The CSMA devices used by the bridge to model the ports of the switch contain an outgoing queue. The defaut type of packet queue created by the CsmaHelper that is used to create the ports of your switch is a DropTailQueue.
i.e.:
CsmaHelper::CsmaHelper ()
{
m_queueFactory.SetTypeId ("ns3::DropTailQueue");
m_deviceFactory.SetTypeId ("ns3::CsmaNetDevice");
m_channelFactory.SetTypeId ("ns3::CsmaChannel");
}
If you need to change this, you can call
CsmaHelper::SetQueue("ns3::RedQueue",
"Mode", StringValue("QUEUE_MODE_BYTES"),
"QueueLimit", StringValue("10MB"))