Is-Is Network Type - HP 4800G Series Configuration Manual

The IS-IS backbone does not need to be a specific Area.
Both the IS-IS Level-1 and Level-2 routers use the SPF algorithm to generate the shortest path tree
(SPT).
Routing method
A Level-1 router makes routing decisions based on the system ID. If the destination is not in the area,
the packet is forwarded to the nearest Level-1-2 router.
A Level-2 router routes packets across areas according to the area address.
Route leaking
An IS-IS routing domain is comprised of only one Level-2 area and multiple Level-1 areas. A Level-1
area consists of a group of Level-1 routers and is connected with a Level-2 area rather than other
Level-1 areas.
The routing information of a Level-1 area is sent to the Level-2 area through the Level-1-2 router.
Therefore, the Level-2 router knows the routing information of the entire IS-IS routing domain but does
not share the information of other Level-1 areas and the Level-2 area with the Level-1 area by default.
Since a Level-1 router simply sends packets destined for other areas to the nearest Level-1-2 router,
this may cause that the best paths cannot be selected.
To solve this problem, route leaking was introduced. A Level-2 router can advertise Level-2 routing
information to a specified Level-1 area. By having the routing information of other areas, a Level-1
router in the area can make a better routing decision for a packet to another area.

IS-IS Network Type

Network type
IS-IS supports two network types:
Broadcast network, such as Ethernet, Token-Ring.
Point-to-point network, such as PPP, HDLC.
For a Non-Broadcast Multi-Access (NBMA) interface, such as an ATM interface, you need to configure
subinterfaces for it and configure the interface type for the subinterfaces as point-to-point or broadcast.
IS-IS cannot run on point to multipoint (P2MP) links.
DIS and pseudonodes
On an IS-IS broadcast network, a router is elected as the Designated Intermediate System (DIS).
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