Domain: section of the network under common administrative control
Area: logical segment of the network composed of contiguous routers and their data links
Intermediate System: A router.
End System: A host device.
The four levels of ISIS routing:
Level 0: ES-IS routing in the same subnet.
Level 1: IS-IS routing in the same area.
Level 2: IS-IS routing in the same domain.
Level 3: Inter-domain routing performed by InterDomain Routing Protocol (IDRP).
ISIS Adjacency Possibilities:
L1: Can form adjacency with any L1 in the same area and any L1/L2 in the same area.
L2: Can form adjacency with any L2 in any area, and with an L1/L2 in any area.
L1/L2: Can form adjacency with any L1 in the same area, L1/L2 in any area, and L2 in any area.
A router interface's SNPA (Subnetwork Point Of Attachment) is its highest DLCI number if it's on a Frame network, and its MAC address if the interface is on an Ethernet segment.
ISIS Hello Types:
ESH: ES Hello - Sent by End Systems to discover a router.
ISH: IS Hello - Send by Intermediate Systems to announce their presence. End Systems listen for these.
IIH: IS-to-IS Hello - Send by one IS to be heard by another IS. These hellos makes IS-IS adjacencies possible.
