An Intermediate System-to-Intermediate System (IS-IS) Level 1 (L1) router can establish adjacencies with any L1 or Level 1/Level 2 (L1/L2) router in the same area.
In ISIS, adjacencies are established by sending and receiving intermediate system-to-intermediate system hello (IIH) packets at the Data Link layer. Since IIH packets are transmitted at the Data Link layer, adjacencies can only be formed if routers share a common network segment. Separate adjacencies are maintained between Level 1 and Level 2 intermediate systems. L1 routers send only Level 1 IIHs, Level 2 (L2) routers send only Level 2 IIHs, and L1/L2 routers send both Level 1 and Level 2 IIHs.
When an L1 router receives an IIH packet on its network segment, the router verifies several details before forming an adjacency:
- Network type
- Routing level
- Area
- Maximum transmission unit (MTU) length
- Authentication parameters

First, the L1 router determines whether the IIH packet is configured for the correct network type and routing level. IS-IS recognizes two network types: broadcast and point-to-point. Broadcast networks use Level 1 LAN IIHs and Level 2 LAN IIHs. If an interface is configured as a broadcast link, such as on a LAN, the interface ignores point-to-point IIHs. Point-to-point networks use point-to-point IIHs. If an interface is configured as a point-to-point link, it ignores LAN IIHs. Point-to-point IIHs indicate the routing level within their type and circuit type fields. An L1 router on a point-to-point link accepts only point-to-point IIHs configured for Level 1 routing.

The L1 router then verifies that the IIH packet originated from a router within the same area in the routing domain. An adjacency is not formed unless the sending router resides in the same area as the L1 router processing the IIH packet. An adjacency is not established unless the router sending the IIH packet is using the same MTU length as the receiving router. By default, IIH packets are padded to the full MTU size unless the no hello padding router configuration command or the no isis hello padding interface configuration command has been issued, in which case only the first five IIH packets are padded. If authentication is used in the area, an adjacency will not be established unless all IIH packets contain the same authentication parameters. If the L1 router determines that the IIH packet satisfies all of the requirements listed above, an adjacency is formed with the neighboring router that sent the packet and routing information is shared.

L1 and L1/L2 routers periodically transmit Level 1 IIH packets onto their network segments. If an L1 router and a neighboring L1 or L1/L2 router send IIH packets with the same network type, area, MTU length, and authentication parameters, an adjacency is established between the routers. If the IIH packet does not meet all of these requirements, it is discarded by the L1 router and an adjacency is not established.

L2 and L1/L2 routers periodically transmit Level 2 IIH packets onto their network segments. L1 routers examine the IIH packets received from L2 routers to determine the packet type before establishing an adjacency. All packets not identified as Level 1 IIH packets are discarded by an L1 router, and an adjacency is not formed. No further processing is performed on the Level 2 IIH packet by the L1 router once the packet has been discarded.

L2 routers form adjacencies only with L2 and L1/L2 routers. L2 routers use the same parameters for establishing adjacencies as L1 routers, except L2 routers do not consider the area parameter? L2 routers can establish adjacencies with L2 and L1/L2 routers from any area in the routing domain. There are two remaining hello packet types: the end system hello (ESH) and the intermediate system hello (ISH). An ESH is sent by an end system (ES), such as a computer, to announce itself to other devices on the network segment. An ISH is sent by an intermediate system (IS), such as a router, to announce itself to other devices on the network segment. ESs discover routers by listening for ISH packets. ISs discover ESs by listening for ESH packets. The ESH and ISH packets are part of the End System to Intermediate System (ES-IS) routing exchange protocol. When an ISIS routing level mismatch, authentication mismatch, or MTU mismatch
occurs, an IS-IS adjacency will not form, but the output of the show clns neighbors command might instead show an ES-IS adjacency.