L2 routers are similar to OSPF backbone routers, and the set of L2 routers (including L1/L2 routers) and their interconnecting links make up the IS-IS backbone, similar to area 0 in an OSPF network.
IS-IS does not use level 3 routers.
Some routers, called L1/L2 routers, belong to both area types.
Unlike OSPF, IS-IS routers are not required to be connected to a contiguous backbone area. In fact, the backbone area can also be segmented in IS-IS.
IS-IS uses the concepts of router levels, which is similar to OSPF areas. L2 routersare similar to OSPF backbone routers, and L1/L2 routers are analogous to OSPF ABRs.
With IS-IS, there is no restriction that all backbone routers (level 2 routers) be contiguous such as the backbone area of OSPF.
In OSPF all areas must be directly linked to area 0, and the backbone area must also not be segmented.
With IS-IS, the backbone area can be more easily extended since all L2 routers need not be linked directly together.
With regard to CPU use and the processing of routing updates, IS-IS is more efficient than OSPF.
In IS-IS, one LSP is sent per IS-IS router in each area (including redistributedprefixes [routes]), compared to the many OSPF LSAs that would be sent.
Not only are there fewer LSPs to process, but the mechanism by which IS-IS installs and withdraws prefixes is less processor intensive.
In IS-IS, the entire SPF table is not refreshedperiodically like OSPF, which does so every 30 minutes by default.
PSNP(Partial Sequence Number PDU) are used to acknowledge the receipt or to request the retransmission of the latest version of an LSP while the CSNP(Complete Sequence Number PDU) are used for synchronizing the LS Database on adjacent neighbors.
The default L1 and L2 priority is 64 in Cisco routers, with a value range of 0-127. A value of 0 means the router is ineligible to become the DIS. The router with the highest priority is chosen as the DIS, and in the case of a tie, the router with the numerically highest SNPA becomes the DIS. DIS is the highest Subnetwork Points of Attachment (SNPA) address on the LAN segment. The SNPA address refers to the data link address, and in this case is the MAC address. On a LAN, one of the routers elects itself the DIS, based on interface priority (the default is 64). If all interface priorities are the same, the router with the highest subnetwork point of attachment (SNPA) is selected. The SNPA is the MAC address on a LAN, and the local data link connection identifier (DLCI) on a Frame Relay network. If the SNPA is a DLCI and is the same at both sides of a link, the router with the higher system ID becomes the DIS. Every IS-IS router interface is assigned both a L1 priority and a L2 priority in the range of 0 to 127. The DIS election is preemptive (unlike OSPF). If a new router boots on the LAN with a higher interface priority, the new router becomes the DIS. It purges the old pseudonode LSP and floods a new set of LSPs.
By default, EIGRP uses only bandwidth and Delay when calculating the metric. EIGRP uses these scaled values to determine the total metric to the network:
Routing 
