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| 7.3 |  | IS-IS Operation |
| 7.3.7 |  | Designated Intermediate Systems (DIS) and Pseudonodes (PSN) |

The idea behind the Designated Intermediate System (DIS) is similar to the one behind the Designated Router (DR) in OSPF. The DIS creates and acts on behalf of a pseudonode, a virtual node. All the routers on the LAN, including the DIS, form an adjacency with the pseudonode, or PSN. Instead of flooding and database synchronization taking place over n x (n-1) adjacencies, the pseudonode enables the reduction in flooding and database synchronization to occur only over the adjacencies formed with the pseudonode. On a LAN, one of the routers will be elected the DIS based on interface priority. The default priority is 64. The configurable range is 0 to 127. If all interface priorities are the same, the router with the highest SNPA is selected. MAC addresses are the SNPAs on LANs. On Frame Relay networks, the local DLCI is the SNPA. If the SNPA is a DLCI and is the same at both sides of a link, the router with the higher system ID in the NSAP address will become the DIS.

A pseudonode LSP represents a LAN, including all ISs attached to that LAN. A non-pseudonode LSP represents a router, including all ISs and LANs connected with the router.

The DIS election is preemptive, unlike DR election with OSPF. If a new router boots on the LAN with a higher interface priority, it becomes the DIS, purges the old pseudonode LSP, and a new set of LSPs will be flooded. The DIS Hello interval, at 3.3 seconds, is three times faster than the interval for other routers on the LAN. This allows for quick detection of DIS failure and immediate replacement on the LAN. Remember that there is no concept of backup DIS in IS-IS.

In IS-IS, a DIS does not synchronize LSPs with its neighbors through acknowledgments. Reliability is ensured when the DIS creates the pseudonode for the LAN. It sends L1 and L2 Hello PDUs every ten seconds and CSNPs every ten seconds. The Hello PDUs indicate that it is the DIS on the LAN for that level. The CSNPs describe the summary of all the LSPs, including the LSP ID, sequence number, checksum, and remaining lifetime. The LSPs are always flooded to the multicast address and the CSNP mechanism only corrects for any lost PDUs. For example, a router can ask the DIS for a missing LSP using a PSNP or, in turn, give the DIS a new LSP. CSNPs are used to tell other routers about all the LSPs in the database of another router. Similar to an OSPF database descriptor packet, PSNPs are used to request an LSP and acknowledge receipt of an LSP.

To restate, the DIS is responsible for conducting flooding over the LAN and also for maintaining synchronization. A router may need an LSP because it is older than the LSP advertised by the DIS in its CSNP. A router may need an LSP if it is missing an LSP that is listed in the CSNP. If either of these is the case, it will send a PSNP to the DIS and receive the LSP in return. This mechanism can work both ways. If a router sees that it has a newer version of an LSP, or it has an LSP that the DIS does not advertise in its CSNP, the router will send the newer or missing LSP to the DIS.