VLANs numbered 1 to 1005 are considered “normal” and ones higher than this are considered extended (VLANs 1006 to 4094). These VLANs cannot be stored in the vlan.dat file which resides in flash as standard range VLANs are, instead they are only stored in the running config. The switch must also be in VTP transparent mode as these VLANs cannot be sent in VTP updates.

VLAN Numbers Summary

VLAN 0 is reserved and not available for use

VLAN 1 is the default  VLAN on all Cisco Switches, the VLAN cant be deleted or changed, its not advertised by VTP as it does not need to be as its a default and cant be removed/changed

VLANs 2 to 1001 Normal range VLANs for allocation

VLANs 1002 to 1005 are used for FDDI and TR translational bridging and shouldn’t be used for anything other than these purposes, they are also not advertised by VTP

VLANs 1006 to 4094 are extended range VLANs which cant be advertised by VTP and the switch must be configured in VTP transparent mode.

Its also an interesting point that when a switch starts up it checks the VTP mode and domain name from the startup-config and vlan.dat file, if they are different it ignores the startup-config and only uses the vlan.dat file

Chapter 1 on Ethernet mentioned about autonegotation using Fast Link Pulses so I wanted to find a little bit more about how this works. Cisco devices detect link speed and duplex settings using Fast Link Pulses (FLP) which is adapted from Normal Link Pulses (NLP) defined in the 10BASET specifications. This negotiation will only occur over 8 wire twisted pair copper and as such wont happen on other media types such as Fiber.

Upon an interface coming up a FLP burst will send 17 pulses of 100ns each representing a Link Code Word (LCW) which informs the other end of the link about the media speed and type and standards to use. The LCW only sends enough information to setup link speed of 100Mbps, so for Gigabit speeds multiple LCWs are sent which contain extra information such as master/slave settings, and multiport capabilities.

Although other media types such as fiber don’t use FLP they do use similar auto negotiation protocols.

Before FLP was defined devices would send an idle signal down the wire and the other end would train to it this is called parallel detection.

Its really important to note that the 802.3u standard does not specify what FLP should do if the link has statically defined speed/duplex settings, so it reverts to parallel detection and correctly sets the speed but assumes the link it half duplex. This results in a duplex mismatch if one end of link is coded for 100Mbps Full and the other for autonegotiate.