In a medium pot of boiling salted water, cook the pasta until al dente, about 2 minutes; drain and divide among 4 bowls.
In the same pot, combine the beef broth, lemongrass and garlic and bring to a boil. Lower the heat and simmer for 15 minutes. Add the carrot, onion, fish sauce and chili paste. Return the soup to a boil, add the beef, then divide the soup among the 4 bowls. Garnish with the cilantro and lime wedges.