ISL is a proprietary protocol to maintain VLAN information in Ethernet frames as traffic flows. As opposed to 802.1q, the VLAN tag under ISL is external to the Ethernet frame, which is the equivalent to encapsulating the frame. ISL adds a 26-byte header and a 4-byte FCS (Frame Check Sequence) trailer to the packets.
If an Exinda is installed on an ISL trunk, frames may be too big and may end up being truncated, resulting in both CRC (Cyclic Redundancy Check) failures and potentially framing errors as the unit tries for a new packet but gets the end of the previous one.
Exinda devices will not bridge large frames without increasing the MTU (Maximum Transfer Unit) on the interface to allow for frames that are 30-bytes bigger than a normal Ethernet frame, causing the units to drop packets due to CRC/framing errors.
Increasing the MTU may allow larger frames to be bridged, however, all functionalities, e.g., including monitoring, QoS, acceleration, packet marking, etc., will work only when using 802.1Q, not ISL.
To increase the MTU, navigate to Configuration > System > Network in the web UI (user interface). Each interface has its own MTU.