from typing import Optional


def fmac_cisco(mac: str) -> Optional[str]:
    """
    Given a string representation of a MAC address in a common format, return it in Cisco format.
    """
    # Fast-like remove ":", ".", and "-" in one go
    mac = mac.translate({58: None, 45: None, 46: None}).lower()
    if len(mac) != 12:
        return None
    return f"{mac[:4]}.{mac[4:8]}.{mac[8:12]}"