Hi, First time poster looking for some input on a debate and I apologise if I've done this completely wrong, but I don't think my colleague will be convinced until he hears it from this community. Granted I'm relatively green when it comes to networking, but it was my understand that other than BGP, the most widely used/implemented IGP would be OSPF. However, my colleague insists that I'm 100 percent wrong and that is IS-IS. I want to be clear, I'm not debating which protocol is better or worse, as they both have their strengths and weaknesses and ultimately it comes down to several factors based on a particular use-case on which routing protocol is the best way forward for a network. However in saying that, I believe that, when it comes to sheer numbers of where an IGP is implemented, OSPF is far more widely used in the world today, whether it be in small to medium to large businesses, Enterprise networks and even Service Provider networks (where as per my understanding, is where IS-IS really shines) I understand that this isn't really quantifiable, but I would like to get the opinions/experiences from this list and see what the outcome of this question is out of sheer curiosity. Thanks! Steve