Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Chapter 4 IS-IS Configuration
4-8
Figure 4-4
DIS in the IS-IS broadcast network
The DIS creates and updates pseudonodes as well as their LSP to describe all routers
on the network.
The pseudonode emulates a virtual node on the broadcast network. It is not a real
router. In IS-IS, it is identified by the system ID and one byte Circuit ID (a non zero value)
of the DIS.
Using pseudonodes can reduce LSPs, the resources used by SPF and simplify the
network topology.
Note:
On IS-IS broadcast networks, all routers are adjacent with each other. The DIS is
responsible for the synchronization of their LSDBs.
4.1.4 IS-IS PDU Format
I. PDU header format
The IS-IS packets are encapsulated into link layer frames. The Protocol Data Unit (PDU)
consists of two parts, the headers and the variable length field, where the headers can
be further divided into the common header and the specific header. The common
headers are the same for all PDUs, while the specific headers vary by PDU type. The
following figure shows the PDU format.
Figure 4-5
PDU format
