How can I connect a Catalyst 5000 to a Catalyst 3900 with ISL?
A. The Catalyst 3900 switch can be connected to the Catalyst 5000 through the dual 100-Mbps ISL expansion module. The Catalyst 3900 Token Ring switch does not support any mode other than ISL, so it is always trunked. The Catalyst 3900 ISL modules also only support a 100 Mb connection and default to full duplex. Be very careful when you connect a Catalyst 3900 and a Catalyst 5000 switch through the ISL link. The main problem is that the Catalyst 3900 does not support Fast Ethernet media negotiation. For this reason, if the Catalyst 5000 is configured for AUTO, it defaults to 100 Mb half duplex. This causes problems, such as the port changing from trunk to non trunk and packet loss.
