Below is the thing:
Fabric Extend Allows you to tunnel your L2 VSNs, IP Shortcuts, and L3 VSNs over a non-SPB network. This could be a carrier network or another backbone network that is not running SPB. There is the IP tunnel method, which uses VXLAN header and that uses point-to-point tunnels. The MTU required size is 1594 or 1600 bytes. This is enforced. We do not do fragmentation on the VSP 7200 or VSP 8000. IS-IS interface won't come up if the connnection does not support this MTU size. If the carrier can not provide a large enough MTU, you will need to use the VSP 4000 with an ONA to perform packet fragmentation and reassembly.
The ONA is only used on the VSP 4000 for Fabric Extend. The VSP 4000 can support L2VSNs without ONA. The VSP 4000 doesn't have the computing power that the VSP 8000 and VSP 7200 have to support IP tunnels. The ONA allows packet fragmenting which is not supported supported on 7200 and 8000.
So Basically, VSP 7200 and 8000 support all FE capabilities except packet fragmentation. For that you need an VSP 4000 with ONA on both sides.
If the tunnels are exclusively L2 and intermediate devices support a 1600 byte MTU, you can use VSP 4000 without ONA
Hope this helps!
DILRAJ
