Network Types - HPE 5800 Series Configuration Manual

Figure 37 shows another IS-IS topology. The Level-1-2 routers connect to the Level-1 and Level-2
routers, and form the IS-IS backbone together with the Level-2 routers. No area is defined as the
backbone in this topology. The backbone comprises all contiguous Level-2 and Level-1-2 routers in
different areas.
Figure 37 IS-IS topology 2
NOTE:
The IS-IS backbone does not need to be a specific area.
Both the Level-1 and Level-2 routers use the SPF algorithm to generate the shortest path tree.
Route leaking
Level-2 and Level-1-2 routers form a Level-2 area. An IS-IS routing domain comprises only one
Level-2 area and multiple Level-1 areas. A Level-1 area must connect to the Level-2 area rather than
other to another Level-1 area.
The routing information of each Level-1 area is sent to the Level-2 area through a Level-1-2 router,
so a Level-2 router knows the routing information of the entire IS-IS routing domain. By default, a
Level-1-2 router does not advertise the routing information of other Level-1 areas and the Level-2
area to a Level-1 area, so a Level-1 router simply sends packets destined for other areas to the
nearest Level-1-2 router. The path passing through the Level-1-2 router may not be the best. To
solve this problem, IS-IS provides the route leaking feature.
Route leaking enables a Level-1-2 router to advertise the routes of other Level-1 areas and the
Level-2 area to the connected Level-1 area so that the Level-1 routers can select the optimal routes
for packets.

Network types

IS-IS supports the following network types:
•
Broadcast network—such as Ethernet and Token-Ring.
•
Point-to-point network—such as PPP and HDLC.
IS-IS cannot run on point to multipoint (P2MP) links.
DIS and pseudonodes
IS-IS routers on broadcast network must elect a DIS.
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.
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