The differences between the two protocols are so small, that people really grasp at straws when 'proving' that one is better over the other. 'IS-IS doesn't work over IP, so its more secure'. 'IS-IS uses TLVs so new features are quicker to implement'. While these may be vaguely valid arguments, they don't hold much water. If you don't secure your routers to bad actors forming OSPF adjacencies with you, you're doing something wrong.Who is running code that is so bleeding edge that feature X might be available for IS-IS, but not OSPF?
Chose whichever you and your operational team are most comfortable with, and run with it.
Basic point I very much agree with. However, if that was all there was to it, nobody would ever switch from OSPF to IS-IS or vice versa :-)
OSPFv3 scaled better than OSPFv2 in 2008. But multi-AF support for OSPFv3 was only developing then, so that was not a viable replacement for OSPFv2.
OSPFv2 should scale better in 2015 (I say "should" because more routers now have x86-based control planes, but I don't run OSPF so I'm hand-waving).
You're right, a single Level-2 domain in IS-IS is akin to a single Area 0 in OSPF. But those "so small" differences between the protocols in 2008 meant I was less eager to try the single area with OSPF than I was the single level with IS-IS.
Some points I've noticed - YMMV. - Needing OSPFv3 for IPv6 when you're alredy running OSPFv2 for IPv4 is less than optimal. I believe nowadays several vendors support OSPFv3 for both IPv4 and IPv6 - but this is not universal. - Probably mostly due to large operators running IS-IS, new features are more likely to show up first in IS-IS. - OSPFv3 security depends on IPsec, while IS-IS uses MD5. You could certainly argue that MD5 is starting to get long in the tooth - on the other hand, it's significantly better than nothing, and significantly less complex than IPsec. - We still have a few cases of needing OSPF towards customers. IS-IS as core IGP makes it slightly easier to ensure that core routing and customer routing are never mixed. I see no reason to mention anything about scaling, since I believe the protocols (both OSPF and IS-IS) nowadays scale to much larger topologies than we're likely to need. Steinar Haug, Nethelp consulting, sthaug@nethelp.no