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PIX IPSEC VPN traffic only goes in one direction (no reply traffic)

I recently had a situation where a pix - pix ipsec tunnel appeared to be set up correctly according to all the examples and discussions on CCO.

It was!

The problem i encountered was that packets were being encapsulated, sent down the tunnel and decrypted in one direction.

My remote site had a client that could send ping packets to a server at a HQ site. The server was seeing these pings and the server was replying. My HQ pix could see the reply packets (debug packet inside proto icmp) but it was not encapsulating them. It was as if the HQ pix was dropping the reply ping packets.

It turns out that the routing on the HQ pix had a route for the remote network pointing back inwards to the HQ lan router.

To solve this I changed the routes on the HQ pix similar to this...

route outside

route inside

This solved the problem.