I've run into an interesting issue and I'm hoping someone here may have some previous experiences or maybe something on best practices I'm missing.
Basically, we have a site-to-site loopback interface set up and we have several tunnels that utilize this and each connects to its own security zone. So far, this has all been working as far as I know. We now have yet another tunnel that utilizes this same type of setup on the same loopback as the others, however, the team that uses it has noticed issues with the applications that go across it... namely that some transactions complete and some do not. Looking at the session logs, I can see a number of tcp-fin but also some aged-out and some tcp server resets.
We got on a call with the team that manages the network/servers on the remote side and found that lowering the MTU on the servers to 1400 seems to resolve it and all transactions work correctly at that point. I'm fairly certain that, on our side of the bridge, the DC networking hardware has jumbo frames configured and so does the datacenter interface on the firewall itself. They wouldn't hit a smaller MTU until the traffic starts to traverse the site-to-site. I don't know what hardware that the remote side uses to terminate or to carry the traffic to the servers.
The site-to-site loopback on our side looks like it is configured with default MTU and Adjust TCP MSS is not configured. The tunnel interface for this particular site-to-site is also using default MTU. I'm guessing I need to either adjust the MTU on the loopback/tunnel (if I have to adjust on the loopback, I wonder how this will impact all of the other tunnel interfaces also utilizing it) or turn on the TCP MSS adjustment?
@BPry thanks for the reply! I'm assuming then that the defaults of 1500 should be fine on the Palo?
This whole thing really seems odd to me because I figured that the Palo must be taking in any overhead into account since it is the one encapsulating for the IPSEC VPN. Furthermore, should the packets not just get fragmented and then re-assembled by the remote client if the do exceed the MTU?
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