From owner-freebsd-stable@FreeBSD.ORG Mon Sep 19 19:36:29 2005 Return-Path: X-Original-To: freebsd-stable@freebsd.org Delivered-To: freebsd-stable@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 0A6F916A41F for ; Mon, 19 Sep 2005 19:36:29 +0000 (GMT) (envelope-from cswiger@mac.com) Received: from smtpout.mac.com (smtpout.mac.com [17.250.248.73]) by mx1.FreeBSD.org (Postfix) with ESMTP id 8AF9843D48 for ; Mon, 19 Sep 2005 19:36:28 +0000 (GMT) (envelope-from cswiger@mac.com) Received: from mac.com (smtpin08-en2 [10.13.10.153]) by smtpout.mac.com (Xserve/8.12.11/smtpout16/MantshX 4.0) with ESMTP id j8JJaKER007437; Mon, 19 Sep 2005 12:36:20 -0700 (PDT) Received: from [10.1.1.209] (nfw1.codefab.com [199.103.21.225]) (authenticated bits=0) by mac.com (Xserve/smtpin08/MantshX 4.0) with ESMTP id j8JJaGVm014034; Mon, 19 Sep 2005 12:36:17 -0700 (PDT) In-Reply-To: References: Mime-Version: 1.0 (Apple Message framework v734) X-Priority: 3 (Normal) Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed Message-Id: Content-Transfer-Encoding: 7bit From: Charles Swiger Date: Mon, 19 Sep 2005 15:35:58 -0400 To: Colin Farley X-Mailer: Apple Mail (2.734) Cc: freebsd-stable@freebsd.org, matt@fruitsalad.org Subject: Re: Gratuitous ARP X-BeenThere: freebsd-stable@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Production branch of FreeBSD source code List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 19 Sep 2005 19:36:29 -0000 On Sep 19, 2005, at 3:08 PM, Colin Farley wrote: > Thanks for your reply. The model of the Cisco router is 2811. Do > you think that lowering the timeout to 5 seconds would be ok? I > have > seen that Cisco does not recommend a timeout below 30 seconds but > after reading your reply and seeing as the re are only a couple > dozen > hosts on this subnet I would think that thi s would be fine. > Please > confirm. Thanks again. Remember that the router is going to have to re-ARP for these hosts whenever something external sends traffic to them, unless the router already has another active connection going. The thing is, ARPOP_REQUESTS use a broadcast MAC address which gets sent to all of the machines on the network, which adds processing overhead not just on the router itself but also on all of these machines. Fortunately, you can see what this overhead is quite easily in order to tune things: Run "tcpdump -nt arp" and see how often your Cisco is making requests with a 5-second ARP cache timeout. So long as your network is only getting, say, a single-digit number of ARP requests per second, this amount of overhead is not going to matter significantly. Adjust as needed. -- -Chuck