Juniper PTX5000 Hardware Manual page 237

The holes in the front-mounting flanges are spaced at 3 U (5.25 in. or 13.3 cm).
Figure 85: Installing the PTX5000 in a Four-Post Rack
1 Four-post rack
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RELATED DOCUMENTATION
Rack Requirements for the PTX5000 | 174
Overview of Installing the PTX5000 | 197
2 Front-mounting flange
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219