cant ping certain host until i issue the trace command...then it works!

[vza]

Hello. I'm having a problem whereby i am unable to ping a certain host(lan interface of my firewall) from certain devices until i issue the trace command and then it mysteriously works. Here's the breakdown. My lan interface on my firewall is connected to a switch which also has other pc's/servers connected to it. I also have a router(called R1) connected to this switch via its E0 port. Within this subnet i can ping any device from any device. The problem occurs with a router(R2) connected to my router(R1) via my R1 s0 interface. From R2 i can ping any interface on the other side of R1's e0 port except for the firewalls lan interface which is in the same subnet as all others off R1's e0 port. Now when i issue a trace to the firewall's lan int from the router(R2) it works. Right after the trace when i issue a ping it starts to work too??? After some time the ping will stop working again until i issue the trace command again. Can anyone tell me why this might be happening? Their are no ACLs on R1 or R2. I really appreciate any assistance. This is driving me insane...

Thanks in advance.

[vza]

got it fixed. Had to add a route on the firewall to the subnet R2 was connected to. i guess it was receiving the ping but couldnt send the response back. not sure why though, cause i figure it would know to send the packet back to the IP of R1's e0 port. The same way all the pc's and servers connected to the same subnet off R1's e0 port would do.