It really doesnt matter that there is a whole class B address space as you should only get arped when someone om the same class B needs to know your mac address. Once the arping device has your mac address it should cache it so it doesn't have to arp for it again for a long time. All the other people on the calls B shouldn't be trying to find your MAC address as they theoretically should only be talking to your service providers DSLAM.
It sounds like somebody has screwed up at the service provider configuring their routers they have probably:- i) Configured a really small arp cache timeout value so the service provider router is permanatly having to re arp for the mac addresses of all the DSL modems or ii) Configured a static route via a broadcast interface (eg etherent) on the cental router. This is a really bad thing as instead of just arping for the next hop address the router will arp every time it needs to send a packet to any address on the network the route is for to try and determine the gateway to that address. This is a really good way to crucify network performance , static routes pointing at interfaces rather than next hop addresses should only be used on point t point networks (leased line etc). Pat -- Cisco CCIE #2305
