Fabric Attach Element Stuck in Discovery
A VSP 7200 acts as a fabric BEB. Port 1/10 connects to an Extreme EXOS access switch that serves 12 wireless access points in VLAN 300. The design requires VLAN 300 to map to I-SID 30000. The VSP engineer enabled auto-sense on port 1/10 but did not enable Fabric Attach globally. The EXOS is configured as a Fabric Attach client and is sending requests. On the VSP, show auto-sense lists the EXOS as discovered, but the element never becomes authorized and no I-SID appears. The APs do not get DHCP addresses. The EXOS uplink is a trunk that allows VLAN 300, and the AP ports are untagged in VLAN 300. The service must work without changing the EXOS hardware.
1. Explain the relationship between auto-sense, Fabric Attach, and I-SID assignment. What must be enabled on the VSP and on the EXOS?
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Auto-sense is the VOSS edge-port automation feature; Fabric Attach (FA) is the signaling protocol that lets an access device request VLAN and I-SID services. Auto-sense can discover an FA-capable neighbor, but discovery alone does not create a working service. The VSP must have FA enabled in the appropriate scope and the port must be allowed to act as an FA server/port; the EXOS must have FA enabled and be configured to send the element/I-SID request. The requested I-SID must exist or be authorized, and the local VLAN must be mapped to that I-SID. If FA is disabled, show auto-sense may show the element as discovered but no I-SID is assigned, which matches the symptom.
2. Give a layered verification sequence with expected evidence and the fix.
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Work layer by layer. First confirm link and LLDP/FA neighbor discovery. On the VSP, show auto-sense should show the port and element; show fabric attach and show fabric attach element should show the element state; show fabric attach i-sid should show 30000 assigned. Then show i-sid 30000 should show the VLAN 300 binding, and show vlan 300 should show the access port and uplink. On the EXOS, verify FA is enabled and the port is an FA port. If the element is discovered but unauthorized, enable FA globally and on the port, authorize the element, then re-check. After the I-SID appears, test DHCP and MAC learning; if DHCP still fails, check that VLAN 300 is allowed on the EXOS uplink and that I-SID 30000 exists on the remote BEB.
3. If the EXOS cannot run Fabric Attach, what is the manual alternative and what pitfalls should you avoid?
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Manual alternative: disable auto-sense on that port, configure VLAN 300 on the VSP, bind it to I-SID 30000, and configure the EXOS port as a normal 802.1Q trunk with VLAN 300 allowed. The VSP access port should be a member of VLAN 300 as appropriate, and the I-SID must be configured on the far-end BEBs that need the service. Pitfalls are a VLAN/I-SID mismatch, forgetting the I-SID on one BEB, and leaving auto-sense enabled so the port flaps between modes. Verify with show i-sid, show vlan, and an end-to-end ping and MAC check; this is less dynamic but predictable and easier to document.