ISL Inter-Switch Link Protocol – Trunking or Frame Tagging protocol

Inter-Switch Link (ISL) is specifically used for tagging information of VLAN onto an Ethernet frame. This tagging information permits multiplexing of VLANs over a trunk link and this takes place through an external covering procedure or method, as a result of which the switch can easily determine/identify the membership of VLAN frame over the trunked link. When you run ISL, then it is easy to interconnect number of switches and it is also possible to maintain the information of VLAN while the traffic is passing on trunk links between switches. If we compare ISL versus Fast Ethernet then we can see that ISL provides a full wire-speed performance, low-latency this is because Fast Ethernet make use of half- or full-duplex mode. ISL actually involves a tagging process externally, in which the original frame is not changed—it’s only covered with a new 26-byte header of ISL. At the end of the frame a second 4-byte frame check sequence (FCS) field is also added. As the frame has been covered with information by ISL so only the devices familiar with ISL can read it. The range of these frames is up to maximum of 1522 bytes long, and the devices receiving them can actually record them as a really big frame since it’s range is more than the allowed range on an Ethernet segment i.e. 1518 bytes. Every single frame is tagged when it enters the switch on the multi-VLAN (trunk) ports on multi-VLAN (trunk) ports,. ISL network interface cards (NICs) permits the servers to receive and send frames that are tagged with multiple VLANs so there is no need of router to pass through number of VLANs. In this way the latency is reduced. With ISL there is no need of router in order to interact/communicate and so the users can efficiently and quickly access to servers without any delay. The use of this technology can serve as network analyzers, investigators and administrators in order to include file servers in more than one VLANs at the same time. When the frame forwards out a port i.e. configured as a trunk link only then the addition of ISL VLAN information takes place to a frame. On the other hand, if the frame is forwarded out an access link then the ISL covering is removed from the frame—do not forget this important ISL fact!

Trunk link

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