GRE Tunnel between 2 switches

Good day everyone.

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I currently have the task of getting our network ready for a sniffing appliance, and as such, need to configure RSPAN on our switching stack.

We use a couple of 9300's as our core switches. Nexus doesn't seem to support RSPAN, so I'm sort of stuck with having to configure ERSPAN to route mirrored traffic to our destination switch.

I'm a bit rough on routing, as it's been a while since I'm mostly doing layer 2 level configs on our stack. What i'm trying to say, is go easy on me.

Here is a quick, dirty sketch of the network:

https://imgur.com/llo7kDg

To test things out, I started by creating a gre tunnel between the 4510-1 on the left to the 4510-2 on the right.

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these are the configs:

###4510-1### interface loopback 0 ip address 1.1.1.1 255.255.255.255 interface tunnel 1 ip address 172.31.0.1 255.255.255.252 tunnel source loopback 0 tunnel destination 3.3.3.3 tunnel mode gre ip ###4510-2## interface loopback 0 ip address 3.3.3.3 255.255.255.255 interface tunnel 1 ip address 172.31.0.2 255.255.255.252 tunnel source loopback 0 tunnel destination 1.1.1.1 tunnel mode gre ip 

both show as up: (exactly the same on the other end, but reverse, obvs)

Loopback0 3.3.3.3 YES manual up up Tunnel1 172.31.0.2 YES manual up up 

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routing is enabled on both switches. Can't ping either side.

I know i'm missing a crucial element to get these two ends to talk, but I just can't think of it.

I feel like there needs to be some sort of manual route entry or something; i'm just plain stumped. Any advice? or is this an exercise in futility?