Just tried an experiment... Powered the BBB from a 5V barrel supply
(since an R-Pi USB probably can't power it), then connected the BBB client
port to the R-Pi host port...
pi@rpi3bplus-1:~$ ifconfig
eth0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether b8:27:eb:fb:d8:30 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet6 fe80::d22b:73ea:8bef:c781 prefixlen 64 scopeid 0x20<link>
ether d0:39:72:18:3e:ea txqueuelen 1000 (Ethernet)
RX packets 32 bytes 5556 (5.4 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 20 bytes 2661 (2.5 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
eth2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet6 fe80::4f35:ad14:88b8:4a1a prefixlen 64 scopeid 0x20<link>
ether d0:39:72:18:3e:e8 txqueuelen 1000 (Ethernet)
RX packets 33 bytes 5376 (5.2 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 18 bytes 2961 (2.8 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.76 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 2600:1700:e630:890:e51e:f96a:9476:3412 prefixlen 64 scopeid
0x0<global>
inet6 fe80::6471:7a90:88f3:79c0 prefixlen 64 scopeid 0x20<link>
inet6 2600:1700:e630:890::1b prefixlen 128 scopeid 0x0<global>
ether b8:27:eb:ae:8d:65 txqueuelen 1000 (Ethernet)
RX packets 19022 bytes 27611083 (26.3 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 10635 bytes 1253318 (1.1 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
pi@rpi3bplus-1:~$
eth1 and eth2 are IPv6 and exist while the BBB is connected, removing
the USB cable leaves the R-Pi with
pi@rpi3bplus-1:~$ ifconfig
eth0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether b8:27:eb:fb:d8:30 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.76 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 2600:1700:e630:890:e51e:f96a:9476:3412 prefixlen 64 scopeid
0x0<global>
inet6 fe80::6471:7a90:88f3:79c0 prefixlen 64 scopeid 0x20<link>
inet6 2600:1700:e630:890::1b prefixlen 128 scopeid 0x0<global>
ether b8:27:eb:ae:8d:65 txqueuelen 1000 (Ethernet)
RX packets 19047 bytes 27613467 (26.3 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 10660 bytes 1261140 (1.2 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
pi@rpi3bplus-1:~$
The lack of IPv4 may be due to the lack of DHCP server. I did not try
to pass traffic (ping) between the two. Merely to demonstrate that it may
be possible to use TCP/IP between the two boards without using a
USB<>Ethernet adapter.