A customer network had a 2620 router trunked to a 2950 switch with some hosts in vlan 2 and some with no access port assignment. Hosts on the native vlan were able to access (per user reports) a server on vlan 2 until we replaced the 2620 with a 3750 mls. I put the untagged ports in vlan 3, setup SVIs for vlan 2 and 3, and trunked to the 2950. The subnets were consecutive /27 that could form a /26. During troubleshooting, I found the customer gave the vlan 2 server the /26 mask. The server could talk to other vlan 2 hosts, ping its SVI gateway, Internet hosts, but not the vlan 3 SVI or hosts. Vlan 3 hosts could ping any vlan 2 host except the server. Once the mask on the vlan 2 server was corrected, all worked as expected. Thoughts? How did the mask affect intervlan routing on a mls but not a router?
The 3750 is not looking at the mask used on the server. The real issue is on the server and how it determines how it chooses to communicate with addresses in VLAN 3. Since it considers the addresses to be "local" it will arp for them. (for any address that is not local the server will forward to its default gateway) If the server arps for them and receives no response (because proxy arp is disabled) then it can not communicate with them.
Consider that the only decision the 3750 has made in this was the decision whether to respond when it receives an arp request for an address in VLAN 3. If it had responded (proxy arp enabled) then the server can communicate with VLAN 3. But when the 3750 does not respond to that arp (proxy arp disabled) then the server can not communicate with VLAN 3.
You certainly should be able to configure no ip proxy-arp on the subinterfaces of the 2620. Any IP interface or subinterface is capable of proxy arp. It is enabled by default. And you can configure any of those interfaces to disable it - on an interface by interface basis. So disabling proxy-arp on the physical interface has no effect on its subinterfaces. If you want it disabled on the subinterface then you must configure on the subinterface.