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ISIS Protocol

DIS and the Pseudonode

What is the DIS?

On broadcast multi-access networks, a single router is elected as the DIS. There is no backup DIS elected. The DIS is the router that creates the pseudonode and acts on behalf of the pseudonode.

Two major tasks are performed by the DIS:

  • Creating and updating pseudonode LSP for reporting links to all systems on the broadcast subnetwork. See the Pseudenode LSP section for more information.
  • Flooding LSPs over the LAN.

Flooding over the LAN means that the DIS sends periodic complete sequence number protocol data units (CSNPs) (default setting of 10 seconds) summarizing the following information:

  • LSP ID
  • Sequence Number
  • Checksum
  • Remaining Lifetime

The DIS is responsible for flooding. It creates and floods a new pseudonode LSP for each routing level in which it is participating (Level 1 or Level 2) and for each LAN to which it is connected. A router can be the DIS for all connected LANs or a subset of connected LANs, depending on the IS-IS priority or the Layer 2 address. The DIS will also create and flood a new pseudonode LSP when a neighbor adjacency is established, torn down, or the refresh interval timer expires. The DIS mechanism reduces the amount of flooding on LANs.

Election of the DIS

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 from 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.

What is the Pseudonode (PSN)?

In order to reduce the number of full mesh adjacencies between nodes on multiaccess links, the multiaccess link itself is modeled as a pseudonode. This is a virtual node, as the name implies. The DIS creates the pseudonode. All routers on the broadcast link, including the DIS, form adjacencies with the pseudonode.

Representation of a Pseudonode:


In IS-IS, a DIS does not synchronize with its neighbors. After the DIS creates the pseudonode for the LAN, it sends hello packets for each Level (1 and 2) every three seconds and CSNPs every ten seconds. The hello packets indicate that it is the DIS on the LAN for that level, and 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 protocol data units (PDUs). For example, a router can ask the DIS for a missing LSP using a partial sequence number packet (PSNP) or, in turn, give the DIS a new LSP.

CSNPs are used to tell other routers about all the LSPs in one router’s database. Similar to an OSPF database descriptor packet, PSNPs are used to request an LSP and acknowledge receipt of an LSP.

Pseudonode LSP

The pseudonode LSP is generated by the DIS. The DIS reports all LAN neighbors (including the DIS) in the pseudonode LSP with a metric of zero. All LAN routers, including the DIS, report connectivity to the pseudonode in their LSPs. This is similar in concept to the network LSA in OSPF.




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