def addPort(self, src, dst, sport=None, dport=None):
'''Generate port mapping for new edge.
@param src source switch name
@param dst destination switch name
self.ports.setdefault(src, {})
self.ports.setdefault(dst, {})
# New port: number of outlinks + base
src_base = 1 if self.isSwitch(src) else 0
dst_base = 1 if self.isSwitch(dst) else 0
sport = len(self.ports[src]) + src_base
dport = len(self.ports[dst]) + dst_base
self.ports[src][dst] = sport
self.ports[dst][src] = dport