GitLab

If you configure your Tor client with

usebridges 1
bridge 128.31.0.34:9009

and you run it and it works, then Tor will end up writing two things to disk: 1) a @purpose bridge descriptor for 128.31.0.34 in your cached-descriptors file:

@downloaded-at 2011-02-08 07:54:52
@source "128.31.0.34"
@purpose bridge
router bridge 128.31.0.34 9009 0 0
...

and 2) an entry guard stanza in your state file:

EntryGuard bridge 4C17FB532E20B2A8AC199441ECD2B0177B39E4B1
EntryGuardAddedBy 4C17FB532E20B2A8AC199441ECD2B0177B39E4B1 0.2.3.0-alpha-dev 2011-02-01 18:43:23

Then if you kill your Tor and run it with

usebridges 1
bridge 150.150.150.150:9009

it will successfully bootstrap -- using the bridge that worked before but isn't your requested bridge.