Technical Tip: ED flag on ICL interface

In a multi-tier MC-LAG ICL topology, where Tier-1 FortiSwitch MC-LAG peers act as the STP root bridge, it is observed that the ICL interface shows the 'ED' (Edge) flag, even though the edge port is disabled by default.

Consider the following CLI output:

show switch interface "_FlInK1_ICL0_"
    set edge-port disabled

end

Despite the edge port being explicitly disabled, the interface '_FlInK1_ICL0_' still shows the ED flag in the STP diagnostic output.

diagnose stp instance list

MST Instance 0 (Common Spanning Tree):

Instance ID 0 (CST)
Config Priority 20480
Bridge MAC 7818ecb9xxxx, MD5 Digest 9999b43d77cc58bba8854f9991c4a487

Root MAC 7818ecb9xxxx, Priority 20480, Path Cost 0
(This bridge is the root)

Regional Root MAC 7818ecb9xxxx, Priority 20480, Path Cost 0
(This bridge is the regional root)

Port Speed Cost Priority Role State HelloTime Flags
------------------ ----- ---- -------- ---------- ---------- --------- -----
_FlInK1_ICL0_ 40G 1 128 DESIGNATED FORWARDING 2 EN ED

MST Instance 15 (VLAN 4094):

Instance ID 15
Config Priority 20480, VLANs 4094
Bridge MAC 7818ecb9xxxx

Regional Root MAC 7818ecb9xxxx, Priority 20480, Path Cost 0
(This bridge is the regional root)

Port Speed Cost Priority Role State Flags
------------------ ----- ---- -------- ---------- ---------- --------
_FlInK1_ICL0_ 40G 1 128 DESIGNATED FORWARDING EN ED

Explanation of Flags:

• EN – STP Enabled.

• ED – Edge Port.

• LP – Loop Protection.

• RG – Root Guard.

• BG – BPDU Guard.

• IC – PVST Port Inconsistent.

• MV – PVST VLAN Mismatch.

Although 'edge-port' is not enabled manually, FortiSwitch’s STP implementation dynamically marks the ICL port as an edge port in some scenarios.

Reasons:

• ICL is a special port, and STP implementation is different from a normal port.

• It is loop-isolated and does not participate in typical STP topological decision-making.

• FortiSwitch dynamically adds the ED flag to reflect that this port itself will not send the real STP BPDU event, though STP is enabled.

This behaviour is commonly seen when the MC-LAG ICL peers are the STP root bridges.

Related articles:

Troubleshooting Tip: MCLAG-ICL interface in STP discarding state

Technical Tip: Spanning tree in LAG and MCLAG