SummaryWhen using an InterSwitch Link (ISL), the Exinda interfaces' MTU needs to be modified to operate properly.
OverviewISL or Inter-Switch Link 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 trailer to the packets.
If an Exinda is installed on an ISL trunk frames may be too big and may be end up being truncated, resulting in both CRC failures and potentially framing errors as the unit tries for a new packet but gets the end of the previous one.
The Exinda will not bridge large frames without increasing the MTU 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 largers frames to be bridge, however all functionality including monitoring, QoS, acceleration, packet markeing will work when using 802.1Q and not ISL. To increase the MTU, it can be done through the web UI on the Configuration > System > Network page. Each interface has its own MTU that can be modified.