3Com Switch 4800G 24-Port Configuration Manual page 331

IS-IS Overview 331
Figure 102 DIS in the IS-IS broadcast network
L1/L2 L1/L2
L2 adjacencies
L2
L1
L1 adjacencies
DIS DIS
The DIS creates and updates pseudo nodes 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 pseudo nodes can reduce LSPs, the resources used by SPF and simplify the
network topology.
n
On IS-IS broadcast networks, all routers are adjacent with each other. The DIS is
responsible for the synchronization of their LSDBs.
IS-IS PDU Format 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 103 PDU format
PDU common header PDU specific header Variable length fields (CLV)
Common header format
Figure 104 shows the common header format.