networking - What happens when Gigabit switches are connected over an old CAT5 cable?

If two gigabit switches are connected using an old CAT5 (not CAT5e) cable, will they slow their connection down to 100 Mbit/s?

Answer

To answer the original question without getting too pedantic, they (the active devices/endpoints/Gigabit switches for the commenters below who got confused) will negotiate and connect at the speed they determine will pass over the wire (CAT5 cable connecting them together). You have three outcomes:

1. 
   

   Quality CAT5 was used for the interconnect, the run was short enough and care was taken in termination (same goes for patch cables). The devices link up at 1Gbps and nobody knows the difference. You also get lucky and both devices are 1000BASE-T and were designed for CAT5 anyway.

   
   


2. 
   

   The CAT5 cabling and termination is marginally capable of handling 1Gbps, the devices negotiate and link up at 1Gbps. Annoyance prevails as packet retries and misery come from a connection that mostly functions. In worse situations, the negotiation occurs, but the link fails and you find you have to manually lock the connections to 100Mbps till you take care of the issues.

   
   


3. 
   

   The CAT5 installation is so marginal that the devices negotiate and connect at 100Mbps with tasty slowness, but are able to pass traffic reliably at the fallback speed.

   
   

Needless to say, scenario 2 is one you don't want.