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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.
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