On point-to-point links, an IS-IS router configured for both Level 1 and Level 2 sends a single, combined Point-to-Point Hello (P2P IIH) packet. A field within this single PDU indicates the router's capabilities (L1, L2, or both), allowing it to form both types of adjacencies efficiently.

Conversely, on broadcast network segments like Ethernet, an L1/L2 router must send two distinct types of Hello packets: an L1 LAN IIH to discover and maintain adjacencies with L1 neighbors, and a separate L2 LAN IIH for its L2 neighbors. This separation is necessary to manage different adjacencies on a shared, multi-access medium.

A. On point-to-point links, a single P2P Hello PDU is used to establish both L1 and L2 adjacencies, not separate ones.

D. On broadcast networks, separate L1 and L2 LAN Hello PDUs are required; a single combined message is not used for this network type.

1. Juniper Networks TechLibrary

"Understanding IS-IS Adjacencies": "On a broadcast subnetwork

a Level 1/Level 2 router sends separate hello packets for Level 1 and Level 2 adjacencies. On a point-to-point subnetwork

a Level 1/Level 2 router sends a single hello packet for both Level 1 and Level 2 adjacencies."

2. RFC 1195

"Use of OSI IS-IS for routing in TCP/IP and dual environments"

Section 3.7.2

LAN IIH PDU: This section states

"A router which is operating as both a Level 1 and Level 2 router shall transmit both Level 1 and Level 2 LAN Hellos." This directly supports the behavior on broadcast interfaces.

3. RFC 1195

"Use of OSI IS-IS for routing in TCP/IP and dual environments"

Section 3.7.1

Point to point IIH PDU: This section describes a single PDU type for point-to-point links where the "Circuit type field indicates whether the sender is a Level 1 router

a Level 2 router

or both

" confirming a single

combined message is used.