Save VLAN.DAT in GNS/Dynamips

You know when you boot your router, the flash: is not saved and the vlan.dat that is saved on it is lost.. To remedy this, use a vlan.dat saved in nvram:

SW0(config)# vtp file nvram:/vlan.dat
SW0#dir nvram:
Directory of nvram:/

  123  -rw-        1683                      startup-config
  124  ----           5                      private-config
    1  -rw-           0                      ifIndex-table
    2  -rw-         780                      vlan.dat

There, now all your vlan/vtp config will be saved..

VLANs #1

VLANs are seperate broadcast domains.

VLAN Numbers are 1-1005 (1 and 1002-1005 are reserved)
Extended VLANs are 0-4095. (0,1 and 4095 are reserved)

ISL Trunking
Cisco proprietary protocol. Encapsulates the whole Ethernet frame.
Adding a 26 byte header and 4 byte trailer. (Total of 30 bytes)

802.1Q Trunking
Standards based. Adds a tagging field to the Ethernet Frame.
This is 4 bytes of overhead. With 2 bytes of TPID (Tag Protocol Identifier, alwasys 0x8100), 3 bits of Priority field for CoS (Class of Service), 1 bit CFI, and 12 bits of VLAN Identifier.

Has concept of Native VLan. Frames that belong to native vlan are not encapsulated with any tagging information.

Because default Ethernet frames are 1518 bytes (1500 bytes of MTU and 18 bytes Ethernet header), with the addition of 802.1Q, Ethernet frame size is increased to 1522 bytes.