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Can someone please explain with an example why reverse path filtering need to be disabled while using IPIP/GRE tunneling?

echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter
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Reverse path filter was introduced to support Strong Send and Receive. Which now most of the Operating Systems have as default setting. In Strong Send and Receive OS transmits outgoing packet only from the same interface from which it has received it. In tunneling packets are received on tunnel interface but they go out from different interface. Hence if RPF is enabled on back-end servers response will not be delivered to clients.

Hence, Reverse Path Filtering should be disabled on routers for the subnet of back-end servers.

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