Reliable Penguin

On redhat, the ETHTOOL_OPTS can provide some control over interface speed. Here's the description from documentation:

 
ETHTOOL_OPTS=...
      Any device-specific options supported by ethtool. For example,
      if you wanted to force 100Mb full duplex:
        ETHTOOL_OPTS="speed 100 duplex full autoneg off"
      Note that changing speed or duplex settings almost always
      requires disabling autonegotiation with 'autoneg off'.

The ethtool utility provides extensive low level control over ethernet interfaces.