Enabling proxy_arp for interface eth2.1

Am trying to enable proxy arp for some of the interfaces, with the normal interface name eth0, eth1, etc [root@eth2.1]# sysctl net.ipv4.conf.eth0.proxy_arp 0 But for interface names such as "eth1.11, eth2.1" its giving the below error. Tried different format "", '' etc. but no help. [root@eth2.1]# sysctl net.ipv4.conf.eth2.1.proxy_arp error: "net.ipv4.conf.eth2\.1.proxy_arp" is an unknown key can anyone please point out the correct way to do this ?