Created: 20 Aug 2013
Status: Not Applicable
Part: Part 90-4 (2013; Edition 1)
Paragraph: Para 3 and Fig 13
Category: Issue for edition 2 of this part
“Nodes are attached by edge links, while bridges are connected among themselves with trunk links that carry a higher traffic than edge links and therefore require more bandwidth."
This statement and figure indicate that each of the switch (a.k.a. bridge) ports for the switch-to-switch links are configured as TRUNK ports.
TRUNK ports allow messages with any VID to pass i.e. there are several, if not all, values of VID that are allowed to pass through the port.
Certainly switch-to-switch links will most likely need to be configured as TRUNK ports since the various messages between the switches could each belong to different VLANs, i.e. their VID might be different but must still pass.
The statement and diagram also indicate that IEDs are only connected to the switch via the EDGE ports on the switches.
EDGE ports support only one specific VLAN i.e. the VID in the message must match the unique PVID of the port.
This therefore means that IEDs only communicate using a single VLAN.
However it is incorrect to assume that the IED will only use one VLAN, i.e. is only connected to an EDGE port.
In fact (assuming VLAN-aware switch configuration is used) the port for switch-to-IED may need to be configured as TRUNK as the IED may need to send/receive messages on multiple VLANs i.e. the acceptable VIDs will need to be more than just a single VID value, e.g. VID=1 for GOOSE and VID=5 for SCADA, in fact I have spoken with one utility who has 200+ VLANs on the one port all for different remote engineering access profiles! These require the switch ports to be configured as TRUNK, not EDGE.
The wording and diagram need to be modified to indicate that the IEDs can be connected via EDGE or TRUNK ports depending on, respectively, whether there is one or multiple VLANs that the IED needs to communicate on.