You can probably get away using relays that have a very flat & short
geometry.NAIS make many that are suitable.
DPDT, tiny geometry. Split the cable out in pairs, and put a pair on
each relay. You'll need 4 since GBE uses
all 4 pairs. If your cable run isn't near the length limit, the losses
introduced will have negligible effect.
You'll lose about 1" of 'twist, but you lose nearly that much with
each plug on a Cat6 cable.

There are micro relays (MEMS) used in cell phones, if you want to keep
the impedance perfect
and are good at doing strip line board layouts.

Manual switches also exist to do this. You didn't say if you needed
electrical control.
http://secureswitch.com/ManualABswitches.htm

and electrical one
http://www.blackbox.com/Store/Detail.aspx/Pro-Switching-System-1U-NBS-RJ-45=
-A-B-All-8-Pins-8-Port/NBSALL8

You need to search for "gigabit failover switch".

Lots of info on 'copper A/B switching' here.
http://img.pr.com/release-file/1108/343451/AB_Switches_5028-02.pdf


Robert
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