Given that you only have one arduino with that address, you should be OK. Ethernet addresses need to be unique in your LAN. Even if you had more, you should get a link light. Are you getting a link light on the switch? You say only one Arduino on this switch, but you earlier mentioned other Arduinos on other switches. Are they on the same LAN (i.e. on the same side of one router)? Everything within a LAN must have a unique ethernet address. If your shields are new enough to have an address sticker, use that in your sketch rather than the "fake" one.