I applied inbound ACL to block ping request from host IP to host IP. The effect after that is ping reply from unblocked IP on the interface that has been applied the inbound ACL is slow/loss.

Any comment? Can it be solved by enabling flow control?

Hardware: Cisco 3700 Fastethernet

  • 3
    What is slow? How do you know that it is slow? Do you know the response times before the ACL? Have you baselined the response times? Have you enabled any logging? Post your configuration.
    – Daniel Dib
    Sep 8, 2014 at 6:16
  • @DanielDib The speed of reply before and after applying the inbound ACL.
    – user49224
    Sep 8, 2014 at 9:22
  • Do you have ip cef enabled on the router? You may be process switching, which would be a lot slower.
    – Ron Trunk
    Sep 8, 2014 at 11:48
  • @Ron I enabled ip cache flow.
    – user49224
    Sep 9, 2014 at 1:48
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Nov 20, 2022 at 0:22

1 Answer 1


The problem is that you're using the finest-grained ACL (host-to-host with a specific protocol (icmp)) and it sounds like you're applying it to an interface that hosts a whole bunch of incoming traffic. You're asking the router to pay attention to three different fields to block or allow traffic, so if you could re-work that to two fields you'd likely see better performance.

Not necessarily as a permanent solution, but as a temporary alleviation to the issue, can you apply that ACL in reverse on the exit interface that leads to the 'victim' you're trying to protect? And by reverse I mean don't allow your victim's icmp echo-replies to make it back to the 'attacker'. It's true this won't stop a flood of icmp inbound from attacker-to-victim immediately, but often echo requests are stopped (automagically or manually) after there aren't any replies. And in this manner you'd free that previous interface from the ACL burden you placed on it.

  • 1
    I think it is backward to allow ICMP request to go through and block ICMP reply. The traffic has been identified useless (by me). I think it is waste of bandwidth to allow that traffic to go into backbone.
    – user49224
    Sep 9, 2014 at 2:04
  • I agree, but you're missing the point. It IS backward, and I went into detail that it is not only backward, but not optimal. It was only suggested as a temporary and testing move, and that intention was stated.
    – danno
    Sep 9, 2014 at 5:52

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