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The best way to do this is via the AXUDP (AX25 over UDP/IP) protocol.

This howto assumes that you are familiar with the basics of XRouter and BPQ.

When both nodes are on the same computer, they can be linked via the operating system's “loopback” interface, usually known as “localhost”.

If we assume that GB7XRP is the XRPi node and GB7BPQ is the BPQ node:

You need a “BPQAXIP” port in BPQ.CFG…

PORT
    PORTNUM=1
    ID=AXIP
    DRIVER=BPQAXIP
    CONFIG
    UDP 10093
    MAP GB7XRP localhost 9393 B
ENDPORT

In the above example, 10093 is the UDP port that BPQ RECEIVES on, and 9393 is the UDP port that BPQ TRANSMITS on to GB7XRP (they are shown as different numbers to avoid confusion, but could be the same).

In XROUTER.CFG you need two things: (a) an AXUDP INTERFACE, and (b) a PORT “attached” to that interface…

INTERFACE=1
   ID=AXUDP
   TYPE=AXUDP
   MTU=256
ENDINTERFACE

PORT=3
   ID=Link with GB7BPQ
   INTERFACENUM=1
   IPLINK=localhost
   UDPLOCAL=9393
   UDPREMOTE=10093
ENDPORT

(if you already have an AXUDP interface, you will only need to add the PORT because the interface will support unlimited ports.)

IPLINK is the ip address or hostname of the other end of the link, in the case BPQ. Since they are both on the same computer, IPLINK should be “localhost” or “127.0.0.1”. The latter is preferable, because it cuts out the domain resolution step.

UDPLOCAL is the UDP port that XRouter RECEIVES on. This must match the UDP port in the BPQ “MAP” line.

UDPREMOTE is the UDP port that XRouter TRANSMITS on. It must match one of the “UDP” lines in BPQ.CFG.

The only difference is that “localhost” in the BPQ MAP line is replaced by the IP address or hostname of the XRouter node, and “localhost” in XRouter's IPLINK statement must be set to the IP address or hostname of the G8BPQ node.

For example, lets assume that GB7BPQ's IP address is 192.168.0.10 and GB7XRP's IP address is 192.168.0.20…

In BPQ.CFG the MAP line would be as follows:

MAP GB7XRP 192.168.0.20 9393 B

And in XROUTER.CFG the IPLINK line would be as follows:

PORT=3
   ID=Link with GB7BPQ
   INTERFACENUM=1
   IPLINK=192.168.0.10
   UDPLOCAL=9393
   UDPREMOTE=10093
ENDPORT

If the two nodes are not at the same location, you must enable (aka “open” or “port-forward”) incoming UDP ports on each internet router.

Thus at GB7BPQ you would open UDP port 10093, and forward the traffic to the LAN IP address of the BPQ node.

And at GB7XRP you would open UDP port 9393, and forward the traffic to the LAN IP address of XRouter.