Cisco Inter-Switch Link (ISL) is a Cisco Systems proprietary protocol that maintains VLAN information as traffic flows between switches and routers, or switches and switches.
ISL is Cisco's VLAN encapsulation method and supported only on Cisco's equipment through Fast and Gigabit Ethernet links. The size of an Ethernet encapsulated ISL frame can be expected to start from 94 bytes and increase up to 1548 bytes due to the overhead (additional fields) the protocol creates via encapsulation. ISL adds a 26-byte header (containing a 15-bit VLAN identifier) and a 4-byte CRC trailer to the frame. ISL functions at the Data-Link layer of the OSI model. ISL is used to maintain redundant links.
Another related Cisco protocol, Dynamic Inter-Switch Link Protocol (DISL) simplifies the creation of an ISL trunk from two interconnected Fast Ethernet devices. Fast EtherChannel technology enables aggregation of two full-duplex Fast Ethernet links for high-capacity backbone connections. DISL minimizes VLAN trunk configuration procedures because only one end of a link needs to be configured as a trunk.
Cisco's ISL competes with the IEEE 802.1Q protocol, a widely used non-proprietary VLAN tagging protocol.
Trunk Ports and the Native VLAN
Clause 9 of the 1998 802.1Q standard defines the encapsulation protocol used to multiplex VLANs over a single link, by adding VLAN tags. However, it is possible to send frames either tagged or untagged, so to help explain which frames will be sent with or without tags, some vendors (most notably Cisco) use the concepts of a) Trunk Ports and b) the Native VLAN for that trunk.
The concept of a Trunk Port is that once a port is designated as a Trunk Port, it will forward and receive tagged frames.
Frames belonging to the Native VLAN do NOT carry VLAN tags when sent over the trunk. Conversely, if an untagged frame is received on a trunk port, the frame is associated with the Native VLAN for this port.
For example, if an 802.1Q port has VLANs 2, 3 and 4 assigned to it with VLAN 2 being the Native VLAN, frames on VLAN 2 that egress (exit) the aforementioned port are not given an 802.1Q header (ie., they are plain Ethernet frames). Frames which ingress (enter) this port and have no 802.1Q header are put into VLAN 2. Behaviour of traffic relating to VLANs 3 & 4 is as to be expected - frames arriving for VLANs 3 & 4 are expected to be carrying tags that identify them so, and frames leaving the port for VLANs 3 & 4 will carry their respective VLAN tag.
Not all vendors use the concept of Trunk Ports and Native VLANS. Annex D to the 1998 802.1Q standard uses the concept of Trunk Links, but the current (IEEE Std 802.1D™- 2004) standard does not use the terms Trunk or Native.
ISL is Cisco's VLAN encapsulation method and supported only on Cisco's equipment through Fast and Gigabit Ethernet links. The size of an Ethernet encapsulated ISL frame can be expected to start from 94 bytes and increase up to 1548 bytes due to the overhead (additional fields) the protocol creates via encapsulation. ISL adds a 26-byte header (containing a 15-bit VLAN identifier) and a 4-byte CRC trailer to the frame. ISL functions at the Data-Link layer of the OSI model. ISL is used to maintain redundant links.
Another related Cisco protocol, Dynamic Inter-Switch Link Protocol (DISL) simplifies the creation of an ISL trunk from two interconnected Fast Ethernet devices. Fast EtherChannel technology enables aggregation of two full-duplex Fast Ethernet links for high-capacity backbone connections. DISL minimizes VLAN trunk configuration procedures because only one end of a link needs to be configured as a trunk.
Cisco's ISL competes with the IEEE 802.1Q protocol, a widely used non-proprietary VLAN tagging protocol.
Trunk Ports and the Native VLAN
Clause 9 of the 1998 802.1Q standard defines the encapsulation protocol used to multiplex VLANs over a single link, by adding VLAN tags. However, it is possible to send frames either tagged or untagged, so to help explain which frames will be sent with or without tags, some vendors (most notably Cisco) use the concepts of a) Trunk Ports and b) the Native VLAN for that trunk.
The concept of a Trunk Port is that once a port is designated as a Trunk Port, it will forward and receive tagged frames.
Frames belonging to the Native VLAN do NOT carry VLAN tags when sent over the trunk. Conversely, if an untagged frame is received on a trunk port, the frame is associated with the Native VLAN for this port.
For example, if an 802.1Q port has VLANs 2, 3 and 4 assigned to it with VLAN 2 being the Native VLAN, frames on VLAN 2 that egress (exit) the aforementioned port are not given an 802.1Q header (ie., they are plain Ethernet frames). Frames which ingress (enter) this port and have no 802.1Q header are put into VLAN 2. Behaviour of traffic relating to VLANs 3 & 4 is as to be expected - frames arriving for VLANs 3 & 4 are expected to be carrying tags that identify them so, and frames leaving the port for VLANs 3 & 4 will carry their respective VLAN tag.
Not all vendors use the concept of Trunk Ports and Native VLANS. Annex D to the 1998 802.1Q standard uses the concept of Trunk Links, but the current (IEEE Std 802.1D™- 2004) standard does not use the terms Trunk or Native.
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