H3C Technologies H3C SR8800 - Manual (Page 157)

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Figure 52 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.
NOTE:
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 PDUs
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. The headers comprise the PDU common header and the PDU
specific header. All PDUs have the same PDU common header. The specific headers vary by PDU type.
Figure 53 PDU format
Common header format
Figure 54 shows the PDU common header format.