Is-Is Pdu Format - HP 4800G Series Configuration Manual

The Level-1 and Level-2 DISs are elected respectively. You can assign different priorities for different
level DIS elections. The higher a router's priority is, the more likelihood the router becomes the DIS. If
there are multiple routers with the same highest DIS priority, the one with the highest SNPA
(Subnetwork Point of Attachment) address (MAC address on a broadcast network) will be elected. A
router can be the DIS for different levels.
IS-IS DIS election differs from OSPF DIS election in that:
A router with priority 0 can also participate in the DIS election.
When a router is added to the network and becomes the new DIS, an LDP flooding is triggered.
As shown in Figure
adjacencies with each other.
Figure 1-4 DIS in the IS-IS broadcast network
The DIS creates and updates pseudonodes as well as generates their LSPs to describe all routers on
the network.
A pseudonode represents a virtual node on the broadcast network. It is not a real router. In IS-IS, it is
identified by the system ID of the DIS and a one-byte Circuit ID (a non zero value).
Using pseudonodes can reduce the resources consumed by SPF and simplify network topology.
On IS-IS broadcast networks, all routers are adjacent with each other. However, the DIS is responsible
for the synchronization of their LSDBs.

IS-IS PDU Format

PDU header format
IS-IS packets are encapsulated into link layer frames. The Protocol Data Unit (PDU) consists of two
parts, the headers and the variable length fields, where the headers comprise the PDU common header
and the PDU specific header. All PDUs have the same PDU common header, while the specific headers
vary by PDU type. The following figure shows the PDU format.
1-4, the same level routers on a network including non-DIS routers establish
1-6