Good question @Jason . Just like any other traffic traversing that NAT device, SVR sessions are discrete (i.e. each one has a unique 5-tuple, or SRC-IP, SRC-Port, DST-IP, DST-Port, and transport protocol) and they are directional (i.e. it initiates from one place to another to begin a bi-directional flow). So a NAT device that is keeping track of flows or connections is going to see the same number of them with or without those sessions being SVR'ed.
By default, two peered routers are going to send TCP/UDP packets between their adjacent waypoint (L3 interface) addresses using the following ports:
1280 (Bidirectional Forwarding Detection)
16348 - 65535 (sessions)
So a NAT is going to need to allow traffic on these ports be default.
Note that it is also going to need to allow connection or flow establishment from one side of it to another in the direction of the service for things to work. For example, most NATs allow a connection to go from ""inside"" to ""outside"", but not the other way around. Again, no different from any other traffic, if you have a service on the ""inside"" of the NAT you'll need to make sure it forwards traffic from ""outside"" (can be limited to just that coming from the peered waypoint address). If the entire range of ports listed above can't be opened between the waypoints, you can configure sessions to be sent to a specific port, or range of ports. You do this using the ""Port Range"" setting on a specific adjacency.
When you set a port range, you'll select from the following set of ports:
1025 - 16383