Inter-Switch Link (ISL)

In the VSX solution topology, an Inter-Switch Link (ISL) is a layer 2 interface between two VSX peer switches. Each VSX switch must be configured with an ISL link connected to its peer VSX switch. It is recommended that this link is peer-to-peer and used for both datapath traffic forwarding and control path VSX protocol exchange. The ISL interface is by default a member of all VLANs on the device. You can change ISL membership through the command line, but you must ensure VLANs that contain VSX LAG members are not excluded from the ISL.

In the datapath, traffic is forwarded natively with no additional encapsulation, unlike VSF. ISL is capable of sending control path data, which requires oversize packets. The ISL MTU is automatically set to the required size to accommodate oversize packets, and cannot be manually overwritten to avoid generating an unintended outage. The token counters of ISL interface show this oversize control path data as part of the ISL operation. The ISL link is the main pipeline for synchronizing data, such as from the following components, during VSX stack join and also permanently between VSX peers:

  • ARP table

  • LACP states for VSX LAGs

  • MAC table

  • MSTP states

The ISL uses version control and provides backward compatibility regarding VSX synchronization capabilities.

The ISL can span long distances (transceiver dependent). The traffic that passes over VSX links has no additional encapsulation.

All ISL ports must have the same speed. The speed can be 1G, 10G, 25G, 40G, or 100G, with 40G and 100G being the preferred speeds. For example: 2x40G.

ISL link between VSX switches