IS-IS Architecture Details
IS-IS has the concept of areas for the purpose of scalability, security, and network separation. IS-IS, being a link state protocol, uses a flooding mechanism to synchronize its databases. As stated previously, when the IS-IS network grows, the number of packets in the network due to flooding increases. This results in the following:
- Increase in the memory consumption in each of the switches.
- Higher number of SPF calculations or recalculations and therefore an increase in the CPU processing time. This affects the network convergence leading to network instability.
- Reduced throughput in the links for data traffic.
Consequently, the IS-IS network is divided into domains. Every domain is identified using an Area Identifier. There are two levels in IS-IS. Level L1 refers to all the switches in an area. Level L2 refers to the backbone, which is for switches connecting two different areas. Switches connected to other switches in the same area are L1 switches. Switches connected to both the switches in the same area and different areas are L1/L2 switches. Switches that are connected only to switches in different areas are L2 switches (see Figure 3-5).
Figure 3-5 IS-IS Area
If a switch is connected to another switch in the same area, it is configured as an L1 link (not shown in the Figure 3-5). If the switch is connected to another switch belonging to a different area, it is configured as an L2 link. LAN links (links connected to a shared media, for example, a bridge), are configured as L1/L2, which means both because it is possible that the shared media has switches connected from the same area as well as different areas. For example, in Figure 3-5, the links connecting the border routers are configured as L2 links. The links connecting the routers within the same area (represented as a cloud in the figure) are configured as L1 links. Figure 3-5 shows four border routers (BR1 to BR4) for their respective areas. As explained previously, the IS-IS flood packets originated in an area do not cross the border routers. The border routers summarize the information of its area before synchronizing it with the border router of another area.