Scalability

In IS-IS, ISO 10589 states that 100 routers per area and 400 L2 routers should be possible. The biggest scaling issue now seems to be the overhead of the flooding in large meshed networks, for example, flat ATM clouds with many attached routers that form a full mesh. OSPF, on the other hand, scales well regardless of the size of the network. However, to make the network operate optimally, you should implement physical and logical areas as needed.

Physical Media Support

Both OSPF and IS-IS support point-to-point links and LANs in similar ways. However, IS-IS has no nonbroadcast multiaccess (NBMA) support and expects the router operating system to present the NBMA network as either a LAN or a set of point-to-point links; this can be problematic. OSPF can overcome this issue, but with Cisco routers, considerable configuration is needed.

Extensibility

Protocols must be able to grow and expand to meet the ever-changing and evolving network environment. OSPF has been able to do this through the recent inclusion of opaque LSAs; however, all routers must understand these LSAs to effectively adapt to the dynamic network environment. IS-IS floods unknown LSAs and ignores them.

Both protocols support traffic engineering, so networks can benefit from Multiprotocol Label Switching (MPLS) regardless of which protocol is chosen.