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| 7.3 |  | IS-IS Operation |
| 7.3.1 |  | High-level view of IS-IS operation |

From a high level, IS-IS operates as follows:

- Routers running IS-IS will send Hello packets out all IS-IS enabled interfaces to discover neighbors and establish adjacencies.
- Routers sharing a common data link will become IS-IS neighbors if their Hello packets contain information that meets the criteria for forming an adjacency. The criteria differ slightly depending on the type of media being used, whether point-to-point or broadcast. The main criteria are matching authentication, IS-type, and MTU size.
- Routers may build a LSP based upon their local interfaces that are configured for IS-IS and prefixes learned from other adjacent routers.
- Routers flood LSPs to all adjacent neighbors except the neighbor from which they received the same LSP. However, there are different forms of flooding and also a number of scenarios in which the flooding operation may differ.
- All routers will construct their link-state database from these LSPs.
- A shortest-path tree (SPT) is calculated by each IS, and from this SPT the routing table is built.

| Web Links Configuring Integrated IS-IS http://www.cisco.com/en/US/products/ sw/iosswrel/ps1828/products_configuration_ guide_chapter09186a00800ca56f.html