I often see questions about redundancy in SharePoint index servers.
Couple of points to keep in mind:
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only one index server per SSP
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outage of index server will not cause a disruption in usage (unless the index server is also the a query server)
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a change to the index server requires central admin change (no automatic failover)
A SharePoint aware backup of the Index file and SSP search database will allow you to quickly restore a lost index server.