icmp is being denied?

Have you tried any other sort of network testing such as trying to telnet to a webserver on port 80. This would help in troubleshooting to tell you whether or not icmp is being blocked by and ACL but web traffic on port 80 is being allowed.

Let me know what you find and I hope this helps.
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Accepted Solution

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Thomas Grassi earned 500 total points
ID: 39962207
run a traceroute  and post results
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Expert Comment

by:Matt Williams
ID: 39962219
try a traceroute to see at which point the ping is failing
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Author Comment

by:leblanc
ID: 39962218
This is a private MPLS network and I am not sure what they have within their network. I can only see my 2 routers. I already opened a ticket. I just want to get your thoughts on this. So it looks like something is blocking the ping

I have the sniffer in A and I don't see the ping coming from B on the WAN interface of A.
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Expert Comment

by:Soulja
ID: 39962244
Okay, if you can only see your routers, then they are have TTL Propogation disabled. This prevents you from seeing the hops withing their cloud.
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Expert Comment

by:nader alkahtani
ID: 39966282
I apply with BigPapaGotti in when he said "Check your ACL's and Firewalls to see if icmp is being denied?"

you should ping A from B then post ping output to analyze that
you should use pingpath
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by:leblanc
ID: 39966415
I found out that the provider does not have the route pointed to us.
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