SWITCH 300-115 1.3 Configure and verify VLANs

1.3.b VLAN database

a vlan is a layer 2 construct, a pointer to layer 3, an association by layer 2 to layer 3…
vlan=network=subnet=broadcast domain

a broadcast domain is the set of devices which may receive broadcasts… a layer 3 device is the defining element of the broadcast domain… a switch creates more and smaller collision domains with each additional port it places within the domain… a layer 3 device creates an additional broadcast domain with each port it introduces…  a switch floods broadcasts, multi-casts and unknown unicasts to every device in the domain… a layer 3 device recieves all broadcasts from a connected switch, and drops them (unless otherwise defined)… a layer 3 device defines the broadcast domain with that action…this layer 3 device is affectionately known as a router... vlan=network=subnet=broadcast domain


a vlan number is the only requirement when creating a vlan. of course it’s fairly useless without ports added to it, and a descriptive name would be nice also, but these are not required…

an end-to-end vlan spans the entire switched network and should be avoided as such, a local vlan remains local (within its group) and is preferred…

a vlan trunk can carry multiple vlans’ traffic across the network

 isl encapsulates a frame for tagged transit whereas the preferred method of tagging is 802.1q, which injects a 4 byte field into the frame… 802.1q does not tag the native vlan; the native vlan is identified by the lack of a tag…

dtp is dynamic trunking protocol.  it is a trunking negotiation mechanism…

the native vlan is a settable parameter and is vlan1 by default… it is recommended to change the native vlan to something other than 1 after bringing the switch up… it is important to match the native vlans between switches to avoid annoying cdp messages, and sometimes derailed traffic…