2860 Router: I have a client setup where they have a /29 WAN IP range. I've configured 2 of their IP's in the WAN IP Alias, the remainder are not used yet. Lets call those two IP addesses WANa and WANb. By default they are both in the NAT IP pool.

In LAN1 they have a e-mail server and there are NAT settings to allow WAN access to it. They have LAN2 as a guest network with ISP DNS set up for that LAN and DHCP from the Draytek.

The majority of the clients in the LAN2 network can access the e-mail system that is in LAN1. The email server sees most of that traffic as originating from WANa and it itself is visible via WANa and, as it happens, it is also visible to WANb. However, the traffic from one specific client in LAN2 appears to originate from WANb and although I can see the packets arriving from that client at the e-mail server, with Wireshark, originating from WANb, the return packets never get back to the client in LAN2. I'm not at all sure the mechanism by which the outgoing IP is selected in the NAT operation. However, the traffic from this particular client in LAN2 always appears to go out over WANb.

Removing WANb from the NAT IP pool fixes the issue as that forces the traffic from the 'errant' client to originate from WANa.

Have I missed something subtle here or is this an 'undocumented feature'

Thanks

Ken