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DEMO: Inter-Switch Link (ISL)

Inter-Switch Link (ISL) is a Cisco-proprietary VLAN trunking protocol that serves as an alternative to 802.1q on select Cisco switches, though it is now deprecated and no longer recommended for new deployments.

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About this video

Inter-Switch Link (ISL) is a Cisco-proprietary VLAN trunking protocol that predates the widespread adoption of the IEEE 802.1q standard. It transfers VLAN information between switches by prepending a tag to the original Ethernet frame, identifying which VLAN that frame belongs to. Because ISL places its tag at the front of the frame while 802.1q inserts a tag within the frame header, the two encapsulation methods are fundamentally incompatible and cannot coexist on the same trunk link — a port must be configured for one or the other. ISL is supported only on Cisco devices and is not universally available even within the Cisco product line. On some Cisco switches it is configured as the default trunking encapsulation, which means administrators working in mixed or standards-based environments may need to explicitly reconfigure affected interfaces. Cisco has deprecated ISL and does not recommend it for any new network deployments, making 802.1q the clear choice for modern infrastructure due to its broader vendor support, open standard status, and continued development. Changing the trunking encapsulation on a Cisco switch is straightforward. From global configuration mode, navigating to the target interface and issuing the switchport trunk encapsulation command provides options to select dot1q, isl, or negotiate. Selecting dot1q and completing the standard VLAN assignments on that interface brings the port into full 802.1q compliance, allowing it to communicate correctly with adjacent switches regardless of vendor.

What you'll learn

What's covered

ISL vs 802.1Q Trunking

Aligned to

Cisco CCNA
2.2 Configure and verify interswitch connectivity
1.13 Describe switching concepts
2.1 Configure and verify VLANs (normal range) spanning multiple switches

Key terms

Virtual LAN
VLAN
A logical grouping of network devices that behave as if they are on the same network regardless of physical location.
Encapsulation
The process of wrapping data with protocol headers as it passes down the layers of the OSI model.
Inter-Switch Link
ISL
A Cisco proprietary trunking protocol that encapsulates Ethernet frames with a 26-byte ISL header and 4-byte trailer to carry VLAN information between switches. ISL has been superseded by the industry-standard IEEE 802.1Q trunking protocol.
802.1Q
The IEEE standard that defines VLAN tagging, specifying how a 4-byte tag is inserted into an Ethernet frame to identify its VLAN membership across trunk links.
Switch
A network device that connects devices within a LAN and forwards traffic based on MAC addresses.

Topics

Networking Vlan Trunking Inter Switch Link 802 1q Cisco Ios Trunk Encapsulation

Transcript

Inter-Switch Link

Some Cisco switches have an alternative to 802.1Q. In fact, on some Cisco switches it's set up as a default, while it's unsupported on other Cisco switches, so we should be familiar with this Inter-Switch Link, or ISL.

802.1Q adds a tag that includes VLAN information that gets transferred from one switch to another. This allows it to transfer the information on what VLAN that frame is a part of. As we see with Inter-Switch Link, ISL, the tag is added at the beginning, so the two don't play well together — you have to configure one or the other on a link.

This is only supported on Cisco devices; it is Cisco proprietary. And even then it's not supported on all Cisco devices. In fact it's being deprecated, so it's not really recommended on any new networks to use ISL. Instead, the recommendation is to use 802.1Q. There are a few advantages to ISL, but ultimately there are more advantages to 802.1Q.

Changing the encapsulation

Since on some Cisco devices ISL is set up as the default, while on other switches it's not, we may need to change between 802.1Q and ISL. Let's take a look at what that looks like.

The configuration for this is really quite simple. What I'm going to do is get into the global configuration mode, and then the interface that I want to change. In this case right here I'm going to do interface fa10, and we'll do 43.

Once I'm in the interface, all I need to do is do a switchport trunk encapsulation, because this is choosing the encapsulation level here. Then I'm going to do a question mark, and you can see I can set it for dot1q, ISL or negotiate. In this case right here, since we want to move to dot1q, I'll type in dot1q and then hit enter on that.

At this point I'd configure it just like any other dot1q port, where I would set up the VLANs on this interface so that it could start communicating with the other switch.

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