Can you imagine Vietnamese pho without its broth? It is probably impossible to do so. The pho broth is what gives the soup its distinct character. There's not necessarily only one way to make pho broth, but a good pho broth must follow certain rules and standards.
In fact, the key to a successful bowl of pho is the creation of a good pho broth. Pho lovers judge the bowl of pho served to them in a restaurant by sipping the broth first without putting in any seasoning or garnishing.
If there's any doubt about the quality of a bowl of pho, just take a sip of the broth to find out. If the pho broth is good, we feel immediately at home; the familiar, the warm, the good, the satisfying. But if the broth is not up to expectation, the pho eating experience becomes labored, beside the fact that one is wasting the time and money with this darn bowl of pho.
You can be forgiven for serving undercooked or overcooked banh pho (the pho noodle). You can be forgiven for not having all the right meat types or garnishing. But mess up the pho broth and your pho is ruined. Conversely, serving a great pho broth will make up for your other shortcomings many times over. There is no one perfect technique for making pho broth, but with practice and attention to to basic techniques, one can create a very good broth, with the right pho taste.