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Routing & Switching · Intermediate

Intermediate System to Intermediate System (IS-IS)

A link-state IGP originally for OSI CLNS that runs directly over Layer 2 and is widely used in service provider cores.

Definition

IS-IS uses Dijkstra SPF like OSPF but organizes routing into Level 1 (intra-area) and Level 2 (backbone) and is extensible through TLVs, making IPv6, TE and segment routing easy to add. It runs directly over the data link layer, not IP, which makes it harder to spoof remotely. Cisco AD is 115.

Example

router isis / net 49.0001.0000.0000.0001.00 ; ip router isis on interfaces

Exam tip

IS-IS areas are defined per router (in the NET) and area boundaries fall on links, unlike OSPF where boundaries fall on routers.

Related terms