In what way does Unferth one of Lord Hrothgar's men challenge Beowulf apex?

Unferth challenges Beowulf's reputation by bringing up an incident where Beowulf lost a swimming match to Breca. Unferth questions Beowulf's bravery and boasts about his own abilities, trying to diminish Beowulf's accomplishments. Beowulf responds by defending his honor and recounting his past victories in battle.