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In one of the Cisco docs, I read:

The FabricPath Layer 2 gateway switch will automatically propagate Topology Change Notifications (TCNs) on all its CE (Classical Ethernet) interfaces. The Layer 2 IS-IS messages will carry TCNs across the FabricPath network if proper STP configuration is made on the different switches.

Questions:

  1. Why do the TCNs need to be carried across the Fabric path network? Is the writer trying to convey that there are two separate STP domains connected via the Fabric path network?
  2. Do I need to know the underlying details of ISIS to understand fabric path? I mean, I just want to know enough to drive the car but not intend to know the working of the car engine.
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  1. Why do the TCNs need to be carried across the Fabric path network? Is the writer trying to convey that there are two separate STP domains connected via the Fabric path network?

Because this creates a single, large layer-2 topology, the writer is conveying the fact that classical ethernet interfaces use TCNs, so FabricPath is backward compatible and can use switches that do not directly support it. FabricPath can connect multiple, separate STP domains in a single layer-2 domain. See FabricPath Interfaces:

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  1. Do I need to know the underlying details of ISIS to understand fabric path? I mean, I just want to know enough to drive the car but not intend to know the working of the car engine.

You would be shielded from many of the IS-IS details, but if you want a job as a network engineer in a data center, you should really understand how things work at a low level. If your car breaks down, you can rent one while you get a mechanic to repairs yours, but if your business is losing millions of dollars each minute your data center is down, that isn't practical; you need to be able to mitigate the damage immediately. IS-IS is an industry standard routing protocol that any network engineer should have some exposure to, anyway.

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