Which three of the following indicate a problem of physical layer? I am not sure about the 'excessive ethernet collisions' and 'router serial line encapsulation failure'. Anyone can help? Thanks.
router serial line down----physical
router serial line protocol down -------Data link
switch port link light is off-----physical
excessive Ethernet collisions ----?
router serial line up, no CDP neighbors----data link
dialup authentication failure-----data link
router serial line ecapsulation failure----?data link?