IS-IS PDU Types And LSPs

Fig 1: IS-IS PDU Types And LSPs

IS-IS PDU Types

Following are the three types of the IS-IS PDU types:

Hello PDU
Link State PDU
Control PDU

Hello PDU (It is responsible for the Neighbor ship)

Local Area Network Layer-1 Hello PDU.
Local Area Network Layer-2 Hello PDU.
Peer to Peer Hello PDU.

Link State PDU (It is responsible for the Routing)

Layer-1 Link state packets and it is for the layer-1 routing updates.
Layer-2 Link state packets and it is for the layer-2 routing updates.

Control PDU (It is responsible for the controlling Routing Updates)

Layer-1 Partial Sequence Number PDU.
Layer-2 Partial Sequence Number PDU.
Layer-1 Complete Sequence Number PDU.
Layer-2 Complete Sequence Number PDU.

Link State Packets

IS-IS protocol uses the link state packets that is pretty similar to the Open Shortest Path First's LSA's so in the link state packets you will find the following things:

One or more than one prefixes
Adjacent neighbors
Metric

Let suppose if two router R1 and R2 are in the same area so they will establish a level neighbor adjacency. These routers will flood their LSPs within the area so that everyone in the area knows all about the LSPs so the two routers add each others LSP in their database and these routers can run the Dijkstra Algorithm on their level 1 database and find out the shortest path to each destination.

And if the two routers are in different areas and will establish a layer 2 neighbor adjacency.

Piece of Concepts of IS-IS

IS-IS uses DIS and Pseudonode that are similar to the OSPF's Designated Router/Backup Designated Router to minimize the unnecessary flooding.

When IS-IS routers becomes the neighbors in between the neighbors they do the election to decide who will become the the Designated IS and that election will base on the following things:

Interface Priority and the default interface priority is 64.
Highest Subnetwork Point of Attachment.
On the Local Area Network Media Access Control Address.
On the frame relay DLCI Number.
The System ID.

The DIS is responsible for the creating the Pseudonode and the Pseudonode is the virtual node and there are two responsibilities of the that virtual node:

Update and create the Pseudonode LSP.
Create a Complete Sequence Numbers Protocol.