Types of frame tagging protocols

Cisco switches support two frame tagging protocols.

  • Inter-switch link (ISL)

  • IEEE 802.1Q


The tagging protocol can be manually specified on a trunk port, or dynamically negotiated using Cisco’s proprietary Dynamic trunking protocol (DTP).


Inter-switch link (ISL)

  • Inter-Switch Link (ISL) is Cisco’s proprietary frame tagging protocol. ISL supports several technologies: • Ethernet • Token Ring • FDDI • ATM

  • ISL encapsulates a frame with an additional header (26 bytes) and trailer (4 bytes). Thus, ISL 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.

  • Normally, the maximum possible size of an Ethernet frame is 1518 bytes. This is known as the Maximum Transmission Unit (MTU). Most Ethernet devices use a default MTU of 1514 bytes.

  • ISL increases the frame size by another 30 bytes. Thus, most switches will disregard ISL-tagged frames as being oversized, and drop the frame. An oversized frame is usually referred to as a giant. Somewhat endearingly, a slightly oversized frame is known as a baby giant.

  • Cisco switches are specifically engineered to support this giant ISL – tagged frames. Note that this is a key reason why ISL is Cisco-proprietary.

  • ISL supports a maximum of 1000 VLANs on a trunk port. ISL is also almost entirely deprecated - most modern Cisco switches no longer support it.


IEEE 802.1Q

  • IEEE 802.1Q, otherwise referred to as dot1Q, is an industry-standard frame-tagging protocol.

  • 802.1Q is supported by nearly all switch manufacturers, including Cisco. Because 802.1Q is an open standard, switches from different vendors can be trunked together.

  • Recall that ISL encapsulates a frame with an additional header and trailer. In contrast, 802.1Q embeds a 4-byte VLAN tag directly into the Layer-2 frame header. Because the Layer-2 header is modified, 802.1Q must recalculate the frame’s CRC value.

  • The VLAN tag includes a 12-bit VLAN ID. This tag increases the size of an Ethernet frame, from its default of 1514 bytes to 1518 bytes. Nearly all modern switches support the 802.1Q tag and the slight increase in frame size.

  • 802.1Q supports a maximum of 4096 VLANs on a trunk port.


Configuring trunk links

To manually configure an interface as a trunk port:

Switch (config) # interface gi2/24

Switch (config-if) # switchport mode trunk


For a switch that supports both ISL and 802.1Q, the tagging or encapsulation protocol must be configured first:

Switch (config) # interface gi2/24

Switch (config-if) # switchport trunk encapsulation ISl

Switch (config-if) # switchport mode trunk

Switch (config) # interface gi2/24

Switch (config-if) # switchport trunk encapsulation dot1q

Switch (config-if) # switchport mode trunk


Important note: Both sides of the trunk must be configured with the same tagging protocol. Otherwise, a trunk connection will not form.

If the switch only supports 802.1Q, the switch port trunk encapsulation command will not be available.


Which is the default mode in switch ports?

  • Older switches are dynamic desirable by default.

  • Modern switches are dynamic auto by default.


Difference between 802.1Q and ISL

  • Cisco ISL (Inter-switch Link) is an old Cisco proprietary that is only supported on some Cisco switches.

  • While 802.1Q (dot1Q) is an open standard protocol that is supported on switches from many vendors and most NICs.


Which protocol encapsulates Ether frames?

ISL encapsulate Ethernet frames while 802.1Q tags Ethernet frame.


Which is the VLAN not tagged by 802.1Q?

Native VLAN.


How to delete VLAN information from switch?

  • VLAN information is not saved in the running-config or start-up-config but in a separate file called VLAN.dat on your flash memory.

  • If you want to delete the VLAN information you should delete this file by typing delete Flash:vlan.dat.


What is the maximum number of VLANS permitted in 802.1Q and ISL?

  • Maximum VLAN permitted in 802.1Q is 4094

  • Maximum VLAN permitted in ISL is 1000


Header size of 802.1Q and ISL

  • 802.1Q header size 4 bytes.

  • ISL header size 26 bytes.


Topics Summary