IS–IS (CLNP) chosen as the OSI intra-domain protocol from DECnet proposal Certainly, this requires extra work with OSPF in terms of configuration. While the overload bit feature of IS–IS is not directly available in OSPF, it can be indirectly induced in OSPF by setting the link metric to the highest value (65,535) on the relevant links to an overloaded router, so that this router is avoided in the short path calculation. Since OSPF runs over IP, it can support virtual links.
This is used, for example, by other routers to not consider an overloaded router in path computation. IS–IS allows overload declaration through an overload bit by a router to other routers. IS–IS keepalive messages can be used for MTU detection since the messages are MTU-sized TLVs that are explicitly checksummed and need to be verified. IS–IS being run directly over layer-2 is relatively safer than OSPF from spoofs or attacks. The OSPF dimensionless link metric value is in the range 1 to 65,535 while IS–IS allows the metric value to be in the range 0 to 63 (narrow metric), which has been extended to the range 0 to 16,777,215 (wide metric). While OSPF packets are encapsulated in IP datagrams, IS–IS packets can be directly encapsulated in link layer frames. In IS–IS, routers are entirely within one or the other areas-the area borders are on links, not on routers.
With OSPF, an area border router can sit on the boundary between the backbone area and a low-level area with some interfaces in the area while other interfaces are in the other area. While there are similarities as noted above, there are several differences:
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