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| 7.3 |  | IS-IS Operation |
| 7.3.12 |  | IS-IS network types |

The types of networks that IS-IS defines include the following:

- Point-to-point networks
- Broadcast networks

Point-to-point networks, such as serial lines, connect a single pair of routers. A router running IS-IS will form an adjacency with the neighbor on the other side of a point-to-point interface. A DIS is not elected on this type of link. The basic mechanism defined in the standard is that each side of a point-to-point link declares the other side to be reachable if a Hello packet is received from it. When this occurs, each side then sends a CSNP to trigger database synchronization.

Broadcast networks, such as Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI), are multiaccess in that they are able to connect more than two devices. All devices connected to routers will receive a packet sent by one router. On broadcast networks, one IS will be elected the DIS. Hello packets on broadcast networks are sent to the AllL1ISs or AllL2ISs MAC-layer broadcast addresses. The DIS is responsible for flooding. It will create and flood a new pseudonode LSP for each routing level it is participating in, whether L1 or L2, and for each LAN to which it is connected. A router can be the DIS for all connected LANs or a subset of connected LANS. This depends on the configured priority or, if no priority is configured, the Layer 2 address. The DIS will also create and flood a new pseudonode LSP when a neighbor adjacency is established or torn down or the refresh interval timer for this LSP expires. The DIS mechanism reduces the amount of flooding on LANs.

Nonbroadcast multiaccess (NBMA) networks, such as Frame Relay, Asynchronous Transfer Mode (ATM), and X.25, can connect multiple devices but have no broadcast capability. All the other routers attached to the network will not receive a packet sent by a router. Special consideration should be taken when configuring IS-IS over NBMA networks. IS-IS has no concept of an NBMA network. IS-IS considers these media to be just like any other broadcast media such as Ethernet or Token Ring. In general, it is better to configure point-to-point networks for IS-IS on WAN interfaces and subinterfaces, such as with ATM, Frame Relay, and X.25.

Unlike OSPF, no configuration is necessary to tell IS-IS what the network type is.