I have an issue with an RB1100AHx2: I'm getting GRE traffic on the interface facing the internet.
The traffic (seen with torch) is between a host on the same subnet as my router(both are connecting to the ISP router) and a different ip from the internet. It is GRE and since it is not meant for my router I can't filter it with the firewall.

The question is: why do I see this traffic on the interface(eating bandwidth) if it is not meant for my router?
Any help is apreciated. Thanks.

Attached diagram below

1. this does not sound like an "issue with a RB1100AHx2" but more like an "issue with your ISP router" or "issue
with router X"?
2. when tracing encapsulated traffic make sure you don't fall in the trap of the simplified dissection where the
output of the trace is not the outer packet but an inside packet that is further encapsulated. make sure you
do a verbose trace to really see what is going on.

I don't quite understand what you are saying. The problem is that this traffic is not sourced by the router or destined to it because it doesn't go on any other interface. It is just on the one facing the internet.

The question is: why do I see this traffic on the interface(eating bandwidth) if it is not meant for my router?

You should address this question to your ISP. There's (a) nothing wrong with your router and (b) nothing you can do by yourself.

Thanks but I just wanted to understand what can cause this so that I address the question to the right side: there is also the router that originates the traffic(X) which does not belong to the isp.

How do you connect to your ISP? Is it some kind of PON (passive optical network)?

I plug my router into a switch which is connected to the isp router. This is also where router X connects.