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Can only ping 1 direction, but it gets weirder...

Tue Jul 20, 2021 9:45 pm

We use OSPF and are having a strange problem where if a path goes down, sometimes it does not re-route correctly and we have to reboot a router on one end of the VPLS tunnel to fix it.

I noticed one of our routers, cannot ping (another router). But it works the opposite direction, has no problem pinging

Ok, then I try to force the Src. Address to see if I get different results. And magically if I put ANY of the IPs in Src Address then can ping just fine. See attachment.

I cannot figure this out. Why would a regular ping be any different then an advanced ping with "Src Address" entered? All Src addresses, from my address list, can successfully ping.

I feel like if I can sort out this ping issue I can sort out my OSPF issue.
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Re: Can only ping 1 direction, but it gets weirder...

Wed Jul 21, 2021 8:23 pm

Are you pinging by machine name, or by IP address, or is it the same both ways?

Are any of these machines performing routing (multiple NICs)?

I'm assuming that when you say they are on the same LAN, you mean that each machine is using a one and only one network card attached to the same switch, and that there are no routers between them. I am also assuming they are on the same logical IP subnet. If this is incorrect, please give more details in the question. I have reason to believe that these assumptions are not 100% correct.

Things to try:

Ping by IP address first. This will rule out name resolution problems. If this is successful, then you probably have a name resolution issue (like a DNS server disagreeing with the hosts file).
Check your interface's subnet mask on each machine. If they don't match, this can happen in some circumstances.
Check for multiple gateways. Machine C may have more than one defined. This shouldn't be an issue if they are on the same IP subnet. The message Destination host unreachable implies that it knows the address of what it is trying to ping, but can't seem to get there with the routes that are defined.
It would help if you could provide your routing table for each machine, which is typically done with route -n on Linux.
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Re: Can only ping 1 direction, but it gets weirder...

Thu Jul 22, 2021 5:01 am

What does your ip route show for that particular route ? Does it specificy a bad src-address? Torch the interface without a src-address and see what src/dst its trying to use.

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