A Level 1 router directly supports one or more individual network nodes. Level 1 routing is the routing that takes place within a single area, between Level 1 routers and their associated nodes.
Each area in a Domain must have at least one Level 2 router. Level 2 routing is the routing that occurs between Level 2 routers, and generally between the areas within a network domain. An area without a Level 2 router is isolated from the rest of the network.
An Alcatel-Lucent 1850 TSS-5 that has IS-IS Level 2 routing enabled, functions as a Level 1-2 router. It continues to provide Level 1 routing within its area, and also provides Level 2 routing to adjacent Level 2 routers. It can be connected to individual network nodes, one or more Level 1 routers, and one or more Level 2 routers.
An adjacent set of Level 2 routers forms a Level 2 sub-domain. For proper OSI routing, every Level 2 router should have at least one neighbor (via OSI LAN or DCC) that is also a Level 2 router. At least one Level 2 router in an area must be adjacent to a Level 2 router in another area, otherwise, the former area is isolated from the rest of the network.
Each Alcatel-Lucent 1850 TSS-5 functions as a Level 1 router by default. You can use the TL1 command ENT-ULSDCC-L3 to also enable Level 2 routing on an Alcatel-Lucent 1850 TSS-5.
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