H3C Technologies H3C S7500E Series Switches - Manual (Page 152)

5-7
Figure 5-4 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.
Figure 5-5 PDU format
Common header format
Figure 5-6 shows the PDU common header format.