For some reason our Cisco is responding to arp requests between machines on the same switch. It's not breaking anything but it is slowing initial connections. I've been making a lot of changes on the Cisco recently to configure a client and site 2 site vpn. The most recent being disabling nat between certain ip ranges. Any ideas why this might be happening?

devperf01b:~ sudo arping devperf01c
   60 bytes from 2c:33:11:2e:xx:xx (10.x.9.230): index=0 
     time=214.100 usec
     60 bytes from 18:03:73:be:xx:xx (10.x.9.230): index=1 
     time=257.969 usec
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    Do you have proxy ARP enabled on the router interfaces? That is something you should disable because it is a security risk. – Ron Maupin Oct 15 '19 at 14:32
  • It doesn't look like you have disabled proxy ARP (no ip proxy-arp). – Ron Maupin Oct 15 '19 at 14:45
  • It's a double negative so I think this means proxyarp is enabled on the inside interface:- no sysopt noproxyarp inside however there are still some machines on this internal network that are natted and apparently disabling proxy arp will break that. So I probably need to try this change out out of hours. Possibly of interest is that I've been adding nat rules like this for the vpn:- nat (inside,any) source static obj-10.1xx.200.0 obj-10.1xx.200.0 destination static obj-10.2xx.0.0 obj-10.2xx.0.0 no-proxy-arp route-lookup – user62679 Oct 15 '19 at 14:53
  • You should not use NAT unless it is absolutely required. That would only be for public<->private addressing and overlapping addressing, but if you have overlapping addressing, then you need to correct that situation. NAT is not a substitute for routing. – Ron Maupin Oct 15 '19 at 14:56
  • Proxy ARP means different things to the ASA (and PIX) Just turning it off can make a lot of things stop working - almost at random. The crap that is "new-nat" removed all the nat0 stuff, so it's a mess to selectively nat traffic. – Ricky Oct 15 '19 at 18:51

I found the cause, I'd added a nat disabling line that didn't include no-proxy-arp:-

nat (outside,any) source static DM_INLINE_NETWORK_3 DM_INLINE_NETWORK_3 destination static AWS_ALL AWS_ALL

  • Please accept your answer so that the question does not keep popping up forever, looking for an answer. – Ron Maupin Mar 14 '20 at 16:58

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