will be sent separately for Level-1 or Level-2. LSP This contains the actual route information. This LSP can contain many type–length–values (TLVs). CSNP This packet will be sent only by the DIS. By default for every 10 seconds, CSNP packet will be transmitted by DIS. This will contain the list of LSP IDs along with sequence number and checksum. PSNP If the router which receives CSNP packet finds some discrepancy in its own database, it will send an PSNP request asking the DIS to send specific LSP back to it. Comparison with OSPF Both IS-IS and Open Shortest Path First (OSPF) are link-state protocols, and both use the same Dijkstra algorithm for computing the best path through the network. As a result, they are conceptually similar. Both support Classless Inter-Domain Routing, can use multicast to discover neighboring routers using hello packets, and can support authentication of routing updates. OSPF was natively built to route IP and is itself a protocol that runs on top of IP, and OSPFv2 is only able to build IPv4 routing tables. IS-IS is an OSI Layer 3 protocol initially defined for routing CLNS. However, IS-IS is neutral regarding the type of network addresses for which it can route, and was easily extended to support IPv4 routing, using mechanisms described in , and later IPv6 as specified in . To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specified in ). Both OSPF and IS-IS routers build a topological representation of the network. This map indicates the subnets which each IS-IS router can reach, and the lowest-cost (shortest) path to a subnet is used to forward traffic. IS-IS differs from OSPF in the way that "areas" are defined and routed between. IS-IS routers are designated as being: Level 1 (intra-area); Level 2 (inter area); or Level 1–2 (both). Routing information is exchanged between Level 1 routers and other Level 1 routers of the same area, and Level 2 routers can only form relationships and exchange information with other Level 2 routers. Level 1–2 routers exchange information with both levels and are used to connect the inter area routers with the intra area routers. In OSPF, areas are delineated on the interface such that an area border router (ABR) is actually in two or more areas at once, effectively creating the borders between areas inside the ABR, whereas in IS-IS area borders