In Cisco Switches for VTP, what frame tagging protocol they support?

Cisco switches support two frame tagging protocols:

  1. Inter-Switch Link (ISL)
  2. IEEE 802.1Q

Inter-Switch Link (ISL)

  • It is Cisco’s proprietary frame tagging protocol.
  •  It encapsulates a frame with an additional header (26 bytes) and trailer (4bytes).
  • It increases the size of a frame by 30 bytes.
  • The header contains several fields, including a 15-bit VLAN ID.
  • The trailer contains an additional 4-byte CRC to verify data integrity.
  • Maximum Transmission Unit (MTU) is 1518 bytes.
  •  It supports a maximum of 1000 VLANs on a trunk port.
  •  It doesn’t support untagged frames, and will always tag frames from allVLANs.

IEEE 802.1Q

  • It is an open standard
  • It inserts a 4-byte VLAN tag directly into the Layer-2 frame header.
  • The VLAN tag includes a 12-bit VLAN ID.
  • This tag increases, from its default of 1514 bytes to 1518 bytes.
  • It supports a maximum of 4096 VLANs on a trunk port.
  • It supports Native VLANS on trunk ports.
  • Management VLAN and configuration of Management VLAN
  • Management VLAN is used for managing the switch from a remote location by using protocols such as telnet, SSH, SNMP, syslog etc.
  • Normally the Management VLAN is VLAN 1, but you can use any VLAN as a management VLAN.