The 'Multi-Exit-Descriminator'. A lower value assigns a higher
preference to the route. Normally, the MED attribute ia attached to all
matched routes. Except in the case of 0xffffffff, which means the MED
will not be sent.
note: It is only used for BGP. If it is used (>0), then the
'xpolicyTxSendOption' can NOT be set to 'never'.