Describing to someone why it is important to correctly configure both sides of the trunk, and, why we specify a native vlan, I was asked a question that I could not answer.
Stage... both sides of the trunk are correctly configured, native vlan is 1, but site does not use vlan1. I explained two reasons for specifying the native vlan.1. Those frame are untagged 2. If the trunk failed for some reason, the vlan traffic specified in the native vlan would still be forwarded.
The result of the customer having native vlan 1 and not having any ports in vlan 1 would be if the trunk failed, they would not be able to pass their data traffic.
The question from the customer was, "what would cause the trunk to fail?" With proper configuration, outside of not saving the config if the switch rebooted, I did not have an answer. I have never seen a properly configured trunk fail.
Does anybody have an answer to this question?
If it is a static trunk configuration then there is likely less or I will say no chance that the trunk will go down especially on our switches, its pretty stable.
But if the trunk configuration is in dynamic desirable mode then due to some negotiation issues trunk may not be operational but at that time data on native vlan will still pass traffic.
If it is dot1q trunk and depending on model of switches you can also avoid native vlan configuration and all vlans can be configured a tagged vlan.
Also you can read this link which tells some more details about usage of vlan 1 and vlan security
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