Connect back to back 3550

I know this is stupid…. but I spent a while on it so if you have to connect two Cisco 3550 switches directly to each other on an Ethernet port you must use a crossover cable. Even though a straight through will do for a connection to an end station or even a 3650… UGHHH

dot1q and ISL

dynamic desirable mode is default on 3550 with ISL
dynamic auto mode is default on 3560 with ISL
Disable DTP:
switch mode port access
switchport nonegoiate

802.1Q and ISL differ in that dot1q doesn’t encapsulate the native VLAN. If it gets a packet without an dot1q header it assumes that it must be in the native VLAN.

Native VLANs only apply to dot1q

If SW1 has a NV is 10 and SW2 has a NV of 20, SW1 will send all traffic in VLAN 10 untagged. SW2 will get that and assume it is for VLAN 20 since it is untagged. The switch will throw a VLAN mismatch error.