Cisco switches support two frame tagging protocols:
- Inter-Switch Link (ISL)
- 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.
- 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.