Is-Is Pdus - HPE FlexNetwork HSR6800 Configuration Manual

The Level-1 and Level-2 DISs are elected separately. You can assign different priorities to a router
for different level DIS elections. The higher the router priority, the more likely the router becomes the
DIS. If multiple routers with the same highest DIS priority exist, 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 the following ways:
•
A router with priority 0 can also participate in the DIS election.
•
When a router with a higher priority is added to the network, an LSP flooding process is
performed to elect the router as the new DIS.
As shown in
adjacencies with each other.
Figure 37 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 an IS-IS broadcast networks, all routers establish adjacency relationships, but they synchronize
their LSDBs through the DIS.

IS-IS PDUs

PDU
IS-IS PDUs are encapsulated in link layer frames. An IS-IS PDU has 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 38 PDU format
Figure 37, the same level routers on a network, including non-DIS routers, establish
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