Comparison of Basic Characteristics
Both protocols are widely used IGP protocols
Both protocols are link state protocols
Both protocols support IP environment
Both protocols adopt hierarchical design and area division
IS-IS supports CLNP environment
IS-IS only supports PPP and broadcast network; OSPF supports PPP, broadcast, P2MP and NBMA network.
OSPF supports virtual link
Different in packet encapsulation format
OSPF routers can be attached to multiple areas with interfaces assigned to different areas. IS-IS routers are associated with a single area and the whole router belongs to the area.
Comparison of Adjacency Relationship
Discover neighbors and form adjacencies via Hello protocol
Elect DR on multi-access network
The conditions for adjacencies formation are different
In OSPF, the formation of adjacency relationship on PPP links is more reliable
In IS-IS, there are 2 types of adjacencies namely level-1 adjacency and level-2 adjacency
In OSPF, all nodes form adjacencies with only DR and BDR but not with each other; In IS-IS, all nodes form adjacencies with each other.
The process of DR and DIS election are different.
Comparison of the Process of Link State Database Synchronization:
Both protocols follow the basic mechanism of LSDB synchronization
OSPF uses many types of LSA whereas IS-IS uses only router LSP and pseudonode LSP
The process of synchronization is different
The remaining time of the LSP in the IS-IS protocol is counted down to 0 from 15 minutes (configurable) to clear the old LSP; while the remaining time in the OSPF protocol is counted up to 60 minutes (not configurable) from 0 to clear the old LSA.
Comparison of the Process of Route Calculation:
Both protocols calculate the route by using SPF algorithm
OSPF treats the IP prefixes as the node of the SPT whereas ISIS treats the IP prefixes as the leaves of the SPT
The metric type of IS-IS is much more complicated compared with OSPF.
Comparison of the Performance:
Loop-free, fast convergence, and support large-scale network
OSPF supports dial on demand link while IS-IS does not support
OSPF advocates for area division; IS-IS generally adopts single area
IS-IS adopts TLV structure and therefore it has high expandability