One of the applications bundled with ExaBGP (/usr/local/lib/python2.7/site-packages/exabgp/application/healthcheck.py) requires devel/py-ipaddress. It should be added to the port dependencies. Or healthcheck.py should be optional but I think it's pointless to make it an option.
See URL, though not documented in setup.py (as a compulsory or optional requirement)
A commit references this bug: Author: zi Date: Tue Nov 7 23:31:02 UTC 2017 New revision: 453711 URL: https://svnweb.freebsd.org/changeset/ports/453711 Log: - Add missing dependency on net/py-ipaddress for healthcheck tool PR: 223468 Submitted by: Borja Marcos Changes: head/net/exabgp/Makefile