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Routing & Switching · Intermediate

IS-IS Level 1

Intra-area IS-IS routing; L1 routers know only their own area and use the closest L1/L2 router for other destinations.

Definition

Level 1 routers form adjacencies only with L1 or L1/L2 routers in the same area. They reach other areas through a default route installed when an L1/L2 router sets the Attached bit in its L1 LSP. This resembles an OSPF totally stubby area.

Example

is-type level-1 under router isis

Exam tip

L1 adjacency requires the same area address; L2 adjacency does not.

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