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### Instruction:
What is the purpose of IS-IS route summarization and where is it configured?
### Response:
Route summarization reduces the size of the link-state database and hides instabilities. It is configured on L1/L2 border routers to summarize L1 routes before advertising them into L2. The command is "summary-address" in IOS and "summary-prefix" in IOS-XR.
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